Sedimentation in the Chile Trench

AN ABSTRACT OF THE THESIS OF

Todd Mark Thornburg for the degree of Doctor of Philosophy in Oceanography presented on December ii, i984

Title: SEDIMENTATION IN THE CHILE TRENCH

ABSTRACT APPROVED: Redacted for privacy LAVERNE D. KULM

The depositional bodies of the Chile Trench--sheet flow basins, trench fans, axial channels, axial sediment lobes, and ponded basins--combine to form a coherent axial dispersal system along the base of the Andean margin. Several trench fans are severely eroded on the down-gradient lobes, implying that hundreds of meters of fan strata have been flushed into the axial channel and removed to distal environments. The sedimentary fades of the Chile Trench are modeled based on the texture and composition of coarse-grained deposits in 27 cores. The lithofacies (Channel, Levee, Basin-i

Basin-2, and Contourite) are used to interpret depositional processes and to model trench st.ratigraphy. The petrofacles (Basic

Magmatic Arc, Acid Magmatic Arc, Metamorphosed Magmatic Arc, and

Cratonic Block) yield information on provenance types in the Andean source region and on sediment transport processes across the continental margin and within the trench.

The Channel fades reflects high-energy processes within the coarse-grained bedload of' turbidity currents and occurs in fan distributary or trench axial channels. The Levee fades represents deposition beneath a rapidly decelerating current; it is deposited on the channel flanks during spillover. The Basin-i facies is characteristic of ponded deposition and occurs where the trench fill is diminished and the seaward trench wall has significant relief.

The Basin-2 facies occurs in a variety of low-energy environments.

The Contourite fades is best developed in the starved trench environment where abyssal currents are accelerated between the trench walls.

The Basic Magmatic Arc petrofacies represents the most chemically immature, least differentiated assemblage and is associated strictly with Quaternary arc volcanism of basaltic andesite composition in South Chile. The Acid Magmatic Arc petrofacies represents a more differentiated continental arc assemblage, associated with acid andesite extrusions and granadiorite intrusions of the Mesozoic and Cenozoic Andean orogeny. The Metamorphosed Magmatic Arc petro- racies represents a greenschist metamorphic assemblage whose provenance lies in the basal marine section of the Andean eugeosyn- dine and in the Paleozoic basement. The Cratorlic Block petrofacies represents mature sedimentation, late-stage granitic intrusion, and amhpibolite fades metamorphism that are associated with the ancient continental crust of northern Chile. SEDIMENTATION IN THE CHILE TRENCH

Todd Nark Thornburg

A THESIS

submitted to

Oregon State University

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Completed December 11, 198' Commencement June 1985 APPROVED

Redacted for privacy

r o Oceanography in charge of' major

Redacted for privacy

Dean of College of Oc

Redacted for privacy

Dean

Date Thesis is presented December 11, 1984

Typed by T.M. Thornburg. ACKN OWLEDGEMENTS

A special thanks goes to Vern Kuim, who during my six years at

O.S.U. made known to me the depth and breadth of oceanography, from

operations at sea with state-of-the-art equipment to the political

and economic aspects--the successes and heartbreaks. Vern put

within reach undreamed of opportunities that include my active

participation on committees that help to determine the course of

future oceanographic research, and a chance to observe first-hand

the spectacular beauty of the sea floor at 2500 meters. Vern has

been generous, willing, and helpful when prompted, yet patient with

my academic wanderings when I needed to find the box canyons by myself. Thanks also to my committee members: Drs. Paul Komar, Dick

Couch, Harold Enlows, and Rubin Landau for their comments and

suggestions on the thesis.

Technical assistance was provided by Mary Jo Armbrust, Dave

Reinert, and Sally Kuim, whose care, efficiency, and skill in word

processing and editing, photographic reproduction, and cartography and layout made my deadlines realizable. Mark 1-fower, Carolyn

Peterson, and Andy Ungerer helped with sample processing in the

laboratory, mineral separations, X-ray diffraction, and with mechanical-electrical monsters whose final digital read-out is about

all I understand. Gracias a mis colegas Chilenos-Esteban Morales

for helping with cruise logistics, Eduardo Valenzuela for providing

river samples and hospitality while in Santiago, and Chileans everywhere for their warmth and friendliness in a strange and wonderful country.

I am grateful to my family--George, Vera, Bob, and my lovely

Dallas ladies--for extending open arms and kind thoughts across these thousands of miles, and keeping the home fires burning. Gina gave me cookies, pasta, and understanding when I couldn't tell my brain from a bucket of overcooked cauliflower.Caleb made me feel calm, by comparison, during my most frenetic crises. Dave shared

Sunday morning funnies and late nights with T.M. Finally, a toast to all my buddies, near and far, who kept me hiking, skiing, boating, dancing, drumming, gambling, generally enjoying life and, especially, smiling.

This research was sponsored by National Science Foundation

Grant No. OCE-7919361. TABLE OF CONTENTS

INTRODUCTION 1

CHAPTER 1. SEDIMENTATION IN THE CHILE TRENCH: LITHOFACIES AND STRATIGRAPHY

ABSTRACT 5

INTRODUCTION 8

BASEMENT STRUCTURE 114

Nazea Plate 1)4 Outer Trench Wall 15 Trench Segmentation 16

SEDIMENTARY ENVIRONMENTS 1 7 Oceanic Plate Environment 19 Trench Axis Environment 22

TRENCH DEPOSIT LONAL BODIES 2 South Chile: Sheet Flow Basins 214 South Chile: Axial Channel 27 South Chile; Trench Fans 28 San Antonio Discontinuity: Axial Sediment Lobe 36 Central and North Chile: Ponded Basins 41

TRENCH LITHOFACIES 143 Channel Facies 145 Levee Fades 51 Basin Fades 53 Contourjte Facies 55

DISTRIBUTION OF TRENCH LITI-IOFACIES 57 South Chile 57 Central Chile 62 North Chile 64

DISCUSSION 66 Depositional Processes 66 Trench Stratigraphy 73 Preservation vs. Destruction of Trench Sediment3 81 Trenches, Fans, and the Ancient Rock Record 85

CONCLUSIONS CHAPTER II. SEDIMENTATION IN THE CHILE TRENCH: PETROFAC]IES AND PROVENANCE 91

ABSTRACT 92

INTRODUCTION

BACKGROUND 97

GEOLOGIC HISTORY 1O The Hercynian Cycle 105 The Andean Cycle 107 Quaternary Volcanism 110

METHODS 111

RESULTS 113 Light Minerals 113 Heavy Minerals 120 Factor Analysis of Light and Heavy Minerals 128

DISCUSSION 135 Petrofacies Distribution in the Chile Trench 137 Sediment Transport in the Chile Trench 1I9 Modification of Petrofacies Composition-- Provenance to Deposit 151 Diagenesis of Trench Sediments 154

CONCLUSIONS 157

CONCLUSIONS 162

BIBLIOGRAPHY 167

APPENDIX 180 LIST OF FIGURES

Figure Page

CHAPTERI

I.'l. Physiography of the Nazca Plate and Andean continental margifl 10

1-2. Axial cross-section of the Chile Trench 11

1-3. ap view of the Chile Trench sediment fill 12

1-14. Trench sedimentary environments 18

1-5. Abyssal plain turbidites 21

1-6. Sheet flow turbidites near 2°S 25

1-1. Bathymet.ry of the Bio Blo and Callecalle fans 29

1-8. Bia Bio fan: transverse seismic reflection profiles across the depositional and erosional fan lobes 32

I-9. Callecalle fan: line interpretations of seismic reflection profiles across the depositional and erosional fan lobes 33

1-10. Axial profiles across Chile Trench fans 3

I-li. San Antonio discontinuity: bathyinetry and seismic reflection of the axial sediment lobe 38

1-12. San Antonio discontinuity: 3-D block diagram

1-13. Ponded basin: line interpretation of a seismic reflection profile across Central Chile

1-14. Ideal lithostratigraphic units

1-15. Chile Trench lithofacies: factor analysis of trench cores described by lithostratigraphic units 146

1-16. Geographic distribution of Chile Trench lithofacies: factor loadings projected onto core locations 58

1-17. Idealized stratigraphic cross-sections through the canyon-mouth and inter-canyon environments 67

1-18. Idealized stratigraphic cross-sections of trench fans in the canyon-mouth environment 78 Figure Page

1-19. Accretion and preservation of trench deposits: stratigraphic control on the positioning of the decollement zone 83

CHAPTERII

11-1. Physiography of the Nazca Plate and Andean continental margin 95

11-2. Map view of the Chile Trench sediment fill 98

II-3. Axial cross-section of the Chile Trench 100

II-. Sheet flow turbidites near 42°S 102

11-5. QFL composition of modern sands from the Andean continental margin 1111

11-6. QFL and QmPk composition of Chile Trench sands: petrologic variations along the strike of the margin 116

11-7. X-ray diffraetograms of Chile Trench heavy mineral separates 121

11-8. Peak height and petrographic count ratios of heavy minerals 127

11-9. Chile Trench petrofacies: factor analysis of petrologic data 130

11-10. Geographic distribution of Chile Trench petro- faoies factor loadings projected onto core locations 138 LIST OF TABLES

Table Page

CHAPTER I

I-i. Chile Trench lithofacies 149

CHAPTER II

11-1. Factor analysis composition model: Estimates of absolute abundances of petrologic components 132 SEDIMENTATIONIN THECHILETRENCH

INTRODUCTION

The plate tectonics theory of a mobile earth lithosphere revolutionized the perspective of marine geological research. This study details modern sedimentation within the Chile Trench, a convergent plate boundary where the Nazca oceanic plate is being subducted and consumed beneath the western margin of the South

American continental plate. Onshore cordilleran belts associated with the convergence process shed coarse elastic sediments across the continental margin and into the trench basin. A trench Is a very dynamic environment where deep-sea sedimentation and convergence tectonics continually interact to produce a complex depositional record.

Sedimentary facies are the tools by which we will characterize the sedimentary environments of the Chile Trench.We will investi- gate the influence of the structure and deformation of the descending oceanic plate, and the geology and physiography of the

Andean margin on the development and evolution of the trench sedimentary environments. The trench facies may be differentiated on the basis of depositional texture and sedimentary structure

(lithofacies) or the petrologic composition of the detrital grains

(petrofacies).

The primary goals of this study are to define the ideal, end- member lithofacies and petrofacies of the Chile Trench, using Q-mode factor analysis, and to describe their distribution along the Andean 2 continental margin from 22°S to i20S latitude.More specific objectives associated with lithof'acies analysis are:

1. To describe the depositlonal bodies of' the Chile Trench

according to their morphologies and seismic stratig-

raphies, and discuss the sedimentary processes that

created them;

2. To present predictive models of trench stratigraphy that

are based on the superposition of lithofacies through

time, as the oceanic basement migrates toward and beneath

the continental slope; and

3. To discuss how the process of sediment accretion may

cause particular sedimentary environments to be preferen-

tially incorporated into the continental slope while

other environments are subducted to greater depth or

cestroyed, thereby imparting a bias to the preservation

record.

Specific objectives associated with petrofacies analysis are:

1. To relate the observed petrofacies assemblages to their

provenance in the Coast Range and High Cordilleras of

Chile, and to establish the influence of volcanism,

geology, climate, and the morphology of the continental

margin on the petrologic signal;

2. To discuss how axial transport affects the distribution of

petrofacies in the Chile Trench; and

3. to discuss how petrologic assemblages may be modified

during Ulagenesis and metamorphism. 3

It is our hope that the concepts presented will provide criteria by which ancient trench deposits can be recognized and interpreted where they are exposed on land. CHAPTER I

SEDIMENTATION IN THE CHILE TRENCH: LITHOFACIES AND STRATIGRAPHY S

ABSTRACT

The sedimentary environments associated with the depositional bodies of the Chile Trench sheet flow basins, trench fans, axial channels, axial sediment lobes, and ponded basins control the spatial distribution of lithofacies along the margin, and the superposition of lithof'acies through time to produce a stratigraphic record Sheet flow basins occur south of 41 °S, perhaps because the trench is fed by more frequent and closely spaced submarine gullies along the base of the slope which approximate a line source of sediment supply. Between 11°S and 33°S, trench fans are built at the mouths of major submarine canyon systems which act as point sources of sediment supply. The Chile Trench axial channel trends northward, along the axial gradient and parallel to the Andean margin, linking the distributary channels from numerous canyon-mouth fans to form a coherent longitudinal dispersal system. On several fans, the down-gradient lobes show evidence of severe dissection by erosional processes, implying that hundreds of meters of fan strata have been flushed into the axial channel and carried along-margin to more distal trench environments. Apparently, periods of local canyon-mouth deposition alternate with periods of massive sediment remobilization and down-axis progradation of the trench dispersal

System. Perturbations of the axial gradient are caused by tectonism in the subducting oceanic basement; the axial channel will tend to erode in areas of high gradient and deposit in areas of low gradient. A large sediment lobe, trending parallel to the margin, has been built at the base of a 600 m high axial escarpment near 33°S where the axial channel traverses a major Andean segment

boundary. In much of Central and North Chile, where the sediment

supply from the continent is diminished, the trench is incompletely filled by ponded basins.

A Q-mode factor analysis of 27 Chile Trench cores yields end member associations of depositional texture and hydrodynainic structure that define the five major trench lithofacies. The

Channel fades consists of thick, amalgamated sand units, massive to laminated or cross-bedded; this facies reflects high-energy processes within the coarse-grained bedload of turbidity currents, and occurs in fan distributary or trench axial channels. The Levee fades consists of rhytñmically-bedded, internally structureless, graded sand and graded silt units; this fades represents rapid deposition from a highly concentrated sediment suspension that has quickly lost momentum, such as occurs during 'channel spillover. The

Basin-i fades consists of more complete Bouma-like sequences that include structures from both the upper and lower flow regimes; this fades is characteristic of ponded deposition and opcurs where the trench is incompletely filled and the seaward trench wall has significant relief. The Basin-2 fades consists of graded and laminated silt units deposited under lower flow regime conditions; this fades occurs in a variety of low-energy environments, such as interchannel areas, on bathymetric features that are elevated above the trench axis, interbedcjed with hemipelagic deposits on the abyssal plain, and in distal basins that are axially removed from sediment supply points. The Contourite facies consists of non- graded silt and sand laminations that are winnowed from hemipelagic 7 muds and distal turbidites by bottom currents; this fades is best

developed in the starved trench environment where bottom currents

are accelerated between the steep inner and outer trench walls.

In a deep-sea trench, the migration of the ocean plate toward

and beneath the continental slope during convergence produces a

predictable sequence of lithofacies in the stratigraphic record. In

the canyon-mouth environment, the migration of the seaward trench

wedge toward the axial channel records a coarsening-upward sequence

that reflects the growing proximity to a zone of high-energy

sedimentation. The trench wedge section is truncated by a time-

transgressive erosional unconformity that marks its passage beneath

the axial channel. As abandoned channel deposits are carried

beneath prograding canyon-mouth fans, the trench stratigraphy

records a riningupward sequence above the basal unconformity of the

axial channel. Trench fans can produce a variety of lithofacies

successions because erosional and depositiorial phases of fan growth

are delicately balanced in the active tectonic environment, and

because flow processes vary from sheeted to channelized. If the

dcollement zone is positioned within the permeable sands of the

transgressive axial channel deposits, trench fan strata will be

preferentially accreted to the overriding continental margin while

the trench wedge strata are carried to greater depth in the

subduction zone. 8

INTRODUCT ION

A deep-sea trench is the surface expression of the subduction zone; it is a basin where sedimentation occurs within the walls of a major lithospheric plate boundary. Onshore cordilleran belts the morphologic complement of' the convergence process -- shed coarse elastic sediments across the continental margin and into the trench basin, via gravity flow processes. Trench fill can be a few kilometers thick beneath a continental arc that is actively eroding within a humid or glacial climate. A trench Is a very dynamic environment where deep-sea sedimentation and convergence tectonics continually interact to produce a complex depositional record.

Concepts and models of modern trench sedimentation have emerged from observations in the Aleutian, Peru-Chile, Hellenic, Middle

America, and Japan Trenches (von Huene, 197I; Schweller and Kuim,

1978; Stanley and Maldonado, 1981; McHillen et al., 1982; and Boggs,

19811). Deep-Sea Drilling Project sites in the Aleutian, Oregon,

Mexico, and Guatemala Trenches (Kuim et al., 1973; Watkins, Moore, et al., 1981; von Huene, Aubouin, et al., 1980) provide vertical stratigraphic sections over the last few 100,000 years. Underwood et al (1980) present a trench sedimentation model based on information from both modern and ancient environments. However, our knowledge of' trench lithof'acies and stratigraphy remains rudimentary and imprecise at present--it is constrained by limited seismic reflection coverage, sparse bottom sampling, and isolated drill hole sections. -J

The objectives of this study are to describe the depositional

bodies of the Chile Trench according to their morphologies and

seismic stratigraphies, to define the various trench lithofacies and

their distribution in the modern environment, and discuss the

sedimentary processes that created them. We present predictive models of trench stratigraphy that are based on the superposition of

lithofacies through time, as the oceanic basement migrates toward

and beneath the continental slope. Finally, we will discuss how the

process of sediment accretion may cause particular sedimentary

environments to be preferentially incorporated into the continental

slope while other environments are subducted to greater depth or

destroyed, thereby imparting a bias to the preservation record. It

is our hope that the concepts presented will provide criteria by

which ancient trench deposits can be recognized and interpreted

where they are exposed on land.

The Chile Trench study area extends from a prominent deflection

in the structural strike of the Andean Cordillera near 18°S to the

intersection of the Chile Rise near5°S latitude (Fig. 1). The

onshore climatic gradient is extreme along this latitudinal

transect, ranging from one of the world1s most arid deserts near

Antofagasta in the north, to the humid and glacially-carved terranes

near Ancud and southward. The volume of sediment in the trench axis

is closely correlated with the annual rainfall on the adjacent

continent, both varying by almost two orders of magnitude from

northern to southern Chile (Figs. 2, 3; Gaul-Oliver, 1969; Scholl

et al., 1970). This along-strike gradient in erosional denudation

and sediment supply to the offshore provides us with an excellent 10

QON

GALAPAGOS RI FT iiV

SOUTH 0° AMERICA FOREARC p. BASINS QUATERNARY ' VOLCANOES J4AGOS I0°8 -:[ISE PER BASIN

iGP' 20° z --..- RIDGE CHILE BASIN - LQ

II 30° JUAN C.) FERNANOEZ ii: ISLANDS

40°

ICHILE I /I'I,

1 I I I 120° 110° 100° 90° 80° 70°W

Figure I-i. Physiography of the Nazea Plate and Andean continental margin. Bathymetry contours after Mammerickx and Smith, 1978. Offahore features include postulated segments of the Pacific- Farallon Rise (Manunerickx et a].., 1980); onshore features include Pliocene-Quaternary strato-volcanoe (Simkin et al., 1961; Paskoff, 1977), and forearc basins as outlined by the distribution of Quaternary alluvium (after Ruiz and Corvalan, 1968). NORTH CENTRAL SOUTH CHILE CHILE CHILE

U) A cr -r I- Ui 0 -J -A

8 _:,k, I S'$ '414 £ t) S'S C I I. 141.qv t U & C D E

20 30 40

DEGREES SOUTHLATITUDE

Figure 1-2. Axial cross-section of the Chile Trench. Bathymetric thaiweg depths, depth to oceanic basement (checked pattern) from seismic reflection, and the thickness ol' the axial sediment fill (stippled pattern) are plotted according to latitude (after Schweller et a].., 1981). Segments in the subducting oceanic plate, as determined from hypocenter locations in the inclined seismic zone, are shown below (C, D, and E, after Barazangi and Isacks,197b). 12

Figure 1-3. Map view of the Chile Trench sediment till. The distribution of axial sediments is shown in stippled pattern; tan distributary arid trench axial channels are unshaded. Asterisks denote core control. The shelf break is represented by the 200-rn bathymetry contour. Submarine canyon morphology Is from Prince et al. (1980), and from data collected on the R/V MELVILLE in 1980. 13

740 730 710 75°W 720 72°W 71° 70° L ri

260

270

73° I . 28° If, 370 J

290

380

SE RE NA

300

39° 314 1 t I 400

IFAN 41° 33°

42° Figure 1-3. 14

opportunity to view trench sedimentation at several stages of d.evelopment along the margin.

BASEMENT STRUCTURE

The igneous lithologies of the Nazca Plate form the crystalline basement of the Chile Trench sedimentary basin. Basement tectonism

exerts a decisive influence on the sedimentary processes which mold

the depositionaj. bodies and structure the lithofacies of the trench.

Subduction deformation of the basement is controlled by: (a) the structural fabric that is imparted to the ocean crust during its

crystallization at the spreading center, (b) tensional stresses

associated with crustal flexure along the outer trench wall, and (c)

compressional stresses that are related to plate convergence along

the Aridean margin and to the structure of the overriding, South

American continental block.

Nazca Plate

Convergence between the Nazca and South American plates occurs

at a rate of between 10 and 11 cm/yr. with a relative orientation of

N 80°E (Chase, 1977; Minster and Jordan, 1978). The trench from

20°S to 45°S latitude maintains a uniform strike, so convergence is

nearly perpendicular and essentially constant throughout the study

area (Fig. 1). The age of the subducting crust is apparently no

older than 50 m.y. (anomaly 19) and decreases in age south toward

the Chile Rise (Pitman et al. , l97). 15

A reorientation of South Pacific spreading occurred about

20-25 m.y. ago. At this time the Pacific-Farallon Rise (striking N

30°W) became extinct, and spreading assumed the orientation of the present East Pacific Rise system (N 10°E) (Fig. 1; Mainmerickx et al., 1980). North of 38°S, the subducting Nazca crust was generated at the ancient Pacific-Farallon Rise, and south of 39°S the crust was generated at the Chile Rise the structural transition from 38° to 390 is not well defined (Herron, 1981).

Outer Trench Wall

As the Nazca Plate bends to descend into the Chile Trench, its

Outer convex surface is flexed and subject to tensional stresses.

From the outer rise to the trench axis, focal mechanism solutions of earthquakes in the upper 25 kilometers of the oceanic lithosphere are characteristically extentional (Chapple and Forsyth, 1979).

Crustal modeling of the Chilean convergence zone by Couch et al.

(1981) reveals a thinning of the oceanic crust beneath the outer trench rise as a result of this flexure.

Tensional stresses cause high-angle normal faults to develop in the seaward trench wall. The style is typically a series of step faults, down-dropped toward the trench axis, with throws of up to

100 meters or more. In North Chile, where the trench is deepest and the plate flexure greatest (dip angles to 8°), horst and graben structures develop on the outer wall, with throws of up to 1000 meters (Fig. 2; Schweller et al., 1981). Although the axis of

tensional stress is approximately perpendicular to the strike of the 16 trench, failure generally occurs along relict faults that were inherited from the spreading center during crustal formation (Hilde and Sharman, 1978; PrInce and Kulm, 1975).

Trench Segmentation

In cross-section and map view, the Chile Trench is distinctly segmented according to its basement structure and sediment distribution (Figs. 2,3). Though the amount of trench sediment fill roughly reflects the onshore climatic gradient from the northern desert to the southern glaciers, basement scarpa and offsets near

27.5° and 33°S latitude cause sharp discontinuities in the regional trench depth, the northward axial gradient, and the thickness and width of the trench fill.

From the Arica Deflection (18°S lat) to the intersection of the

Chile Rise ('5S), the trench can be subdivided into 3 provinces

(Figs. 1,2,3): (1) North Chile (18 to 27.5°S): isolated sediment basins are ponded within depressions of the faulted oceanic basement; (2) Central Chile (27.5 to 33°S): the trench sediment wedge is nearly continuous and the volume of the fill is moderate

(less than 10 km in width and several hundred meters in thickness); and (3) South Chile (33 to 45°S): the trench fill swells in volume and eventually buries the structural trench completely (20 or more kilometers wide and 1 to 2 km thick).

The observed trench segmentation appears to be convergence related because the seismic Benioff zone beneath the South American plate, the Pliocene-Quaternary volcanism in the High Cordillera, and 17 the physiography of the Andean forearc region reveal identically segmented along-strike variations. The forearo regions adjacent to both North and South Chile have developed a trench-parallel geomor- phology that defines a coastal range, an intracratonic forearc valley, and an active volcanic arc. In Central Chile, however, both the longitudinal valley and Quaternary arc volcanism are absent

(Muoz Cristi, 1956; Lomnitz, 1962). Rather sharp contortions or tears in the subducting oceanic lithosphere near 27.5° and 33°S define a zone of shallow, near-horizontal subduction beneath the volcanic gap of Central Chile (Fig. 2; Stauder, 1973; Barazangi and

Isacks, 1976).

SEDIMENTARY ENVIRONMENTS

As an oceanic plate descends into the trench axis of a continental arc, it typically carries a conformable sequence of pelagic and terrigenous strata on top of the basaltic basement; these strata constitute the deposits of the oceanic plate

environment (Fig. ; Schweller and Kulm, 1978). Because the subducting plate enters the trench with a dip of several degrees, the trench axis strata will onlap the oceanic plate deposits with slight angular unconformity to produce a seaward-thinning, wedge- shaped deposit known as the trench wedge (Piper et al., 1973). In areas of high sediment supply, proximal to the mouths of major submarine canyons, trench fans may develop near the base of the slope that prograde across the trench wedge environment. The trench 18

- STARVED TRENCH -

A.North Chile 0 0 a-

) , I. I

TRENCH AXIS FACIES Trench Fan B. Central Chile I- 1Trench Wedge

( OCEANIC PLATE FACIES ') c _..Terrigenous 11111 Pelagic '.t- Oceanic Basalt C. South Chile

t.) I..

- FILLED TRENCH -

Figure I-4. Trench sedimentary environments. Schematic cross- sections through the Chile Trench sediment fill, showing the stratigraphic relationships that develop as an oceanic plate migrates across the abyssal plain and descends into the trench axis. iUong the latitudinal gradient from the North Chile to South Chile provinces, the sediment supply to the deep-sea environment varies from low (starved trench conditions) to high (filled trench conditions). 19

wedge and trench fan strata together constitute the deposits of the trench axis environment (Fig. J).

The Oceanic Plate Environment

The oceanic plate section records the plate's passage across the abyssal plain, from its genesis at the spreading center to its destruction at the subduction zone. Typical lithofacies of the pelagic environment include oxidized metalliferous clays with authi- genie mineralogy, and biogenous chalks and cherts (Berger, 197k; van

Andel, 1975). As the plate nears a continental landmass, a rain of terrigenous hemipelagic mud, carried in suspension along density interfaces in the water column, will greatly dilute pelagic sedimentation. The rate of hemipelagic mud accumulation (centimeters to

O's of centimeters per 1000 years) is roughly an order of magnitude greater than that of true pelagic rain (millimeters to centimeters per 1000 years), though the transition between these two environments is typically gradual. Reduced gray and green muds with a continental mineralogy are the prevalent lithofacies in the terrigenous section of the oceanic plate environment. At high latitudes, ice rafting may cause coarse, poorly-sorted glacial debris to be interbedded with the hemipelagic muds on the abyssal plain (Schweller and Kulm, 1978; Kuim et al., 1976).

Gravity flow deposits (turbidites, grain flows, etc.) may become interbedded with the more typical lithofacies of the oceanic plate environment by at least two mechanisms: (1) During its journey toward the subductiorl zone, an oceanic

plate may come within reach of terrigenous distributary systems that supply coarse sediment (silt and sand) to the distal abyssal plain, either laterally across transform plate boundaries or via deep-sea

channels such as those found in the northeast Pacific (Fig. 5;

Nelson and Kuim, 1913; Ness and. Kuim, 1973). The Paleogene "Zodiac

Fan" beds of the Aleutian Abyssal Plain, sandwiched between pelagic

chalk sequences, are an example of a turbidite deposit that is

isolated within the oceanic plate stratigraphy (Stevenson et al.,

1983). Such deposits, if eventually overlain by similar lithofacies

in the trench axis environiient, will likely be characterized by the unique petrology of a distant source terrane.

(2) When the sediment influx from the adjacent continent is very high, a trench may become completely buried as a topographic feature, and turbidity currents can spillover onto the adjacent abyssal plain. Examples of filled trenches occur offshore from

Oregon-Washington (Kuim et al., 1973) and South Chile of the present study. Stratigraphically, the frequency and coarseness of turbi-

dites should gradually increase upsection in the terrigenous oceanic

plate deposits, reflecting the migration of the plate toward the filled trench during convergence. Because they were derived from

the same provenance region, these deposits should be very similar in petrologic composition to those of the overlying trench axis section. 21

V '. V '. " ..\ ..' \ \ \ .' " \ '\ ,.-, 0 NJ 1\ _\, \/.\/./s,_\/.\, k ,_ At. -I :...... :.,. :... . , "S -- II.If "t'% I . - -- , I, I, / Ft - ___ I - - I, - - - 'I- % . II ... 1 1, 1, U . '- -v-v-i.

- - .: I ::: "2

'.5

Trench Axis Turbidites [jAbyssal Plain Turbidites I--iPelagiclHemipelagic Muds

Figure 1-5. Abyssal plain turbidites: mechanisms by wilich coarse- grained turbidites may be laid down on the abyssal plain, incorporated into the oceanic plate section, and later overlain by trench axis turbidites. These include (a) distal transport from a continental margin via deep-sea channels, and (b) lateral input from a continental landmass across a strikeslip plate boundary. 22

The Trench Axis Environment

Sedimentation in the trench axis environment is dominated by catastrophic gravity flow events that are superimposed on a more steady state rain of hemipelagic material through the water column.

During interglacial high stands of sea level, when submarine canyon

heads are isolated from nearshore sediment sources, the trench axis

environment may be reduced to finegrained, hemipelagic sedimentation

(von Huene, 1974; Scholl and Marlow, 19714). But during glacially-

lowered sea level, when coarse terrigenous detritus is actively

by-passing the shelf to the deep-sea environment, the sedimentation rate in the trench axis (meters per 1000 years) can be an order of

magnitude greater than the rate of hemipelagic sedimentation on the

abyssal plain. In the Chile Trench, up to two kilometers of trench axis strata can accumulate in several hundred thousand years, but

the few hundred meters of strata that constitute the entire oceanic

plate section may represent tens of millions of years of sedimentation.

The nature of the depositional environments in the trench axis

are primarily controlled by the rate of sediment supply to the

trench basin relative to the convergence rate (Schweller and Kuim,

1978). While the convergence rate remains roughly constant along

the Chilean margin, the latitudinal gradient in sediment supply

creates a complete spectrum of trench settings that vary from sediment-starved to sediment-filled (Fig. 4).

In the North Chile province, the trench axis is often

completely devoid of sediment, and the terrigenous deposits of the 23 oceanic plate environment are poorly developed (Fig. '4a). Under such harsh arid conditions, the sediments that are shed from the

High Cordillera are trapped behind the Coast Range in the forearo valley, and drainage to the offshore is confined to the seaward flank of the Coast Range (Mortimer and Saric, 1975). Island arc trenches, by comparison, are also commonly barren of terrigenous sediment, and for basically the same reasons: the subaerial drainage basins are quite small and much sediment is trapped behind uplifting structural highs in the forearc (Karig and Sharman, 1975;

Underwood and Karlg, 1980; Boggs, 198fl.

With increasing sediment supply, as in the Central Chile province, a trench wedge of significant volume develops in the trench axis, and the terrigenous section on the oceanic plate expands in response to the higher concentration of suspended particulates in the water column (Fig. nb). Trench fans are well- developed only in the South Chile province, where major submarine canyons cross the continental slope at regular intervals to provide point sources of large-volume turbidity currents to the trench basin

(Fig. 1c). The terrigenous oceanic plate environment'benef its from the increased sedimentation rate in South Chile, especially where the trench is completely filled and turbidity current spillover is prevalent. 24

TRENCH DEPOSITIONAL BODIES

South Chile: Sheet Flow Basins

South of 11°S the trench fill is flat and unchannelized, and deposition occurs by sheet flow turbidity currents (FIgs. 3). These sheet flows are limited only by the total width of the trench wedge

(30 or more km), or they evolve completely uriconfined and spill onto the adjacent abyssal plain. In seismic profiles across the sheet flow region, shallow reflectors are near horizontal, conformable, and continuous across the entire width of the trench wedge (Fig.

6a). Deeper reflectors appear warped and offset; these older beds have been deformed by extensional movements of the underlying oceanic basement. Some normal faults propagate up through the sedimentary section and break the surface, especially near the

seaward wall where the axial sediment fill is thin and plate flexure

is most pronounced. Extensional deformation has created a subtle relief of perhaps ten meters or less on the depositional surface.

Sheet flow turbidjtes will tend to thicken within depressions and

thin above bathymetric highs (Ricci Lucchi and Valmori, 1980), so

surface relief is continually smoothed during periods of active

deposition.

tn seven cores near 1120S, individual turbidites can be

correlated across the 30-km-wide trench wedge using a variety of

criteria, including stratigraphic position, sedimentology, radio-

carbon dating, and ash content and mineralogy (Figs. 6a,b). An

upper turbidite the Black Ash unit is rich in volcanic lithic 25

Figure 1-6. Sheet flow turbidites near I2°S. (A) Seismic reflection profile showing extensional faulting and mild deformation of the conformable sequence, with location of coring transect. (B) Lithologic correlation of individual sheet flow units (the Black Ash and Sand Lens units) in piston cores across the 30-km-wide trench wedge. Carbon-14 age control is shown. Note the lateral lithofacies changes in the Sand Lens unit, reflecting a channelization of velocity during turbidity current deposition (CLF = Channel lithofacies, LLF Levee lithofaclés, BLF = Basin Lithofacles). The less disturbed section in the gravity core trigger weight is spliced on top of the piston core section of core M-09. 26

STRATIGRAPHY OF SHEET TURBIDITES, 42°S w E.

0 0 0 0 o 0 0 I 2I 5 2I 25I I 5I SEC -5

V.E. 18:1 KM

N-09 M-Oe M-05 M-O8 M-07 11-04 M-1O

*E 0 8lack Ash 2

Unft s,aso ± 90

.. Sand Lone BLF r-:__-

kin

Figure 1-6. 27 grains, while a lower turbidite the Sand Lens unit--has a more quartzofeldspathic composition and shows evidence for lateral lithofacies changes induced by an along-axis channelization of velocity during deposition (Fig. 6b).

South Chile: Axial Channel

The continental slope of the South Chile province is extensively dissected with submarine canyons fron41°S to 33°S (Prince et al., 1980; Fig. 3). The inception of the trench axial channel emanates from the mouth of an unnamed submarine canyon at 41°S. The channel initially follows the base of the Slope, but it is Soon displaced seaward by prograding fans at the mouths of submarine canyons. Canyon-mouth distributary channels apparently braid and become tributary to the main axial channel.

The Chile Trench axial channel maintains a fairly uniform northward gradient of 1:650 (Fig. 2). Channel width and relief vary from 1 to 5 km wide and from 50 to 200 m deep (Sohweller and Kulm1

1978). These dimensions compare quite favorably with both submarine f an and abyssal plain channels (Nelson and Kuim, 1973; Griggs and

Kulm, 1970). South Chile: Trench Fans

Detailed surveys--including bathymetry, seismic profiling, and reconnaissance coring--were conducted on two submarine fan systems

in the Chile Trench. These are the Bio Bjo fan (near 37°S) and the

Callecalle fan (near I0°S) (Fig. 3). A third submarine fan, which remains unnamed, was discovered near Il0S. The morphology and structure of these fans are unique and extraordinary when compared to conventional submarine fan systems (i.e., Norrnark, 1970; Mutti and Ricci Luochi, 1972; Nelson and Kuim, 1973; Walker, 1978; Normark et al., 198l; and many others).

Basement Tectonics and Sedimentation

The oceanic crust that forms the basement ror the Blo Bio fan was generated at the ancient Pacific-Farallon Rise; the basement of

the Callecalle fan was generated at the Chile Rise. The orientation of extensional rupture along the seaward trench wall appears to reflect the structural grain of the spreading centers where the

crust originated (i.e., N20°W for the Blo Blo fan and N-S for the

Callecalle fan, Fig. 7). Magnetic anomalies indicate that the oceanic crust beneath the Blo Bio fan is approximately 35 m.y. old, about twice the age of the basement of the Callecalle fan (Herron,

1981).

The trench axial channel trends northward, down the gravita-

tional gradient, along the base of both the Bio Blo and Callecalle

fan systems (Figs. 3,7). The location of the axial channel is often

controlled by extensional tectonism in the underlying oceanic 29

Figure I-?. Bathymetry of the Bio Bio and Callecalle fans. The transverse seismic reflection profiles in Figures I-B and 1-9, and the axial bathymetric profiles in Figure 1-10 are located. Note axial channel, which truncates the seaward development of these fans. The bold arrows point to the erosional scarp which divides each fan into an up-gradient depositional lobe and a down-gradient erosional lobe. A. BlO BlO FAN B. CALLECALLE FAN

74M0'W 7430' 7420' 741C 1520 1510 7500' 1450 7440 3620 S

363O

64O

3650

37OO

I(M DEPTH IN MATTHEWS CORRECTED METERS CONTOUR INTERVAL 5m 0 10 20

Figure 1-7. 0 31

basement (Figs. 7,8,9). Step faults and grabens create axial depres-

sions in the trench floor that apparently concentrate the coarse

bedload of turbidity currents. Buried channel deposits beneath the

Callecalle fan, evidenced by highly reflective bodies in the

subsurface, can also be correlated with basement offsets (Fig. 9).

Depositional and Erosional Morphologies

An abrupt, cross-trench erosional scarp has developed between

the canyon mouth and the axial channel on both the Blo Bio and

Callecalle fan systems (Fig. 7). The scarpa are 150 to 200 meters

high (near 36°'10' and 1400001) and they clearly divide the fans into

a southern (up-gradient) lobe of depositional morphology and a

northern (down-gradient) lobe of erosional morphology. Axial

profiles across the fans--striking nearly parallel to the margin--

illustrate well the morphologic differences between the depositional

and erosional fan lobes, and the relief on the erosional scarp that

separates the fan lobes (Figs. 7, 10, sections X-X',Y-Y',Z-Z').

Hundreds of meters of fan strata have apparently been removed from

the northern lobes and carried down the axial gradient to more

distal reaches of the trench, via the axial channel.

On the Bio Bio fan, the depositional lobe displays flat-

floored distributary channels with well-developed flanking levees

(Figs. 7a, 8, section A-A'; Figs. 7a, 10, sections X-X', Y-Y'). A

major deposjtjcjnal fan channel extends S-SW from the canyon mouth

near the base of the continental slope and eventually becomes

confluent with the axial channel (Fig. 7a). A levee has been built

on the flat, seaward trench wedge across the axial channel from the 32

BIO BlO FAN

A- DEPOSITIONAL LOBE (36.8OS) - A' LVe Lfl.sS 6.0 T.ncf. Wdgs c:::n.I Aia$ / Chnns

7".. I I- 6.5

7.0

It 7.5

10 KM

B- EROSIONAL LOBE (6.59S) - B' 6.0

6.5

7.0

'I Batem4flt I I I . II 'I 7.5 I I VE 12 10KM

Figure 1-8. Bio Bic fan: transverse isznic reflection profiles across the depositional and erosional fan lobes. See Figure1-7 for locations. 33

CALLECALLE FAN C C,

- ___ 10km

I- I V.E. 10:1

5.5 seconds

r o e 1) U) 3 o -

... tj.i a

Figure 1-9. Callecalle fan: line interpretations of seismic reflection profiles across tte depositional (C-') and erosional (D-D', E-E') fan lobes. See Figure 1-7 for locations. Tne oceanic basement is how in flecked pattern. Reference lines are 5.5 seconds 2-way travel time in all profiles. 34

S N Depositional Lobe Erosional Lobe

BlO 610 FAN (37°S) --0-- V x'

e z CALLECALLE z, (40°S)

"i F- OMt

UNNAMED FAN

AXIAL PROFILES OF CHILETRENCH FANS

Figure 1-10. Axial profiles across Chile Trench fans. Three different fan systems show a remarkable parallel in asymmetric, bilobate morphology, such that the down-gradient fan lobes are severely di.ssected by erosional processes. Profiles are oriented so that the viewer is looking seaward (down-channel) from the canyon mouth, with the axial gradient to the right. See Figure 1-7 for locations. Subbottom data are from 3.5 KHz and single- channel seismic reflection records. 35

mouth of this distributary. A possible crevasse splay breaches the distributary channel near the confluence with the axial channel. In contrast, the erosional lobe is characterized by apparent braided and V-shaped gullies that have cut deep into the fan sediments, tending to reduce the fan's relief to the base-level depth of the axial channel (Figs. 7a, 8, section B_Bt; Figs. 7a, 10, sections x-x' ,Y-Y').

On the CallecaJ.le fan, the depositional (or up-gradient) lobe is built of sheeted bodies that are lens-shaped in crosssection, with surfaces furrowed by small distributaries (Figs. 7b, 9, section C-C'; Figs. 7b, 10, section Z-Z'). We suspect that these are depositional sand lobes; the coarseness of the deposits is suggested by the lack of seismic penetration beneath the surf icial reflectors. On the down-gradient side of the fan, a network of braided channels has carved the erosional lobe into a series of bars

(Figs. 7b, 9, sections D-D', E-E'; Figs. Tb, 10, section Z-Z'). The coarsest material recovered from the entire Chile Trench was taken from an erosional channel here--the core penetrated a one-meter- thick, massive sand bed, and common laminated and cross-bedded sands.

Submarine Fan at 41°S

An unnamed fan at 41°S (Fig. 3) displays a bilateral, morphologic asymmetry that is remarkably similar to the other two fan systems, as seen in the axial profiles of Figure 10. All three fan systems exhibit an up-gradient depositional lobe that is truncated by a cross-trench scarp which attests to pronounced erosion of the 36

down-gradient tan lobes. Superimposed on the obvious morphological

similarities between the three fan systems are more subtle

differences in depositional and erosional style that may reflect

differences in the grain size or composition of the sediment supply.

For example, the total relief on the erosional fan lobes--from the

crest of the erosional scarp to the thalweg of the axial channel--

appears to increase systematically toward the south, perhaps

reflecting an increase in the grain size of the sediment supply as

glaciation of the source regions becomes more intense.

The 1°S fan is unique because it represents the transition

from sheeted to channelized turbidity current deposition in the

filled trench of South Chile. Distal deposits of the depositional

fan lobe interfinger with sheet flow deposits to the south, and the main distributary channel of the erosional lobe is deflected northward against the continentalsiope to conceive the axial channel (Figs. 3,10).

San Antonio Discontinuity: Axial Sediment Lobe

San Antonio Canyon is apparently tectonically controlled by

the same transverse mega-structure or shear zone that creates a

discontinuity in the trench near 33°S (Figs. 2,3). Here, a 600- rn-high, basement-involved scarp in the trench axis causes an abrupt

increase in the axial gradient (to greater than 1:200), and forms

the boundary between the Central and South Chile provinces (Fig. 2;

Schweller and Kuim, 1978). The sharp gradient increase causes the

axial channel to become very erosional in character, incised, and V-shaped in cross section (Figs. 11,12). The confluence of San

Antonio Canyon with the axial channel just above the axial scarp at

33°S probably contributes to the pronounced erosion here.

At the foot of the scarp the gradient abruptly flattens to

1:650. Turbidity currents that cross the 33°S scarp into Central

Chile likely undergo an hydraulic jump that swiftly curtails

momentum and forces the rapid deposition of much of the transported

sediment load (Komar, 1971). Over time, turbidity currents have

built a lobe-shaped body of sediment at the scarp's base (Figs.

11,12). The lobe is approximately 15 km wide and 15 km long, with more than one second of sediment above the basement reflector. The

lobe deposits are sometimes conformable and sheet-like (Fig. ha),

and sometimes they exhibit depositional discordance produced by

levee sedimentation (Fig. lib). Normal fault offsets reflect

tectonic instability in this trench subbasin (Fig. ha).

The axial sediment lobe superficially resembles a submarine

fan in that both of these depositional bodies are produced by rapid

sedimentation following an brupt loss of gradient. The lobe

differs from typical fans because current directions are parallel to

the margin rather than perpendicular and radial, and distributary

channels are not present. Instead, the axial channel continues

uninterrupted across the lobe and north into the Central Chile

province, where the axial gradient flattens and the channel

eventually disappears. 38

Figure I-il. San Antonio discontinuity: bathymetry and seismic reflection of the axial sediment lobe. Top: Bathymetry from Prince et al. (1980); see Figure 1-3 for location. The trench fill is stippled. Bottom: Seismic reflection profiles across the sediment lobe at the base of the axial scarp. Note extensional faulting in profile A, depositional discordance caused by levee sedimentation in profile B. 39

SAN ANTONIO DISCONTINUITY

33°

73° 720 71°

A. 32.52°S B. 32.50°S w E W E

Late Axial Z .' Levee Axial . i Channel I Channel

: ii j. 'f t

90 F o 5 km SECONDS 0 5km V.E. 1$: 1 Two- Way) V.E. 22: 1

Figure I-li. Figure I-2. San Antonio discontinuity; 3-D block diagram. Note the confluence of the axial channel with the San Antonio fan" channel in the flat-lying trench flu, of the South Chile province, the basement-controlled axial scarp at the segment boundary, and thesediment lobe at the base of the scarp. Coarse-grained axial channel deposits are stippled; oceanic basement Is checked pattern. 41

Central and North Chile: Ponded Basins

Because San Antonio Canyon opens into the trench at the head of' the 33°S axial scarp, it provides an apical point source for. the trench sediments of southern Central Chile (Fig. 3). However, petrologic studies show that trench sediments north of 31°S share a common provenance that is distinct from the southern San Antonio source(Chapter II, this study). The sediments of northern Central

Chile are probably derived from a submarine canyon that heads at L.a

Serena near 3Q05 (Fig. 3).

The North Chile province, adjacent to the Atacania Desert onshore, represents the most sediment-starved environment of the entire Peru-Chile Trench (Figs. 2,3). SubmarIne canyons are rare and mostly inactive. Isolated sediment ponds are presently disjoined from the northward longitudinal transport system that controls sedimentation in the Central and South Chile provinces.

The porided trench fill of Central and North Chile generates horizontal and conformable seismic reflection sequences (Fig. 13), suggesting that deposition occurs via sheet flow turbidity currents.

However, these ponded sheet flow basins differ fundamentally from

the sheet flow basins south of 41°S, in the South Chile province, because: (1) there is no axial gradient, and (2) turbidity currents are severely limited by the confines of the structural trench owing to a substantial reduction in the volume of the trench fill the

depositional surface is much narrower and the seaward trench wall has much greater relief. 42

PONDED BASIN 8.0 E 29.31°S ___1

. .' 8.5 \ . -

0 5 KM V.E. 14:1 9.5 SEC

Figure 1-13. Pondecl basin: line interpretation o' a seismic reflection profile across Central Chile. 43

TRENCH LITHOFACIES

Twenty seven cores (25 piston, 2 Kasten) were recovered from

trench axis and trench wall environments In the Chile Trench, the majority of these having been collected during an Oregon State

University cruise in November 1980 aboard the R/V MELVILLE.

Radiocarbon dates indicate that our cores rarely sampled material

older than twenty or thirty thousand years. Most of the coarse-

grained lithologies were probably deposited during lowered sea

level, and are perhaps associated with the latest Pleistocene

glacial maximum near 18,000 y.b.p. (Imbrie et al., 198J4).

The Chile Trench sediments were described by grain size and

hydrodynainic structure, because these parameters are commonly used

to interpret the energy of the depositional environment in terms of

flow velocity and bed stress (Komar, 1985). Permutations of silt

and sand, grading and lamination, yielded the ideal lithostrati-

graphic units of Figure 14. Each core, or stratigraphic section, was digitized by the percentage of the vertical dimension that is

composed by each of the seven coarse-grained lithostratigraphic

units (non-graded, non-laminated silt is so rare that it is

insignificant in the analysis). These data were scaled to constant means and subjected to Q-mode factor analysis with varimax rotation

(Kiovan and Imbrie, 1971) to isolate end-member associations of

lithology and sedimentary structure.

A four factor model produced the most reasonable breakdown of

the lithologic data, with an equitable distribution of sample

variance among the factors: Factor I, 23.2%; Factor II, 21.6%; 44

CHILE TRENCH LITHOSTRATIGRAPHIC UNITS

NON-GRADED GRADED LAMINATED GRADED and SAND SAND SAND LAMINATED SAND

GRADED LAMINATED GRADED and SILT SILT LAMINATED SILT

Figure 1-14. ideal lithostratigraphic units. These units represent permutations of grain size (silt and sand) and sedimentary structure (grading anlamination), with the exception of non- graded silt which is rare in ChiJ.e Trench cores. 45

Factor III, 19.0%; Factor IV, 24.7%. These factors define the ideal sedimentary lithofacies of the Chile Trench: the Channel, Levee,

Basin-i and Basin-2, and Contourite lithofacles, respectively (Fig.

15 a-d; Table 1). Our discussion of Chile Trench lithofacies includes a comparison of sedimentary structures with the ideal hyd.rodynamic sequence of Bouma (1962) for turbidity current deposition, and a comparison of our facies associations with those described by Mutti and Ricci Lucehi (1972) for ancient deep-sea fan deposits.

Channel Fades

As defined by Q-mode factor analysis, the Channel fades

Consists of laminated sand, internally graded or non-graded, and massiveor structureless non-graded sand; laminated units are occasionally cross-bedded (Fig. 15a; Table 1). Bed thickness ranges from iO cm to >1 m (medium to very thick bedded), sand is typically fine to medium grained, the sand to mud ratio exceeds 1:1 (sections may locally reach 3:1 or 4:1), and amalgamated sand bodies are

common. This facies occurs within fan distributary channels (core

M-16), in the trench axial channel (cores M-13 and M-17), and on channel walls, particularly on the outside of channel meanders.

This facies also develops in the sheet flow basins of South Chile, beneath the high-velocity zones of unchannelized turbidity currents

(Fig. 6b).

Some units of the Channel fades suggest truncated Bouma sequences Tae, The, and Tee (Fig. 15a; Table 1). Beds of' 46

Figure 1-15 (a through d). Chile Trench lithofacies: factor anaJ.ysis of trench cores described by lithostratigraphic units. Core sections that are heavily loaded by each factor provide representative stratigraphic columns. See text for discussion. CHANNELfactor variance: 23.2 % LEVEEtacfor variance : 21.6 % NON-GRADED SAND, LAMINATED SAND, GRADED SAND, GRADED SILT GRADED and LAMINATED SAND -.4 Figure 1-15. I a fllP14 I!T

i-- -:

IJ-_J

BASINfactor variance:19.0% CONTOURITE(actor variance24.7 % GRADED and LAMINATED SILT LAMINATED SILT 00 Figure 1-15 (con't). Table I-i. Chile Trench Lithofaci.es

BASIN-i BASIN-2 COHTOURITE CHANNEL LEVEE Reworked silt and fine Graded but structurelessMore complete Graded and laminateG PESCRIPTION Thick, massive sandS silt; no scour; may sand lajninae; often in Structureless or Sands and silts; some sequences of graded clusters or truncating basal scour; associated hydrodynamlc (Booma) grade Into Basin-I laminated, some cross- facies or Channel/ the tops or graded beds Dedded; riongraded with Channel fades in atructures, common to graded; common proximal environsent scour below basal Levee association scour sands toward high-energy en v irormients

Medium-bedded (10- Thin to medium bedded Commonly bnterbedded with SEEmING Tum to very thick Thin-bedded (5-15cm); hemipelagic muds or low- characteristically 30cm) (5-30cm); Interbedded bedded (lOom to >lm); with hemipelagics energy turbidites amalgamation common rhyti-mic (Bastn-2 factes) Tl -1:1 Much less than 1,1, Variable; depends on SAND;MUD >1:1; can be as high commonly

T baSe cut- N.A. SL)UFIA includes E, T56 Tb Tde Tbe TpcdeIbde our, l&rd flow regime STRUCTURES truncated sequences, more compeen,e am- upper Flow regime quences, Including structures tram both upper and lower flow regimes

MulTI and A and B E C and 0 P RICCI LUCCHI FAC I ES 50 non-graded sand with planar cross-bedding, and non-graded structureless sand are more suggestive of grain flow deposits than typical turbidites. While pure inertial grain flows can exist only on very steep slopes (greater than 18° according to Middletori and

Hampton, 1976), grain flow transport may occur on more gentle slopes

if the grainsupporting dispersive pressure is enhanced by the bed shear of an overlying unidirectional current.We expect that grain flow processes operate within the bedload of turbidity currents, similar to the "traction carpet" concept of Sanders (1965) or the

"modified grain flow" of Lowe (1976). By this mechanism, cohesion-

less sands may be reworked by one or more turbidity current events

to form non-graded, laminated to cross-laminated sands. Pure grain flows and liquefied flows may also be generated laterally on the steep channel walls to form structureless, non-graded sand.

The Chile Trench Channel facies correlates well with the highly channelized Facies A (our structureless, non-graded sands) and Facies B (our laminated to cross-bedded, non-graded to graded

sands) of Mutti and Ricci Lucohi (1972); these authors also infer

grain flows to be the primary mechanism of deposition.Massive

coarse sandstone, pebbly sandstone, and conglomerate are probably

found within inner fan channels, most proximal to the mouths of

major submarine canyons (Nelson and Kuim, 1973; Stanley et al.

1978; Walker, 1978), though we did not core these environments in

the Chile Trench. From seismic records, it is evident that the walls of' the trench axial channel commonly fail by slumping and

sliding into the channel axis (see profiles of Fig. 9). In this

way, contorted or chaotic beds that have deformed during 51

remobilization (Fades F of Mutti and Rioci Lucehi) may become

interbedded with the Channel facies.

Levee Fades

The Levee fades Is characterized by graded but rather struc-

tureless (non-laminated) sands and silts (Fig. 15b; Table 1). These

are thin-bedded deposits (5-15 cm thick) with a sand to mud ratio

near 1:].; scour marks are common at the base of sand beds. A

diagnostic trait of the Levee fades is the very rhythmic bedding

that develops in the stratigraphy. As the name implies, this facies

is strongly represented on the modern levees of fan distributary and

trench axial channels (cores F-Ui and M-19) and in fining-upward

sequences within channels (core M-14). The tacies is poorly repre-

sented in the ponded regions of the Chile Trench (i.e., those regions with no axial gradient).

The Levee fades does not fit well into a Bouma scheme, but it

is best described as a basal graded sand ("a" Interval) succeeded

immediately by a graded pelite (Ttetinterval). The "a" to "e"

transition may be gradual or abrupt; if abrupt, the basal sand will

have a sharp upper contact. Structureless graded beds apparently

occur when a highly concentrated sediment suspension congeals very

rapidly and without the development of bedforms during deposition

(Micldleton and Hampton, 1976; Banerjee, 1977). This situation

probably results following the release of a turbid suspension from a

channelized high-velocity environment to an unconfined levee

environment where energy dissipates rapidly. 52

The Levee facies correlates remarkably well with Mutti and

Ricci LuccYij' Fades E (Table 1). These workers describe rhythmic,

thin-bedded (commonly less than 15 cm thick) Bouma Tae sequences with a sand: shale ratio near 1:1. They also note the common associ-

ation of this fades with Fades A and B (our Channel facies) In the

inner and middle tan environments.

Eight radiocarbon age determinations from various Chile Trench

environments yielded sedimentation rates of between 10 and 30 cm/i 000 yr for the last 12,000 years of Holocene time. Sedimenta-

tion rates of between 1and 3 meters/bOO yr are more typical during

glacial periods, (Kuim et al., 1973; Schweller and Kuim, 1978).

Levee sedimentation rates of between 25 cm/i 000 yr and 200 cm/i 000

yr can be produced by varying the thickness of the individual turbi

dites between 5 and 10 cm, and the frequency of turbidite events

between one in 50 to one in every 200 years. It is interesting that

the frequency of great earthquake events (M>8) along the Chilean margin is roughly one in 100 years (Kelleher, 1972); such shocks may

provide a triggering mechanism for large-scale turbidity currents.

The levee environment is unique because it is close to a zone of'

high-energy sedimentation but it does not suffer the erosion and

reworking that occurs in the adjacent channel environment. The

proximal levees appear to provide the most accurate and complete

record of resedimentation events in the associated feeder canyons. 53

Basin Fades

The Basin fades is described by graded and laminated silt units according to the Q-mode factor analysis (Fig. 15c). The Basin fades is generally thin to medium bedded (5 to 25 cm) and displays sharp, but not scoured, lower contacts and graded upper contacts.

Normal grading may occur either by an upward decrease in the average grain size of the silt la.minae or by an upward increase in the thickness of the clay interbeds. Hesse and Chough (1980) observed similar grading in silt turbidites flanking an abyssal channel in the NW Atlantic. Distal turbiditic muds differ from hemipelagic muds by showing evidence of rapid burial (higher organic carbon content or preservation of carbonate below the CCD) or the presence of allochthonous mixed fossil assemblages from the adjacent margin

(Piper, 1978).

The Bouma sequence of hydrodynamic structures very neatly describes the Basin facies -- graded and laminated silts represent base cut-out Tde units (Fig. iSa, core F-03). Occasionally the beds will include basal cross-laminated silts (core 1-81), plane- laminated sands (core M-03), or graded sands (core W-22), suggesting more complete Bounia sequences or less severe base cut-out (Tode,

Tbcde, Tabcde). The sand to mud ratio appears to decrease with increasing base cut-out, from near 1:1 with basal Bouma "b" units to

1:5 or less with basal Bouma "d" units.

It is a unique environment in the Chile Trench where structures from both upper and lower flow regimes are preserved for a given flow event to form more complete Bouma-like sequences. 54

However, the factor analysis was unable to identify this association

as an illustrative end-member lithofacies that formed under a peculiar set of' physical circumstances. The upper silty portions Of

the sequences simply factored into the Basin end-member, and the

basal sands factored into the Channel or Levee member, i.e.,

individual turbidites were artificially split into fades couplets.

To correct this, we have subdivided the Basin fades into two sub-

facies (Table 1). The Basin-i subfaeies consists of beds with rather complete hydrodynainic sequences that include structures from

both the upper and lower flow regimes (Tabede, Thode, Tbde), and the

Basin-2 subfacies contains beds of graded and laminated silt that were deposited under lower flow regime condition8 (Tde, Tcde).

The Basin-i fades is best developed in areas of ponded depo-

sition, such as in North and Central Chile where the sediment fill

is sparse to moderate, and it Is restricted to the proximal areas of

the trench floor (Fig. 15c, cores M-03 and W-22). It also occurs in the sheet flow basins of southern Chile where high- and low-energy lithofacies overlap (Fig. 6b, core M-Ofl. The asin-2 facies is much more widespread and diverse In its environmental affinities.

It is well developed in the ponded trench environment, especially on trench walls (core F-03) and bathymetric ridges (core Y-81) that are

elevated above the trench floor. In the filled trench, the Basin-2 fades can be found in interchannel areas of the proximal, canyon- mouth environment (Fig. 15b, interbedded with the Levee fades in core F-al), in areas that that are axially removed from up-gradient sediment supply points, and interbedded with hemipelagic deposits on

the abyssal plain. The Basin-2 facies is also found on the fringes 55 of lenticular sheet sands in the sheet flow basins of southern Chile

(Fig. 6b). The Basin-2 fades may grade into the Basin-1 fades toward the high-energy environments of the ponded trench, or it may grade into the Channel/Levee association toward the high-energy environments of the tilled trench.

The Basin-i fades is analagous to Fades C of Mutti and Ricci

Lucohi (1972), but these authors are more strict in their application of the facies (Table 1). They limit ita use to those turbidites that contain the complete sequence of classical Bourna structures: Tabcde. The Basin-2 fades correlates well with Facies

D. Mutti and Riaci Luechi describe Fades D as a series of base cut-out beds (Tode and Tde common) with thicknesses ranging from 3 to 14Q cm (excluding the graded pelite interval) and sand to shale ratios from 1:2 to 1:9. They emphasize that Facies D beds are strictly plane parallel and traceable for tens of kilometers in some cases.

Contourite Fades

The Contourite fades is characterized by laminated silt in the factor analysis model (Fig. 15d; Table 1).The silt laminae

(less than 1 cm thick) occur singly, in clusters (interbedded with mud), or in laminated silt beds up to 10 cm thick with no obvious grading. The silt laminae appear to be a lag deposit resulting from bottom current winnowing of hemipelagic silty clay; winnowing of turbidites can produce coarser, sandier lag laminae atop truncated graded beds (as in core M-12). The Contourite I acies displays many 56 of the features that Stow arid Lovell (1979) consider characteristic of coarse-grained coritourites sharp upper contacts, bioturbated laminae, lack of a regular structural sequence (such as the Bourna sequence), and reworked tops of graded turbidites. The Contourite fades is well represented on the lower continental slope, and near the base of the slope in the filled trench of South Chile (core M-

12), and on the walls and floor of the incised, starved trench of

North Chile and parts of Central Chile (cores W-24 and M-01).

The resedjmented fades of Mutti and Ricci Lucchi are not applicable to contourites (Table 1). Bottom currents differ fundamentally from sediment gravity flows in that fluid motion drives the sediment rather than vice versa (Middleton and Hampton, 1976).

Several researchers have noted increased winnowing and erosion of deep-sea sediments during glacial periods, reflecting more vigorous bottom water circulation (Shanmugam and Molola, 1982; Barron and

Keller, 1982). Thus, contour currents are likely intensified during lowered sea level, coincident with enhanced deep-sea sedimentation by gravity flow processes.

In natural deposits, contourites and turbidites are found inti- mately interbedded, and a continuum probably exists between these two processes (Stow and Lovell, 1979). Dilute, flne-grained turbidity currents may be particuarly susceptible to modification by bottom currents because rapidly deposited muds with high water content and porosity are easily eroded (Einsele et al., 1971). In combination, bottom currents and low-energy turbidity currents may

Interact to produce amalgamated deposits of the Basin-2 fades.

Continuous grading is only developed within intervals of millimeters 57

to a few centimeters because the sequences are interrupted bY

numerous truncation surfaces. Such amalgamated, low-energy deposits

were observed by the author in DSDP cores from the Aleutian Trench.

DISTRIBUTION OF TRENCH LITHOFACIES

The Q-mode sample loadings of the lithostratigraphic factors

described in the previous section are plotted in map view in Figs.

16a through d. These loadings provide information on (a) the geo-

graphic distribution of lithofacles in the modern trench environment

and (b) the relative contribution of each lithofacies to the vertical stratigraphy of a particular trench core.

South Chile

Severa]. cores near 42°3 provide an effective cross-section of

sheet flow deposition in the trench adjacent to the glaciated

southern Andes (Fig. 16a). Two centrally located cores 01-07 and

M-09) show significant loadings of the Channel fades, and there is

a lateral gradation to strong loadings of the Levee and then Basin-2

fades toward both the landward and seaward fringes of the flow,

implying margin-parallel, along-trench transport. The factor

loadings directly reflect the lateral lithofacies changes that can

be observed visually in core sections through the Sand Lens unit

(Fig. 6b). Thus, there is a subtle channelization of velocity

within the sheet flow that produces a lateral (and presumably 58

Figure 1-16 (a through d). Geographicdistribution ofChileTrench lithofacies: factor loadings projected onto core locations. Trench fillis stippled; fan and axial channels are unshaded. J1 (U-tie) (M-04) (M-051 Figure 1-16. to 73 72° 71 W 78

RE NA

LOADINGS KEY

I : NAZCA ffl co IILl I PLATE SLOPE - - I '. I- z -- 3 mzo ' C ISO rn CORES1 r 4 ?XA(. / \JRENCH SUBMARU4E LflRNNtL FILL IãAIITUN

Figure 1-16 (con't). 61 longitudinal) segregation of hydrodynainic lithofacies in the resultant sheet turbidite.

Two cores near the base of the slope at 142°S (M-05 and M-lO) and one core on the lower slope near 140.5°S (M-12) record pronounced contour current activity (Fig. 16a). Presumably, abyssal currents touch down along the foot of the slope to cause winriowing of the deep-sea muds. According to Lonsdale (1976) , cold and saline bottom water passes through fracture zone gaps in the Chile Rise near 11105 and5°S to flow northward near the base of the Chilean margin

(Fig. 1).

As expected, both fan distributary channel and trench axial channel cores show strong loadings of the Channel fades (Figs.

16a,b,c; cores M-16, M-13, M-17, and M-20) , while the flanking channel wall and levee environments show strong loadings of' the

Levee fades (cores M-111 and M-15). A variation to this pattern exists where flanking levees were cored on opposite sides of distributary channels associated with the Bio Bio Canyon near 37°S

(Fig. 16b; cores M-18 and M-19) and the San Antonio Canyon near 33°S

(cores Y-73 and F-al). In both cases, the levee core on the outside of a channel meander was heavily loaded by the Channel fades, while the levee core on the inside of a meander was heavily loaded by the

Levee facies, indicating a lower energy environment. In a deep-tow study of the La Jolla upper fan channel, Normark (1910) found that gentler wall gradients and pronounced terrace development prevailed on the inside of thannel meanders, and he compared this environment to subaerial point bars. 62

The levee stratigraphy on the outside of channel meanders sometimes consists of thin (J. to 5 cm), rhythmically bedded, non- graded sands that presumably represent small-scale, truncated Bouma

"a" units or grain flows. Though distinct from the thicker- bedded, coarser-grained channel axis deposits, the nongraded sands of the outside levees nevertheless factored into the Channel member (Figs.

16a,b; cores M-1i and M-18). However, the thin, rhythmic bedding suggests that these deposits are more closely related to the Levee facies. They may be considered as a variation of the Levee fades in which the transition from the basal sand to the upper pelite is sharp and abrupt.

Central Chile

In the ponded basins of the Central Chile province, there is a tendency to develop a vertical stacking of successive hydrodynan"ic structures within individual turbidites, to form more complete

Bouma-like sequences (our Basin-i facies). In the sheet flow basins of South Chile, by contrast, there is a tendency toward lateral segregation of hydrodynamic structures across the width of the trench wedge. Unfortunately, a continuous vertical succession of

Bouma structures is artificially divided by the factor analysis into a basal Channel fades and an upper Bain-2 fades, though the sequence evolved during a single flow event (Fig. 16e, core M-03, for example).

The significant reduction in the volume of trench till in

Central Chile, as compared to South Chile, represents a reduced rate 63 of sediment supply to the trench axis, given that the convergence rate is essentially constant between the two provinces. Consider, however, that the gross sedimentation rate of a trench wedge that is

10 km wide and 500 m thick (i.e., Central Chile analog) is identical to that of a wedge 30 km wide and 1500 m thick (I.e., South Chile analog) under conditions of constant convergence. That is, although the smaller wedge has one third the stratigraphic thickness of the larger wedge, the sediments have also accumulated in one third of the time (the width of the trench fill divided by the convergence rate equals the residence time or "age" of the trench wedge deposits; see Schweller and Kuim, 1978). In Central Chile, lateral confinement by the trench walls must induce ponding and thickening of turbidity currents in order to produce a sedimentation rate comparable to that in South Chile. In other words, a turbidite of given thickness can be laid down by a flow of one third the volume in a basin that is one third as wide, given comparable axial flow dimensions. In terms of sedimentary fades, the exaggerated sedimentation rate that results from ponding and thickening of turbidity currents corresponds to a vertical stacking of' hydrodynamic sedimentary structures.

The turbidity currents of Central Chile use the entire width of the trench floor as an effective "channel," and deposit material several hundred meters up onto the landward and seaward walls. As the flows gain elevation above the trench floor, basal Bouma units are progressively eliminated from the turbidite deposits such that the low-energy Basln-2 fades (Tcde, Tde) is dominant in the trench wall environment (Fig. 16c, cores F-03 and 1-Si). MoMillen and 64

Haines (1982) also note Tede sequences in the sediment-starved

Mexico Trench, both elevated above the trench axis on the inner wall, and in an axial sediment pond that is apparently removed from direct canyon supply.

A 1,000-rn-high basement scarp at 27.5°S constitutes the boundary between the Central and North Chile provinces (Fig. 2).

Along the sill above this boundary soarp the Contourite fades becomes significant (Fig. 16c, cores M-02 and Y-81).

North Chile

Turbidites in the small ponded basins of the North Chile province tend to exhibit a stacking of hydrodynamic structures to form more complete Bouma sequences, as in Central Chile (Fig. 15c, core W-22). Also, true grain flows, evidenced by structureless and non-graded sands (Fig. 15d, core M-01) probably contribute material to the trench axis for two reasons: (1) the adjacent continental slope is the steepest found on the Pacific margin of South America

(Prince et al., 1980), and (2) the dominance of mechanical weathering in the arid mountains combined with the complete absence of a continental shelf should provide a large contribution of sand-sized material to the deepsea environment.Massive units of coarse, clean sand were also recovered from a ponded basin in the Mexico Trench, and attributed to grain flow processes (MoMillen and Haines, 1982).

The Peru-Chile Trench is deepest and most incised in North

Chile (Fig. 2, Schweller et al., 1981). The Contourite fades is strongly represented both in the ponded axis fill and on the trench walls; it is the exclusive factor loading in lower continental slope cores that are elevated above turbidity current deposition (Fig.

16d, cores W-2i4 and Y-87). Presumably, base-of slope bottom currents are accelerated within the trench's narrow passageway to cause sediment wirtnowing, similar to current accelerations that are observed in other constricted passages such as the Vema Channel (Ledbetter,

1979). Coarse, well-sorted volcanic sands were dredged from a basaltic scarp on the seaward trench wall in North Chile, suggesting strong current reworking (K. Scheidegger, pers. comm.).

Lonsdale (1976) describes a northward flowing eastern boundary current that parallels the lower slope of South America In the abyssal circulation of the SE Pacific. Lonsdale notes that the

Aridean trench provides a major passageway for bottom water flow from the Chile Basin to the Peru Basin, and from the Peru Basin to the

Panama Basin (Fig. 1). Trench currents can be quite strong and erosive, especially near the sills between basins: a current meter station near the sill of the Ecuador Trench recorded an average velocity of 33 cm/s (Ibid). DISCUSSI ON

Depositional Processes

At the heart of the Chile Trench sediment dispersal system is the axial channel which follows the northward axial gradient parallel to the base of' the slope, apparently uninterrupted f or many hundreds of kilometers (Fig. 3). Trench fans are built at the mouths of major submarine canyons, and fan distributary channels become tributary to the axial channel. Trench fans provide the link, then, between off-margin and along-margin sediment dispersal systems.

On the ideal, passive margin submarine fan system, the proximal-to-distal fades evolution develops radially from the canyon mouth to the abyssal plain environment. In a trench system, the axial channel truncates the seaward development of the fans; it pirates sediment from the fans and carries it down the axial gradient. Proximal-to-distal fades relationships are then developed parallel to the continental margin, from the canyon-mouth to the inter-canyon environment (Fig. 17).

Sheet Flow Basins off Glaciated South Chile

The axial channel begins at1°S, but south of this latitude sheet flow deposition extends across the entire width of the trench wedge (Fig. 3). The axial gradient throughout the entire South

Chile province is essentially constant at 1 :650 toward the north

(Fig. 2; Schweller et al., 1981), and is maintained regardless of 67

Canyon-Mouth (Proximal)

Trench Fan IIAxial Channel J-ITrench Wedge Oceanic Plate 111111

Figure 1-17. Idealized stratigraphic cross-sections through the canyon-mouth and inter-canyon environments. Note the changes in the axial channel and trench fan strata between the proximal and distal settings. See text for discussion. the pronounced change in depositional morphology near 111 °S.

However, the sheet flow region does correlate with the occurrence of the inland passage (the submergence of the longitudinal valley), and with the extension of the Pleistocene ice mass to below present-day sea level, onto the exposed continental shelf (Paskoff, 1977).

We hypothesize that the glacial terminus at these latitudes was drained by a complex network of outwash streams, which discharged into the deep-sea through a series of small-scale, closely spaced submarine gullies on the continental slope. By increasing the linear frequency of dispersal pathways across the margin, the sediment supply to the trench will approximate a line source, and sheeted deposition should occur. In contrast, trench fans and axial channels develop where sediment is supplied as fixed point sources from entrenched submarine canyon systems associated with large subaerial drainage basins; this is the more typical situation north of

41 05 Sediment supply from a line source will tend to smooth and dampen depositional relief in the trench, while sediment supply from a point source will intensify depositional relief through channel down-cutting and levee build-up, because the avenues of high-energy flow are established and consistent through time.

By analogy, Mutti (1977) describes sheet turbidites from the

Eacene of Spain that are fed not by submarine canyons and fan valleys, but by a multitude of small-scale gullies cut into the

Slope mudstones at frequent intervals. The slope gullies are generally shallower than 50 m and less then 300 m wide. Seaward Trench Wedge

Constructional levees are often poorly developed or absent on the trench wedge seaward of the axial channel, in the proximal trench environment (Fig. 17a). However, late Pleistocene to

Holocene deposits of the Levee fades were sampled from this environment (Fig. 16a, cores M-1I1 and M-15), implying sedimentation contemporaneous with that on the trench fan and axial channel -- the seaward trench wedge is not a remnant feature but an integral part of the canyon-mouth depositional scheme. Apparently, channel spillover occurs dominantly via sheet flow processes on the seaward trench wedge, evidenced by parallel and conformable seismic reflection sequences (Fig. 9). We expect that the Levee fades on the channel flank will grade seaward into the Basln-2 fades, as it does in the sheet flow region of South Chile (Fig. 16a, cores M-07 and

M-08).

Deposition and Erosion on Trench Fans

The erosional, braided channel complexes on the downgradient

Side of the Chile Trench fans are, to our knowledge, a unique situation in the modern deep-sea. The closest modern analogs may be the braided channel complexes that form on subaerial alluvial fan systems associated with high relief, sporadic discharge, and high bedload/suspended load ratios (Boothroyd, 1976). A base-of-slope braided channel sequence, skewed parallel to the margin, in the lower Paleozojo of Quebec may provide an ancient analog (Hem and

Walker, 1982). 70

It appears that the depositional fan lobes are remnant features, and that the last phase of fan evolution was erosional, because channel courses on the depositlonal surface and seismic reflectors in the subsurface of the depositional lobes are truncated by the erosional scarps (Figs. 7,10). However, contemporary turbidites were recovered from the depositional lobe of the Bio Bio fan, indicating continued but curtailed activity, and some amount of minor deposition by spillover from the erosional channels.

It is riot, clear how or why the South Chile fan systems evolved from conditions of local deposition at the canyon mouth to severe dissection and remobilization of sediment along the axial gradient.

Since at least three different fan systems have been affected by massive erosion of their down-gradient lobes, the causative processes must be of considerable magnitude to produce a coherent response along more than 400 kilometers of the trench system. On the Bjo Bio and Callecalle fan systems, we estimate that roughly 150 cubic kilometers of fan sediment (-25 km x 25 km x 250 m, per fan) have been removed to more distal trench environments.

Most researchers believe that submarine fan deposition is enhanced during glacially lowered sea level when there is a more direct supply of sediment from fluvial and littoral systems to the deep-sea environment (Nelson and Kuim, 1973; Scholl and Marlow,

1971; Shanxnugani and Moiola, 1982). We must assume that currents powerful enough to erode enormous quantities of sediment from the

South Chile fans were also generated during lowered sea level. We conclude that both deposition and erosion of Chile Trench fans occurred during low stands of sea level when these systems were most 71 active. Perhaps a depositional phase of fan growth occurred during one glacial period, followed by changing sedimentologic or tectonic conditions during an interglacial transition that stimulated fan erosion and dissection during a subsequent glaciation.

The change to a destructive phase of fan development might have been caused by an increase in the carrying capacity of turbidity currents that entered the fan systems. This is accomplished most easily by increasing the volume of denuded material that discharges into the submarine canyons at the shelf edge, though at least three different subaerial drainage basins were involved. Rapid uplift of a large portion of the southern Andes is one possible mechanism; an increase in the severity of the glacial climate is another.

A tectonic explanation provides an alternative. A simple increase in the trench's axial gradient could have triggered a change from local fan deposition at the canyon mouth to erosion and down-trench transport of fan sediments. The northward axial. gradient is fundamentally related to the buoyancy of the subducting Chile

Rise at the southern boundary of the study area, and to the gradual northward increase in the age of the subducting lithosphere (Fig.

1). Because the Chile Rise migrates northward as it subducts, the axial gradient should steepen following (a) a pulse of magmatic activity along the spreading center, or (b) the subduction of a spreading center segment as opposed to a fracture zone segment.

Distal Basins and the Axial Channel

The distal environment in the Chile Trench is composed of conformable sequences of sheet flow turbidites, derived from an up- 72 gradient canyon-fan system by way of the axial channel (Fig. lTb).

We have insufficient control to establish how the axial channel evolves into sheet flow deposition in its distal reaches. Perhaps the channel sequentially bifurcates into small distributaries, by analogy with conventional submarine fan systems, or perhaps the channel becomes gradually wider and shallower, and yields an increasing volume of sheeted spillover as it shallows.

In most seismic reflection records across the inter-canyon areas of South Chile, the axial channel is present as a modern surface feature but there is no evidence for any channels of similar dimensions buried in the subsurface stratigraphy. It appears that the axial channel did not evolve into a continuous feature until the culmination of the present trench fill sequence, at which time it incised the conformable sheet turbidites in inter-canyon areas. We hypothesize that the pervasive encroachment of the axial channel into inter-canyon areas, to integrate and coordinate the longitudinal sediment dispersal system, is correlated with the severe erosion of trench fan deposits in the canyon-mouth areas.

An increase in the axial gradient or in the sediment supply may cause the trench dispersal system to actively prograde down axis.

The axial channel may encroach the inter-canyon environment and incise the distal sheet flow deposits (Fig. lTb). If the trench system retrogrades, then the axial channel will infill with distal sheet turbidites, identical to the lithology that the channel has cut into. Little evidence of' the channel's existence may remain aside from a truncation surface or a thin lag deposit in the monotonously bedded inter-canyon sequence. We expect that 73 intercanyon deposition is dominated by fine-graineci, low-energy deposits ot' the Basin-2 facies, but our lithologic control is very poor in the distal trench environment.

Structural and tectonic disruptions of the oceanic basement will constantly modify the axial gradient of the trench. In response, the channel may tend to erode in areas of high gradient and deposit in areas of low gradient, as turbidity currents accelerate and decelerate respectively. An extreme example of this phenomenon occurs across the tectonic discontinuity at 33S, where a large sediment lobe has been built at the foot of a major structural escarpment (Figs. 11, 12).

While hundreds of cubic kilometers of fan strata have been removed from the proximal trench environments and carried down the axial gradient, we cannot provide quantitative evaluations of the transport distances involved. The present density of bathymetry and seismic reflection is too sparse to establish the absolute continu- ity of the axial channel. However, the petrologic composition of the trench sands suggests that axial transport on the order of 300 kilometers may occur (Chapter II, this study).

Trench Stratigraphy

In traditional stratigraphy, lithofacies successions are produced by the migration of' depositional environments across the surface of the earth. In a deep-sea trench, it is the migration of' the very surface of the earth through a predictable sequence of 74 depositional environments that produces the observed lithofacies successions.

Role of Convergence Tectonics

In a deep-sea trench, sediment is constantly removed from the depositional basin, through accretion to the continental slope or subduction beneath the slope during convergence, even as younger sediment is deposited; the compressional deformation is syn-depositional. At a convergence rate of 10 cm/yr, a 30-km-wide trench wedge can be completely recycled in 300,000 years. Under steady- state conditions a constant rate of sediment supply and a constant rate of convergent deformation -- the volume of the undeformed trench axis fill should reach an equilibrium quantity at which the rate of sediment input just balances the rate of sediment removal (Schweller and Kuim, 1978). Though able to maintain an equilibrium volume, the depositional bodieS in the trench axis must continually grow and develop in a constant struggle to counter the destructive forces of subduction.

An equilibrium volume of trench axis fill is a somewhat unstable condition, since many factors will tend to upset the steady-state fluxes in and out of the system. The rate of sediment supply to the trench axis is controlled by the fluvial. and littoral sediment discharge into the submarine canyon heads at the edge of the continental shelf, and these processes are strongly dependent on sea level fluctuations. The volume of sediment at a particular trench locality is affected by perterbations in the axial gradient, and by changes in the relative roles of local fan sedimentation in 75 the proximal environments versus along-axis sediment transport to distal environments. Changes in the rate of sediment deformation may be caused directly by changes in the actual rate of plate convergence, or by changes in the relative orientation of convergence through time. More short term variations will be caused by fluctuations in the rate of slip along the major dcollement zone long periods of stress accumulation may be relieved during catastrophic deformation events -- and the nature of the stress release will be controlled by the rheological properties of the lithologies along the dcollement zone.

While equilibrium conditions are seldom maintained for long periods of time in the trench axis because of the inherent

Instabilities of the system, we will assume that these conditions are approached when averaged over the length of one complete trench cycle. Based on this assumption, we will predict lithofacies successions and model trench stratigraphy. Drawing from the available lithologic, morphologic, and seismic stratigraphic data, we present ideal stratigraphic sequences for trench wedge, axial channel, and trench fan deposits in the proximal environment.

Trench Wedge and Axial Channel Sequences

If trench fans are able to maintain an equilibrium volume in the trench axis, they will systematically prograde relative to the converging oceanic plate and the strata it carries. The prograding tans will drive the axial channel seaward into the trench wedge

(Fig. 17a). The seaward channel wall may oversteepen and fail, allowing blocks of the trench wedge strata to become reworked into 76 the high-energy channel deposits. The relative seaward migration of the axial channel will scour a time-transgressive erosional unconformity across the top ot the trench wedge section.

The seaward trench wedge will migrate toward the axial channel during convergence (Fig. 17a), and the lithofacies succession will record this approach to a zone of high-energy sedimentation. The vertical stratigraphy will consist of a coarsening-upward sequence, up to several hundred meters thick above the oceanic plate section.

The sequence would ideally grade from low-energy deposits of dilute, near-expended turbidity currents to rapidly congealed spillover deposits from the axial channel i.e. from the Basin-2 facies to the Levee facies--and the section will be capped by an erosional surface that marks the base of the axial channel sequence. Because channel migration is controlled by normal faulting to a large degree, episodic seaward jumps in channel position may be recorded by a sudden influx of coarser spillover deposits into the trench wedge stratigraphy.

During convergence, abandoned axial channel deposits will be carried toward the slope and buried beneath the prograding fans

(Fig. 17a). Their gradual burial will be recorded by a fining- upward sequence above the basal erosional unconformity. The sequence would ideally grade from the Channel fades to the Levee fades to the Basin-2 fades -- the last representing low-energy, iriterchanriel deposition but it may be interrupted by Channel or

Levee deposits from the overlying trench fan sequence. Seismic records indicate that the fining-upward sequences are on the order of fifty to a few hundred meters thick. 77

Trench Fan Sequences

Trench fans may form under the presence of a strong axial

gradient, and axially asymmetric fan morphologies may develop. In

the extreme ease, a trench fan may be split into an upgradient lobe

that is dominated by depositiona]. processes, and a down-gradient lobe that is dominated by erosional processes (Figs. 7, 8,9, 10).

The depositional fans may be of either channelized or sheeted morphology (Fig. 18). Deposits of meandering distributary channels

(Channel facies), flanking constructional levees (Levee facies), and

interchannel basins (Basmn-2 I acies) are probably typical of chan-

nelized depositional fan lobes (Fig. 18a; compare Fig. 8, section

A-A'). The fades which develop on sheeted depositional fan lobes

(Fig. 18b; compare Fig. 9, section C-C') are probably similar to

those which develop in the sheet flow basins of the South Chile

province, south of 41°S (Figs. 3, 6). We envision a lateral and

longitudinal evolution from Channel to Levee to Basin-2 facies, as

in the channelized environment, but the fades changes are more

gradual and congruous. Where structures from upper and lower flow

regimes overlap, more complete Bouma sequences are constructed

within the sheet turbidites (our Basin-2 facies), but such sequences

are probably better developed in the ponded sheet flow basins where

sedimentation is confined by the seaward trench wall.

The lithofacies that develop within the channelized and sheeted

environments of depositional trench fans (Figs. 18a,b) may be

analagous to those developed in fan valley and sand lobe environ-

ments, respectively, on more conventional submarine fans (Mormark, 78

Figure 1-18. Idealized stratigraphic cross-sections of trench fans in the canyon-mouth environment. Note changes in the style and geometry of the subsurface lithofacies succession as fan processes vary from depositional to erosional, and from sheeted to channelized. These sections are based on the seismic reflection profiles of Figures 1-8 and 1-9. Note that fan sections lie above coarse-grained deposits of the transgressive axial channel. Vertical exaggeration is approximately 10:1. STRATIGRAPHY OF TRENCHDEPOSITIONAL FANS EROSIONAL L1 Basin-2 - L)THOFACIES: _ Channel .11' Levee Oceanic plate Pigure I-iS. 1970; Mutti and Ricci Luochi, 1972; Nelson and Kuim, 1973; Walker,

1978; Bouma, Coleman, et al., 1983; and many others). To carry the analogy further, we predict that deposition via meandering distributary channels will produce fining-upward sequences in the fan stratigraphy, while deposition via aggrading sheet flow lobes will produce coarsening-upward sequences.

On the channelized erosional fan lobe, a network of braided channels has cut deeply into the older fan stratigraphy, creating irregular truncation surfaces and a patchwork stratigraphy (Fig.

18c; compare Fig. 9, section D-D'). If preceeded by a depositional phase of fan growth, the erosional fan channels should initially conform to the course of the remnant depositional channels. In the more mature stages of erosion, or with alternating episodes of deposition and erosion, nonoontemporary deposits may be superposed across numerous truncation surfaces -- any fades sequence from a previous period of fan growth may be truncated and overlain by erosional channel deposits. The petrologic composition of the sands may help to discriminate between major periods of fan evolution, though the erosional channel sands will include a petrologic signal reworked from the strata it has cut into.

Finally, if the erosional fan lobe equilibrates with the prevailing axial gradient, it may develop a sheeted morphology that

grades smoothly and gently from the canyon mouth to the thalweg depth of the axial channel (Fig. 18d; compare Fig. 8, section B-B').

Small, ephemeral, braided gully systems could migrate freely across

the surface of the lobe to winnow and scour sheet-like lag deposits.

We expect neither fining-upward nor coarsening-upward sequences to 81. be well-developed on the erosional fan lobes, whether channelized or sheeted, due to the destructive nature of the process.

Preservation vs. Destruction of Trench Sediments

Ancient trench sequences have been cited from the early

Paleozoic (Leggett, 1980; MeKerrow and Cocks, 1978) so their potential for preservation extends through much of Phanerozoic time.

Mesozoic and Cenozoic trench deposits are commonly exposed along the circum-Pacific region; these include the Chugach terrane of Alaska

(Nilsen and Zuffa, 1982), the Franciscan terrane of California

(Bachman, 1982), and the Torlesse terrane of' New Zealand (MacKinnon,

198I). The interpretation of ancient trench deposits is often confounded by structural disruption and metamorphic overprinting of the stratigraphic record, producing in extreme cases isolated sandstone boudins in a pervasively sheared mudstone matrix, recrystallized under greenschist or bluesehist fades conditions. In some instances, however, relatively coherent and unaltered stratigraphic sequences up to a few kilometers in thickness are preserved, though they are isolated by faults from neighboring sequences.

From studies of modern convergent margins, two end-member schemes of trench-slope evolution may be considered: subductiori accretion arid subduction erosion (Kulm et al., 1977; Scholl at al,

1980). Subduction erosion is a destructive process by which material is removed from the foot of the continental slope or from beneath the foreare of the overriding plate, and the subducting plate is consumed in its entirety. Oceanic material may be 82 incorporated into a continental margin by the process of subduct ion accretion, and it is only by this mechanism that trench sediments may be preserved. Accretion may occur at considerable depth in the subduction zone (a process referred to as underplating), but it 18 during shallow accretion near the toe of the continental slope that metamorphism and tectonic disruption of trench sediments will be minimized.

During the accretion of oceanic material to a continental margin, convergence stresses are typically accommodated along seaward-verging thrust faults that splay off the master subduction zone thrust and propagate into the oceanic stratigraphy (Seely et al., l97J; Karig and Sharman, 1975; J.C.Moore, Biju-Duval et al.,

1982). Compressional failure is localized along mechanical diacon- tinuitie or zones of weakness in the oceanic basalt or overlying sedimentary section. The position of mechanical decoupling

(d'ecollement) within the oceanic stratigraphy is of prime importance because material above the dcol1ement zone will be accreted and preserved while material below the dcol1ement will be carried to greater depth in the subduction zone where it is metamorphosed or destroyed. The volume, composition, and physical properties of the trench axis sediment fill exerts a specific control on the positioning of convergence-related mechanical failure.

In starved trenches, such as that found in the North Chile province and in many island arc situations, failure may propagate along weak zones in the Layer 2 volcanics of the descending oceanic plate (Fig. 19a; Kuim et al.,, 1981). Since trench sediment serves as a aubduction zone "lubricant", starved trenches allow enhanced 83

TRENCH AXIS FACIES OCEANIC PLATE EAGlES Trench Fan Terrigenous Axial Channel Pelagic Trench Wedge ." Oceanic Basalt

Figure 1-19. Accretion and preservation of trench deposits: stratigraphic control on the positioning of the decollement zone. The nature of the lithofacies succession, as trench conditions vary from starved to filled along a margin, may infiuence which sedimentary environments are preferentially acoreted to the adjacent continental slope. Compare with Figure 1-4. 84 mechanical coupling between the oceanic and continental plates and this facilitates rupturing within the crystalline oceanic basement.

Under these conditions, the sequence preserved on the continental slope may include slices of oceanic basalt with overlying pelagic sections, and isolated bodies of ponded trench fill (Fig. 19a).

A discontinuity in the physical properties of the oceanic plate section may develop between the terrigenous strata (hemipelagic and turbiditic sands and muds) and the underlying pelagic strata (calcareous and siliceous oozes and muds) (Fig. 19b; J.C.

Moore, 1975; J.C. Moore1 Biju-Duval et al., 1982). Differences in age, diagenetic history, and sedimentation rate cause the biogenic sequence to be generally denser, less porous, and of greater strength than the overlying clastic sequence. A dcollernent zone may develop along this lithologic interface, causing the prefereri- tial subduction of pelagic strata and the accretion and preservation of hemipelagic deposits and trenchf ill turbidites (Fig. 1gb). This style of accretion may be prevalent in areas of moderate to abundant trench fill, such as in the Central Chile province or in the inter- canyon environment of the South Chile province.

Finally, the sedimentary fades that develop within the channelized canyon-mouth environment may control the position of convergence-related thrust faulting. Failure by thrusting is greatly facilitated by increased pore pressures that partially or wholly compensate the lithostatic load (Hubbert and Rubey, 1959; von Huene,

1984) . Increased pore pressures can develop in permeable sand beds that are adjacent to compacting, dewatering muds. In trench stratigraphy, the trarisgressive axial channel sands that develop between 85 the trench tan and trench wedge strata are a likely site for the localization of high pore pressures (Fig. 19c). Failure along tills lithology, then, would allow the accretion and preservation of canyon-mouth tan deposits, but not the seaward trench wedge.

Trenches, Fans, and the Ancient Rock Record

Many aspects of trench sedimentation may be directly compared with submarine fan sedimentation, which ideally occurs in the continental rise environment of a passive margin. The dimensions

(width, relief, and gradient) of channels from both submarine fans and trenches exhibit almost complete overlap in the modern environment (Schweller and Kuim, 1978). This study has shown that the lithofacies of the modern Chile Trench may be compared on a one-to-- one basis with those described in ancient submarine fan deposits

(Mutti and Riccj Lucehi, 1972). However, trench systems differ from submarine fan systenis in (a) tile spatial distribution of lithofacies along the continental margin, (b) the nature of the depositional bodies which mold lithofacies relationships and associations, (c) the superposition of lithofacies through time to produce a stratigraphic sequence, and (d) the mechanism by which the deposits are preserved in the rock record. The distinction of the trench environment results primarily from the presence of an axial gradient that redistributes sediment parallel to the cordilleran arc, and from the unique structural foundation that frames the strata between the opposed walls of converging lithospheric plates. A sound and rigorous interpretation of ancient flysch terranes should address the regional aspects of basin analysis--paleocurrent patterns, depositional geometries, lithofacies distributions, and stratigraphic successions. Still, similarities may exist between trenches and other active tectonic settings where elongate deep-sea troughs receive sediment from lateral sources but distribute sediment along-strike down an axial gradient. Depositioflal processes that operate in trenchslope basins (G.F. Moore et al., 1980; J.C.

Moore et al., 1980; Bachman, 1982), in troughs developed along collisional sutures (Pescatore, 1978), in translational basins (Hsd et al., 1980), and in aulacogen rifts (Surlyk and Hurst, 1983) may bear some resemblance to trench processes. Ultimately, the recogni- tion of ancient trench sequences may be based on the structural and stratigraphic relationships to surrounding oceanic and continental terranes, arid the mechanism by which the deposits have been moor- porated. into the continental margin and preserved in the rock record.

CONCLUSIONS

1. A Q-mode factor analysis of Chile Trench core sections

yielded end-member associations of depositional texture and

hydrodynainic structure that represent sedimentary

lithofacies. The late Pleistocene-Holocene lithofacies of

the Chile Trench correlate well with the ancient submarine

fan lithofacies of Mutti and Ricci Luochi (1972): the

Channel facies corresponds to their combined Facies A and 87

B, the Levee fades to their Fades E, and the Basin-2

facies to their Fades D (Table 1). We find that more

complete sequences of graded Bouma structures occur in the

ponded basins of Central and North Chile, and to seine

degree in the sheet flow basins of South Chile; this

corresponds to the Basin-i fades, or Fades C. In

addition to gravity flow deposits, the Contourite fades

consists of winnowed silt and sand laminations. The

Contourite fades is best developed within the walls of the

incised, starved trench in North Chile, where bottom

currents are presumably constricted and accelerated, and

near the base of the continental slope in South Chile.

2. Several types of depositional bodies occur in the

Chile Trench, including sheet flow basins1 axial channels,

and trench fans. South of 1°S, the trench is filled by

sheet turbidites, deposited under the influence of a

northward axial gradient. We hypothesize that sheet flow

is correlated with a higher density of sediment supply

points along the base of the slope, and that such a

configuration may be related to the extension of the

Pleistocene ice mass onto the continental shelf at these

latitudes.

Near I1°S latitude, the trench axial channel emanates

from the mouth of an unnamed submarine canyon and continues

northward along the base of the continental slope for

possibly hundreds of kilometers, linking the distributaries

from several canyon-mouth fans to form a coherent axial dispersal system. Tectonism in the oceanic basement causes

perterbations of the axial gradient; the axial channel

responds by eroding in areas of high gradient and

depositing in areas of low gradient. Near 33°S, a large

sediment lobe has been built at the base of a 600-rn-high

axial escarpment that appears to be related to a major

transverse discontinuity in the Andean continental margin.

The axial channel truncates the seaward development of

the trench fans and causes proximal-to-distal facies

relationships to develop parallel to the continental

margin, from the canyon-mouth to the inter-canyon

environment. Three different fan systems in South Chile,

though separated by more than I00 km along the margin,

exhibit a similar bilobate morphology -- an up-gradient

lobe of depositlonal morphology and a down-gradient lobe of

erosional morphology, separated by. an erosional scarp which

signifies that hundreds of meters of fan strata have been

swept into the axial channel and down the trench dispersal

system. The down-gradient lobes tend to erode down to the

base level of the axial channel. This most recent phase of

fan erosion is correlated with active along-axis

progradation of the trench dispersal system, and with the

encroachment of the axial channel into distal sheet

turbidite sequences of the inter-canyon environment.

3. Though trench sedimentary facies can be directly

compared with submarine fan facies, the spatial

distribution of these fades and the manner in which they are superposed to produce a stratigraphic sequence is distinct in the trench environment. If a trench basin receives sediment at a rate which exactly balances the rate of sediment loss through convergent deformation (subduction or accretion), then the depositional body is said to be at equilibrium. Over the long term, equilibrium may be upset by changes in the convergence rate and direction, or by relative fluctuations in sea level. Short term instabilities may be caused by axial redistributions of sediment in response to tectonic perterbations of the gravitational gradient, and by variations in the rate of

/ slip along the subduction decollement.

At equilibrium position, a trench fan will systematically prograde with respect to the converging oceanic plate. Consistent and steady fan outbuilding drives the axial channel seaward into the trench wedge, creating a time-transgressive erosional unconformity at the top of the trench wedge section. With plate convergence, the conformable trench wedge sequence will migrate toward the zone of channelized deposition, recording a coarsening- upward sequence that culminates in spillover deposits from the axial channel. Abandoned axial channel deposits are carried toward the slope and buried beneath the prograding fans, recording a fining-upward sequence above the basal erosional unconformity. The Chile Trench fans imprint a complex and variable stratigraphy because both depositional and erosional environments may exist within a single ran,

and both thannelized and sheeted morphologies are common.

4. The volume and composition of' the trench-axis sediment

fill may influence the localization of compressional

stresses during subduction accretion and thus determine the

position of the dcollement zone within the trench

Stratigraphy. All strata above the decoupling zone will be

preserved at shallow levels in the forearc aecretionary

complex, while those strata below will be subducted to

deeper levels where they are metamorphosed or destroyed.

In starved trenches, compressional failure may propagate

along weak zones in the shallow volcanic layering of the

oceanic basement. In areas of' moderate trench fill, the

dcollement may develop along a physical discontinuity

between the t.errigenous strata and the pelagic strata of

the ocean plate section, due to differences in porosity,

density, and cementation. In the proximal canyon-mouth

environment, decoupling may be localized within the

permeable sands of the transgressive axial channel

deposits, causing the subduction or trench wedge strata and

the preferential accretion and preservation of trench fan

deposits. CHAPTER II

SEDIMENTATION IN THE CHILE TRENCH: PETROFACIES AND PROVENANCE 92

ABSTRACT

The Chile Trench Study area extends over 2,000 kilometers along the Andean continental margin, from 23°S to I2°S latitude. On the

QFL diagram, the compositional field defined by the suite of Chile

Trench sands encompasses the average compositions of modern sands from strike-slip, continental arc, oceanic backaro, and oceanic forearc types of active margins (Valloni and Maynard, 1981), and ranges over the complete spectrum from dissected to undissected continental arc settings (Dickinson et al., 1983). Trench sands along North Chile, Central Chile, and the glaciated archipelago of

South Chile are derived from a more plutonic source region, or

"dissected" magrnatic arc, as shown by abnormally low lithic and calcic plagioclase content, but high quartz and alkali feldspar on

QFL and QmPK plots. The heavy mineral fraction is characterized by an amphibole: orthopyroxene ratio greater than unity. In Central

Chile, the plutonic nature of the provenance is correlated with an absence of Quaternary volcanism. Ashore from the Chilean archipelago, Pleistocene glaciation has carved deep into the magmatic roots of the Andes, effecting arc dissection in spite of active volcanism.

In arid North Chile, the volcanic debris of the High Cordillera Is trapped in the longitudinal forearc basin, allowing an exagerrated contribution to the deep-sea from the crystalline rocks of the Coast

Range.

The light and heavy, mineral and lithic composition of the trench sands is described by four ideal petrologic assemblages, derived by Q-mode factor analysis. Factor I the Basic Magmatic 93

Arc petrof'acies -- is rich in volcanic lithics, cable plagioclase, obivine, clinopyroxene, and orthopyroxene; it represents the most chemically immature, least differentiated assemblage, and is associated strictly with Quaternary arc volcanism of basaltic andesite composition in South Chile. Factor II the Acid Magmatic Arc petrof'acies is rich in volcanic lithics, alkali feldspar, biotite, hornblende, and magnetite; it represents a more sialic and differentiated continental arc assemblage, and is associated with acid andesite extrusions and granodiorite Intrusions of the Mesozoic and Cenozoic Andean orogeny. Factor III the Metamorphosed Mag- matic Are petrcifacies is rich in mica, alkali feldspar, chlorite, actinolite, epidote, and blue-green hornblende; it represents a greerisahist metamorphic assemblage whose provenance lies in the basal marine section of the Andean eugeosyncline, and In the

Paleozoic basement. Factor IV the Cratonic Block petrofacies is rich in quartz and the accesory minerals andalusite, garnet, and tourmaline. It represents mature sedimentation, late-stage granitie intrusion, and amphibolite facies metamorphism that are associated with the cratonic block; it is strongly represented along northern

Chile, where the continental crust is thickest and presumably most ancient.

Actual petrofacles In the Chile Trench are typically mixtures of the four Ideal petrofacies. Their composition can be related to

(1) the geology of the source terranes, (2) the presence or absence of' arc volcanism, and its composition, (3) the climate of the adjacent landmass, and (1!) the morphology of the continental margin.

Contemporary volcanism dominates the petrofacies assemblage to a 94 much greater degree than other provenance types, but other factors

tend to dilute this signal. Lithologies that underlie heavily glaciated regions may be preferentially denuded, arid the extent of

onshore glaciation affects the diversity of the offshore petrologic assemblage. The frequency of sediment supply points along the base

of the slope probably controls the heterogeneity of the trench

petrofacies in geographic distribution.

INTRODUCTION

The Chile Trench provides an outstanding opportunity to study

petrofacies within a linear tectonic basin adjacent to an active

magmatic arc. Several physiographic gradients exist along this

portion of the Andean margin that are expected to influence the

petrologic composition of deep-sea sands (Fig. 1). These gradients

include: (a) abrupt terminations and a spatial gap in the distribu-

tion of Quaternary volcanism in the arc, which define a geomorpho-

logic segmentation in the overriding plate (MüiozCristi, 1956;

Lomnitz, 1962), (b) an extreme climatic gradient that ranges from

one of the world's most severe deserts in northern Chile to the

glacially-dissected terranes of the southern Chile archipelago

(Galli-011ver, 1969; Scholl et al., 1970), and (c) a thinning of the

continental crust from 70 km in the north to ZI0 km in the south

(Couch et al..,1982; Ocola et al., 1971), accompanied by a trend

toward a more "oceanic" composition of the pre-Andean basement

toward the south. 95

110°N

GALAPAGOS RIFT

SOUTH 00 AMERICA FOREARC BASINS

QUATERNARY 4àVOLCANOES 4L '00$

200

R'iD CHILE ( , RbINl-# vI-J

-J z 30 JUAN .) FERNANCZ ISLANDS

z,'iI,

400 /1 CHILE I -6OO

I I I //'1I I I I 50° 70°W 120° 110° 1000 90° 80°

Figure 11-i. Physiography of the Nazea Plate and Andean continental margin. Batnymetry contours after Marnmerickx and Smith, 1978. Offshore features include postulated segments of the Pacific- Farai.jon Rise (Mammerickx et al., 1980); onshore features include Pliocene-Quaternary strato-volcanoes (Simkin et al., 1981; Paskoff, 1977), and forearc basins as outlined by the distribution of Quaternary alluvium (after Ruiz and Corvalan, 1968). The objectives of this study are to characterize the light and heavy, mineral and lithic composition of late Quaternary sands from the Chile Trench, to describe the distribution of mineral and lithic species in terms of end-member associations, or petrofacies, and to relate the observed petrofacies assemblages to their provenance in the Coast Range and High Cordilleras of Chile.By comparing the mineralogy of modern trench sediments with onshore source rock terranes from which they were derived, we can establish the influence of volcanism, geology, climate, and the morphology of the continental margin on the petrologic signal. We will discuss how axial transport affects the distribution of petrofacles in the Chile

Trench, and how petrologic assemblages may be modified during diagenesis and metamorphism.

The concept of petrofacies has endured extensive application in convergent margin settings, both modern and ancient. Previous studies include Recent sediments from the Middle America (Enkeboll,

1981) and South America continental margins (Yerino and Maynard,

198!); Neogene-Quaternary sediments from DSDP drill holes on the

Mexico (Bachman and Leggett, 1981). Guatemala (Prasad and Hesse,

1982), and Oregon continental margins (Scheidegger et al., 1973), and on the Bengal/Nicobar fan complex (Ingersoll and Suazek, 1979); and ancient subduction complex! forearc basin deposits from

California (Ingersoll, 1983; Dickinson et al,, 1982), Indonesia

(G.F.Moore, 1979), Alaska (J.C.Moore, 1973), and the circum-

Pacific region In general (Dickinson, 1982).

Comparisons and summaries of sand composition from diverse tectonic envi ronrnents--cratoni c, passive, transform, and convergent 97 margin settings--are provided for ancient sandstones (Dickinson and

Suczek, 1979; Dickinson et al., 1983) and modern deep-sea sands

(Va.11oni and Maynard, 1981 ;Dickinson and Valloni, 1980; Maynard et al., 1982). In several studies, the use of modern statistical tech- niques (Q-mode factor analysis, discriminant analysis) has facilitated the identification of petrofacies assemblages within a basin

(Imbrie and van Artdel, 19611; Scheldegger et al., 1971; Ingersoll,

1983).

BACKGROUND

The Chile Trench study area extends from 18°S to 115°S latitude, from a prominent bend in the strike of the Andean cordilleran system to the intersection of the Chile Rise (Fig. 1). Although the volume of sediment in the trench axis parallels the onshore climatic gradient (i.e., increased precipitation and erosional denudation toward the south), the structure and morphology of the trench is obviously segmented by tectonic discontinuities near 27.5°S and 33°S latitude (Figs. 2,3). The segments are referred to as (a) North

Chile province (20°S to 27.5°S lat) -- isolated sediment basins are ponded within depressions of a broken basaltic basement; (b) Central

Chile province (27.5°S to 33°S lat) -- the trench sediment fill is continuous but thin (<10 km wide) and shallow (several hundred meters thick); and (c) South Chile province (33°S to '15°S lat) the trench fill swells in volume to greater than 30 km in width and up to 2 km in thickness, and completely buries the structural trench south of about 38°S (Schweller et al., 1981). Figure 11-2. Map view of the Chile Trench sediment fill. The distribution of axial sediments is shown in stippled pattern; fan distributary and trench axial channels are unshaded. Asterisks denote core control. The shelf break is represented by the 200-rn bathymetry contour. Submarine canyon morphology is from Prince et al. (1980), and from data collected on the R/V MELVILLE in 1980. 75°W 74° 73° 72° 71° 72°W 71° 70°

28°

29°

30°

31°

32°

33°

Figure 11-2. NORTH CENTRAL SOUTH CHILE CHILE CHILE

U) A ( -r I Lii 0 %CA j 6 V < , TRENCHAXISDEPTHS ,1,

'a ' S ç 515 ç C,1,'.' & a I 1 j. sgv C D E

20 30 40

DEGREES SOUTH LATITUDE

Figure 11-3. Axial cross-section of the Chile Trench. Bathymetric thalweg depths, depth to oceanic basement (checked pattern) from seismic reflection, and the thickness of the axial sediment fill (stippled pattern) are plotted according to latitude (after Schweller et al., 1981). Segments in the subducting oceanic plate, as determined from hypocenter locations in the inclined seisnic zone, are shown below (C, D, and E, after Barazangi and Isacks, 1976). 0 101

An identical segmentation is apparent In the physiography of

the overriding continental plate, and in the configuration of the

dipping seismic zone (Stauder, 1973; Barazangl and Isacks, 1976);

evidently the margin segmentation is genetically related to the

subduetion process. In both the North and South Chile provinces, a

trench-parallel forearo basin separates the Coast Range from the

High Cordillera, which is formed by a chain of active stratovol-

canoes (Fig. 1). In the Central Chile province, both the longitu-

dinal depression and Quaterriary arc volcanism are absent.

South of 141°S the trench is filled by a sequence of flatlying,

conformable, sheet flow turbidites (Fig. 2). Two turbidites can be

correlated across the 30-km-wide trench wedge in seven cores near

42°S, based on stratigraphic position, sedimentology, radiocarbon

dating, and ash content and mineralogy (Fig. 14). The youngest sheet

turbidite--the Black Ash unit--is composed of nearly pure (>70%),

fresh, basaltic andesite lithic grains with a scoriaceous texture.

The older turbidite--th Sand Lens unit--is composed of quartz and

feldspar mineral grains and reworked volcanlelasticS. The coarsest

and thickest sands are centralized in the Sand Lens unit (cores M-09

and M-07) and grade both landward and seaward into finer, siltier

deposits, reflecting a cross-trench gradient in flow velocity during

deposition. Much of the Sand Lens unit was deposited under upper

flow regime conditions, while the Black Ash unit was deposited under

lower flow regime conditions.

At 141 °S, the trench's axial channel emanates from the mouth of

an unnamed submarine canyon and trends northward down the gravita-

tional gradient, integrating and centralizing the longitudinal 102

Figure II-I. Sheet flow turbidites near 2°S. (A) Seismic reflection profile showing extensional faulting and mild deformation of the conformable sequence, with location of coring transect. (B) Lithologic correlation of individual sheet flow units (the Black Ash and Sand Lens units) in piston cores across the 30-km-wide trench wedge. Carbon-11 age control is shown. Note the lateral lithofacies changes in the Sand Lens unit, reflecting a channelization of velocity during turbidity current deposition (CLF Channel lithofacies, LLF = Levee lithofacies, BLF Basin Lithofacies). The less disturbed section in tiie gravity core trigger weight is spliced on top of the piston core section of core M-09. 103

STRATIGRAPHY OF SHEET TURBIDITES, 42°S w E

I- SEC

V'E. 18:1 KM

M09 M-O6 U M-04 M10 Mo5

- a410 t210

- -. 0Ia BIRck Ash 2 T Unit

Lens BLF

4_ -4 5km

Figure 11-4. 104

sediment dispersal system for many hundreds of kilometers (Fig. 2).

The channel provides a major avenue f or long-distance, along-margin

transport of material. Numerous submarine canyons in South Chile

deliver sediment across the margin from subaerial drainages. Trench

tans are built at the mouths of these canyons (i.e. the Bio Blo and

Callecalle fans), and the fan channels become tributary to the axial

channel. The axial channel continues across the tectonic discon-

tinuity at 33°S (the San Antonio Discontinuity) and partway into the

Central Chile province, where the axial gradient eventually disap-

pears and sedimentation becomes truly ponded.

GEOLOGIC HISTORY

A brief sketch of the geologic history of Chile Is provided to

better understand the provenance of mineral suites in Chile Trench

sands. Comprehensive reviews on the geology of Chile and the Andean

cordilleran system are given by Zeil (1964, 1979) and Munoz-Cristi

(1973).

Basement terranes of Precambrian age are essentially absent

from Chile. To the north they outcrop in the Arequipa massif on the

southern coast of Peru (Cobbing et al., 1977) and to the east in the

Sierras Pampeanas Range in the foreland of Argentina (Caminos et

al., 1982). It is unknown how far these ancient terranes extend

beneath the Phanerozoic continental crust of Chile. 105

The Hercynian Cycle

Paleozoic rocks outcrop primarily in the Coast Range of Chile, in isolated exposures through the Andean cover in the High

Cordillera, and extensively in the eastern cordilleran belts across the continental divide in Bolivia and Argentina. The Paleozoic stratigraphy is broken by unconformities that represent Hercynian orogenic movements during the late Devonian/ Early Carboniferous

(The Eohercynian Phase, Ca. 330-350 m.y.a.) and during the Permian

(The Tardihercynian Phase, ca. 260-265 m.y.a.) (Dalmayrac et al.,

1980). The most severe deformation and uplift occurred during the

Eoheroynian Phase.

Paleozoic exposures in the Coast Range of Chile are nearly barren of fossils and stratigraphic relationships are not well established. In northern Chile, sediments of flysch-like aspect may be predominantly Devonian in age (Zeil, 19614; Berg et al., 1983), while in central Chile, between latitudes 30° and 32°S, sporadic fossil occurrences have been assigned to the Carboniferous and

Permian for the most part (Caniinos et al., 1982, references therein).

The structural trend of the Hercynian paleogeography is NNW

SSE. Hercynian structures are truncated by the Pacific coast of

Chile and are oblique to the present orographic elements which trend

N10°E (Aubouin et al., 1973). In Peru, Bolivia, and northern Chile, sedimentation occurred within a subsident intracratonic trough. The

Hercynian basin was underlain entirely by ancient continental crust, limited to the east by the Brazilian Shield, and to the west by a 106

"Pacific Paleocontthent" of which the Arequipa Massif is a remnant

(Dalmayrac et al.,1980;Isaacson,1975;Berg et a]..,1983;Miller,

1970). South of about29°S,the Hercynian subsident zone became more marginal or extracontinental in character, and oceanic terranes have been incorporated into the Paleozoic basement. Occurrences of bluesohist minerals, metabasalt of tholelitic composition, meta- chert, and serpentinite bodies may respresent a subduction complex that was aecreted to the South American continent during316to329 rn.y.a. (Gonzalez-Bonorino and Aguirre,1970;Herve et al.,197k,

1g76;Coira et al.,1982;Caminos et al.,1982). Radlometric dating of pre-Andean plutonic bodies from diverse localities indicates a wide range in age of apparent crystallization from about 170 to320m.y.a. (Berg et al.,1983;Caininos et al.,

1982;Drake et al.,1982;Herve et al.,1976;McBride et al.,1975). The magrnatism may be associated with both of the Hercynian deformation phases, and it is post-orogenic to a large degree.A great

Paleozoic batholith outcrops almost continuously from330to38°S latitude along the eastern flank of the Coast Range in southern Chile. The metamorphism of the pre-Andean basement has been studied mainly in southern Chile (south of33°S)where these rocks outcrop extensively in the Coast Range.The basement is composed primarily of slate, phylljte, mica schist, and metasandstone that has been regionally metamorphosed under greensahist fades conditions

(Gonzalez-Boriorino and Aguirre,1970;Gonzalez-Bonorino,1971). Regional metamorphism under low P/T conditions locally reaches the 107 amphibolite and granuJ.ite fades near the contact with the Paleozoic batholith.

The Andean Cycle

The central Andean Cordillera differs from typical alpine-type cordilleras in that flysch sequences, ophiolites, extensive thrusting and riappe formation, and metamorphism under elevated pressure have contributed very little to the orogenic development of this mountain belt (Zeil, 1979; Aguirre et al., 197k; Aubouin et al., 1973).These differences emphasize the fact that t'ormatiori and deformation of Mesozoic/Tertiary Andean basins occurred strictly within the confines of the South American continental block--limited by forelancl and backland basement elements of Precambrian and Paleozoic age, today represented by the Eastern Cordilleras of Argentina and Bolivia and the Coastal Cordillera of Chile. Several orogenic phases, generally represented by angular uncoriformities and continental molassic sequences in the stratigraphy, appear to have regional significance in the central Andes; these include the Oxfordian-Kimmeridgian (Late Jurassic), Albian- Cenomanian (Middle Cretaceous), Paleocene, Late Eocene Early Oligocene, and Late Miocene events (Charrier and Vicerite, 1970; Aguirre et al., 197').The compressional phases are short-lived and followed by longer periods of intervening quiescence, or possible extension, during which calcalkaline volcanism, continental arid marine sedimentation ensued.Major episodes of plutonic emplacement are broadly associated with each of the tectonic events, though they 108 display a postkinematic relationship and lag slightly in time

(Aguirre et al.,197I). The central Andes display a clear polarity in time arid space, such that there is a relative west to east nhigra- tiori (toward the continent and foreland) in the locus of tectonisin, volcanism, and plutonism from the Jurassic to the present (Halpern,

1978;MeNutt et al.,1975;Levi,1973;and others). The geosynolinal period of the Andean orogeny includes two cycles of marine sedimentation in Jurassic and Early Cretaceous time that are separated by a phase of mild teetonism and emersion during the late Jurassic (Aguirre et al.,19714;Aubouin et al.,1973;

Charrier and Vicente,1970). A pair of adjacent and contiguous basins occupied the geosynclinal depression--a sequence of volcanica and volcaniclastic sediments, many kilometers thick, accumulated in the eugeosyncline (western zone), while non-volcanic clastic and carbonate sediments accumulated on a continental platform in the miogeosyncline (eastern zone).The trend of the geosynclinal basin pair (N-S) is slightly oblique to the present Andeari morphostructural units (N100E).An orogenie phase in the middle Cretaceous brought an end to marine sedimentation and thrust the eugeosynclinal series over the miogeosynclinal series along the paleogeographic boundary; this constitutes the most significant deformational event in the occidental zone of the Andean Cordillera (Charrier and

Vicente,1970). Sedimentation younger than the Neocomian epoch is characterized by thick sequences of continental voloanics, volcaniclastics, and red beds.Dominantly andesitic sequences with minor rhyolitic rocks accumulated during the late Cretaceous until the Paleocene 109 orogeny. In central and southern Chile, there is an apparent magmatic hiatus (volcanic and plutonic) throughout much of the lower

Tertiary between 30 and 60 m.y.a. Folded Miocene volcanic strata outcrop extensively in the High Cordillera, and are separated from

undeformed Pliocene-Quaternary volcanics by an angular unconformity which represents a significant deformatiorlal event in the late

Miocene (Drake et al., 1982; Lahsen, 1982; Drake et al., 1976;

Vergara and Munizaga, 197k). Late Miocene compression followed by

postorogenic extension in the early Pliocene formed the present morphostructural units--the Coast Range, the longitudinal depres-

sion, and the High Cordillera (Aguirre et al., 197k).

During the late Cretaceous and Cenozoic, non-volcanic marine

basins of limited extent developed directly on crystalline Paleozoic

basement, associated with the tensional breakdoWn of the backland

terrane. In southern Chile these marine strata outcrop along the western flank of the Coast Range and extend onto the continental

shelf (Mordojovich K, 197k; Aubouin et al., 1973).

The generally post-orogenic Andean plutons were emplaced at

shallow crustal levels with sharp, discordant contacts to the

country rock and narrow metamorphic aureoles; they were passively

intruded under a tensional stress regime and are commonly bounded by high-angle normal faults (Damm et al., 1981). The plutonic bodies

belong to the cab-alkaline series, with granodiorite, quartz monzO-

diorite, tonalite, and granite being the most common compositional

varieties. Typical mineral phases are oligoclase-andesine, ortho-

clase, quartz, biotite, hornblende, augite, and magnetite (Aguirre

et al., 19714). 110

Regional metamorphism associated with penetrative deformation

and syntectonic, migmatitic plutonisrn is absent from the Mesozoic!

Cenozoic Andean orogeny (Aguirre et al.., 197k). There is only a

low-grade, generally non-schistose, burial metamorphism that affects

the thick volcanic sedimentary sequence; it ranges from the zeolite

to the greenschist fades and its isograds are subparallel to

bedding planes (Aguirre et al., 1978; Levi, 1970). The most exteri-

sive greensohist fades metamorphism occurs In the older and deeper

stratigraphic units of Jurassic and lower Cretaceous age.

Quaternary Volcanism

The composition of the Pilo-Quaternary lavas from the volcan-

ically active North Chile province can be contrasted directly with

those from the South Chile province. In general terms, the North

Chile voleanics are more acidic in composition and more differentiated. North Chile volcanics are predominantly andesites and dacites

that range from 61to 66% SIO, while South Chile volcanics concen-

trate in the basaltic andesite and high-Al basalt fields, with 50 to

55% Si0 (Deruelle, 1982). Extensive sheets of' rhyolitid ignim-

brites (average 71% SlOz) have contributed immense volumes of

detritus to the Upper Cenozoic volcanic pile in North Chile, but

rhyolitic ash flow deposits of this magnitude are absent from South

Chile (Klerkx et al., 1977; Pichier and Zeil, 1972). Typical marie

phenocrysts in North Chile lavas and ignimbrites include hornblende

and biotite (hydrous minerals), but olivine and pyroxene (anhydrous

minerals) are most common in South Chile (Deruelle, 1982). North 111

Chile volcanjos are more enriched in potassium, and in incompatible

elements such as thorium, barium, strontium, and rubidium, and

exhibit higher strontium isotope ratios (87Sr/86Sr: 0.705-0.709)

compared to the South Chile volcanics (ca. 0.70) (Thorpe and

Francis, 1979; Klerkx et al., 1977; Deruelle, 1982, Palacios and

Oyarzun, 1975). The main exception to this pattern occurs in the

South Chile arc near the termination of active volcanism at the San

Antonio discontinuity, between 330 and 34°S latitude, where amphi-

bole andesites (about 61% Si0) are chemically very similar to the

andesites from North Chile, but unlike the more basic rocks from the

rest of the province (Lopez-Escobar et al., 1977).

METHODS

Twenty-six cores were utilized to characterize the mineralogy

of the trench axis and trench wall environments in the Chile Trench,

the bulk of these being piston cores obtained on the R/V MELVILLE

during an Oregon State University cruise in November 1980. Sixty-

four sand samples were extracted from these cores for processing,

taken mainly from the basal portions of coarse-grained turbidites.

Carbon-l! dates indicate that our samples are younger than 50,000

years old.

The sands were disaggregated with HO2, then sieved to isolate

the fine to very fine sand fraction (62-250 microns). This fraction

was then split into light and heavy mineral separates using tetra-

bromoethane (specific gravity of 2.97 g/cc).A portion of each

heavy mineral split was crushed to powder, mounted and X-rayed from 112

15 to 110 degrees 29 at 0.02°/3 sec, using copper radiation. The

X-ray diffractograms were stacked according to geographic location, and major mineral peaks were identified qualitatively (see Borg and

Smith, 1969). Both representative and anomalous diffractogram sig- natures were selected for petrographic quantification -- 36 samples in all.

The light mineral split was mounted on microscope slides with epoxy resin, and stained for plagioclase and potassium feldspar according to the method of Laniz et al. (1961!). Pure albite grains were included with potassium feldspar in the alkali feldspar sub- group, since the plagioclase stain treats calcium but not sodium and many of the albite grains are microperthitlo. For each slide, at least 1!00 mineral grains were identified under reflected light. The counting method of Dickinson (1970) was followed so that our research could be directly compared with other recent work on light mineral petrology.

The heavy minerals were mounted in Canada balsam, and more than 20 species were identified petrographically under transmitted and polarized light. Because of the preponderance of magnetite-rich volcanic lithic grains in the heavy mineral fraction, an initial survey count of 3004- grains was made to establish the relative proportions of glassy volcanic fragments, opaque minerals, and translucent minerals. Then 250 translucent grains were identified by their optical properties. The heavy mineral X-ray diffractograis served as a qualitative check on the petrographic determinations.

The grain counts of the light and heavy mineral fractions were each normalized to 100% (see Appendix I).These represent 113

independent data sets because the relative proportions of the light

and heavy fractions in the parent sand populations were not calcu-

lated. The compositional data were combined and all mineral species

were sealed to constant means; the resulting matrix was subjected to

Q-mode factor analysis with varimax rotation (see Kiovan and Irnbrie,

1971) to isolate end-member petrologic assemblages, which we inter-

pret as ideal petrofacies. With further modelling, -the factor

scores were transformed into estimates of end-member composition in

terms of the original units of measurement, that is, in normalized

percentages of mineral and lithic species (Kiovan and Miesch,

1976).

RESULTS

Light Minerals

The range of QFL values (quartz-feldspar-lithio composition)

for the Chile Trench samples is basically coincident with values

obtained from continental slope and abyssal plain environments along

the Andean margin (Fig. 5; Yerino and Maynard, 1981!). The Chile

Trench data encompass the average compositions of modern sands from

strike-slip, continental arc, oceanic baekarc, and oceanic forearc

types of active margins (Valloni and Maynard, 1981). The data also

span the entire compositional range within the magmatic arc field,

from undissected arc (supracrustal volcanic cover) to dissected arc

(unroofed plutonic roots) tectonic settings, as defined by Dickinson

et al. (1983) for ancient Phanerozoic sandstones. The Q/F ratio is 114

F L MODERN SANDS o CHILE TRENCH - ThuStudy ANOEAN MARGIN - Verino and Maynard (1984) + ACTiVE MARGINS (AVG.) - VallonI and Maynard (1981> ANCIENT SANDS MAGUATIC ARC - DckInaon at ii (1983)

Figure 11-5. QFL composition (quartz-f'eldspar-lithic) of modern sands from the Andean continental margin.Chile Trench samples (solid circle) and Andean margin samples from continental slope and abyssal plain settings (open circle) are plotted. Also shown are the average compositions of modern deep-sea sands from several different active margin settings (crosses), and the magmatic arc compositional field f or ancient Phanerozoic sandstones. 115

Consistently less than unity, but nearly the entire range of lithic composition is represented, implying that immature, quartzofeldspathic sands suffer varying amounts of dilution by volcanoclastic grains. It is clear that diverse geotectonic environments and sedimentary provenances may exist along a single continental arc, and that the QFL composition of the sand fraction is a poor discrimina- tor between different types of active margin settings.

In Figure 6a, we consider the distribution of samples on the

QFL diagram according to their geographic position along the Chile

Trench. Samples from the sheet flow basin at2°S show a bi-modal distribution. The compositional split reflects the difference between two sheet turbidites that were deposited under contrasting flow regime conditions, and which represent different hydrodynamic facies (see Background). Samples that are rich in volcanic lithic grains represent the composition of the Black Ash unit, while the more quartzofeldspathic mode represents the coarser, higher energy deposits of the Sand Lens unit. Apparently, vessicular volcanic scoria Is concentrated in the low-energy portions of' a turbidity current due to its low density and irregular shape; quartzofeldspathic grains are largely diluted by this lithic component in the

Black Ash unit.

Most samples from the South Chile province contain subequal amounts of feldspar and lithic grains, and less than 20% quartz.

Sands from the North Chile province (aside from a single ash-rich sample), the Central Chile province, and from the high-energy litho facies of the sheet flow basin at 42°S exhibit markedly higher quartz content and lower lithic content (Fig. 6a); these samples 116

Figure 11-6. QFL and QmPK composition of Chile Trench sands: petrologic variations along the strike of the margin. Note that sanples from the North Chile and Central Chile provinces, and from the sheet flow basin at2°S in the South Chile province, are enriched in quartz and alkali feldspar, and depleted in lithics and plagloclase feldspar, reflecting a derivation from more plutonic source terranes. rai I'III BheitBloBan Blo Antonio Flow FanaIalaIl.c.lI. Channe Fa F L P K L P I1 F A Figure 11-6. B 118 plot within the "Dissected Arc" field of Dickinson et al. (1983).

The quartzofeldspathic nature of the Central Chile sands is not surprising since Plio-Quaternary volcanism is absent along this segment of the Andean margin (Fig. 1). Erosion has cut through the

Mesozoic and Cenozoic volcanic carapace and exposed the crystalline rocks of the underlying batholith.

The relatively low lithic content of North Chile and some 112°S samples is anomalous since both regions lie adjacent to active volcanic arcs. In the North Chile province, the aridity is so extreme that many rivers do not discharge into the ocean, but evaporate on the plains of the longitudinal depression in ephemeral lakes and salt flats. Much of the debris from the volcanic High Cordillera is thus trapped as alluvium in ponded interior basins (Mortimer and

Saric, 1975). The crystalline lithologies of the Coast Range may provide an exagerrated contribution of quartz and feldspar minerals to the offshore under these harsh desert conditions.

At 1120S, dissection of the volcanic arc is probably enhanced because Pleistocene glaciation was severe at these southerly latitudes (Paskoff, 1977). Mechanical erosion beneath a moving ice mass will reduce plutonic rock much more effectively than river processes, especially in an area of steep relief and high maritime precipitation, such as southern Chile (Flint, 1971). The And.ean batholith crops .out extensively on the western flank of the High

Cordillera in tile high latitudes of the South Chile province, and the Quaternary stratovolcanoes of the arc are built directly upon it

(Ruiz and Corvalan, 1968). 119

The monocrystalline mineral grains of quartz, plagioclase, and alkali feldspar were normalized to 100% and displayed on the ternary

QmPK diagram (Dickinson and Suezek, 1979) (Fig. 6b). The majority of samples from the South Chile province are very plagioclase-rich, with <30% quartz and <15% alkali feldspar, reflecting their derivation from the contemporary volcanic arc. This includes samples from the low-energy, ash-rich deposits of the sheet flow region, since many of the glassy volcanic lithics contain sand-sized plagioclase phenocrysts. The influence of a plutonic provenance in samples from North Chile, Central Chile, and from the high-energy deposits of the sheet flow basin at 42°S is more clearly indicated by the monocrystalline population. Sands from North Chile and the

1120S region are characterized mainly by a significant increase in the quartz component over plagioclase.

The Central Chile samples show not only an elevated quartz contribution but, above all, the greatest enrichment of alkali feldspar. The anomalous alkali-enrichment can be attributed, at least in part, to a volcanic provenance rather than a strictly plutonic one. In the basal section of the Andean eugeosyncline (pre-middle

Cretaceous), volcanic sequences were laid down in a submarine environment, metamorphosed during burial under greensahi.st fades conditions, and altered to spilitic/keratophyric compositions

(Aubouin et al. 1973; Aguirre et al., 197I; Levi, 1970). This has caused the in situ replacement of much calcic plagioclase by pure to microperthitic albite. These older volcanic strata crop out extensively along the Central Chile province. Apparently, the erosional dissection of the Andean Cordillera that is associated with volcanic inactivity in Central Chile has also cut deeply into the eugeosynclinal sequence and exposed the basal, alkali-enriched strata.

A group of samples near the San Antonio Discontinuity at 33°S is displaced toward the "K" pole as well. The drainage in this region cuts through the Faleozoic batholith of the Coast Range, and the Paleozoic plutons are generally more alkaline in composition than the Andean intrusions (Berg et al., 1983).

Heavy Minerals

X-Ray Diffractograms: Qualitative Mineral Analysis

The heavy mineral content of the trench sands can be determined qualitatively by analyzing the X-ray diffractogranas of this density fraction for major mineral peaks (Figs. 7a, Tb).

Samples from the sheet flow basin at 12°S plot as two distinct fields on the QFL and QmPK diagrams, each light mineral field defin- ing the composition of a particular sheet turbidite--the low-energy

Black Ash unit, and the high-energy Sand Lens unit (Figs. 1, 6a,

6b). However, the heavy mineral assemblages from both the quartzofeldspathic and ash-rich turbidites are markedly similar, and represent a mixed igneous/metamorphic provenance (Fig. Ta). 011- vine, both ortho- and elinopyroxene, and hornblende are indicative of basic to acidic extrusive, and intermediate to acidic intrusive igneous rocks. Epidote coupled with hornblende suggests a meta- ulorphic contribution at' low to medium grade.

The heavy mineral assemblage appears relatively insensitive to changes in the energy at' the depositional environment. The only 121

2O2 34 30 2V

u.as 49243 U. 0 I-le 3L98 z

U. i16-l1I sa.rs

30.18 I 12-I 14

M11 1631U 40.53 x

311-313 42.V$

u-a? 21?220 42.18

I 0) u-el 61-32 42.1S 0 u-to I- al-OS 42.1S I a,

Figure lI-la. X-ray diffractograxns of Chile Trench heavymineral separates. Diffractograms are stacked according to latitudinal position along the trench. Major mineral peak occurrences are located in the legend on the following page, and scaledby their relative peak intensities. Core numbers and sample intervals are noted. 122

I-

I ZQ-S W-24 23.38 A__ /'\._)7-I L F

23.38 -I______

25.78 u-at 242-243

27.es - d. a ._4__,_ illITT

30.68 __

32.58 _._ -S%._ M-20 41-43

033.2S - . i..

ourNEl.. CUNOPYROXENE ORThOPYROXENE *ELAflVI HORNBLENDE PEAK 1EN EHOOTE Q>76%

NAGNETITE BIOTITE ilCHLDRI1

Figure 11-Tb. X-ray diffractogranis of Chile Trench heavy mineral separates. See Figure 11-Ta for caption. 123 consistent difference between the two turbidites at I2°S is that the

Black Ash unit contains substantially more biotite (Fig. 7a); this probably represents an effect of hydraulic sorting during deposition, and not a change in provenance. Both platy mica and vessicular volcanic scoria are easily entrained, and likely become concentrated in the low-energy portions of the turbidity current.

Evidently, minerals from a broad geographic provenance, including both the Coastal and High Cordilleras, become mixed in the fluvial, littoral, and shelf environments before progressing to the deep-sea at 42°S. These sands are then diluted by a fluctuating content of volcanic lithics that may represent (a) lithofacies changes imposed within the basin of deposition during the evolution of turbidity currents, or (b) actual changes in the intensity of arc volcanism.

The mineralogy of sands taken near the axial channel at IO.5°S, and from the Callecalle submarine fan off Valdivia at 39.7°S, contains a mixed provenance assemblage similar to that of the sheet flow basin at 42°S (Figs. 2, 7a). However, one notices a gradual increase in olivine content relative to hornblende in progressing north along this part of the margin. Presumably this reflects a northward decrease in the intensity of glaciation, and in the extent to which mechanical erosion has carved into the plutonic roots of the arc.

Diffractograins from the Bio Bio fan at 39.7°S show exceptionally well-defined peaks for olivine and both pyroxenes. with clinopyroxerte apparently dominant over orthopyroxene (Figs. 2, 7a).

The contribution of hornblende, a common to abundant heavy mineral in almost all other Chile Trench environments, is minor or 124

insignificant on this particular submarine fan. The composition of

these sands is the most chemically immature and least differentiated of the entire trench; the source of this unstable sediment can be traced onshore to the Bio Bio River (Fig. 7b; Baba and Scheidegger,

in press).

Several core samples straddle the San Antonio Discontinuity near 3303, and all are apparently derived from the structurally-

controlled San Antonio submarine canyon system (Fig. 2). These sands share a mineralogy that is characterized by abundant horn-

blende, orthopyroxene, and rnagnetlte components, reflecting an

intermediate to acidic igneous provenance (Fig. 7b).

For hundreds of kilometers along the Central Chile province--

from at. least 27.6° to 30.6'S latitude--the heavy mineral assemblage

is quite homogenous, and dominated by hornblende and epidote of

plutonic/metamorphic provenance (Figs. 2, Tb). Secondary pyroxene minerals attest to intermediate volcanics in the source region, necessarily reworked from an older stratigraphic sequence because

Plio-Quaternary volcanism is absent along this segment of the Andean

Cor dill era.

Sands from the North Chile province exhibit a similar assemblage, but pyroxene minerals have gained importance relative to the

hornblende-epidote association in conjunction with the reappearance of active volcanism onshore (Fig. 7b). It is interesting to compare

a core sample from a ponded basin in the trench axis at 8080 m

(W-22) with one taken 20 km directly upsiope, on the lower continen-

tal Slope at 5100 m (w-24). The diffractograms from these two

samples are exceptionally dissimilar. The trench axis core contains 125 a heavy mineral assemblage that compares well with sands recovered from the trench axis farther south, but in a different ponded basin

(core M-01). The lower slope core contains significant quantities of hornblende, biotite, andalusite, and garnet, suggesting derivation from an intermediate-grade metamorphic provenance. Core W-24 represents the most chemically mature, compositionally differentiated, craton-like assemblage recovered from the Chile Trench.

The remarkable contrast between the mineralogy of samples W22 and W-211 suggests that the trench axis sands may have been derived from a longitudinal, axially-removed source (along-margin transport), while the lower continental slope sands represent local submarine weathering and recycling of slope deposits, through upsiope gravitational failure (across-margin transport). In support of this hypothesis, Neogene quartzofeldspathic sandstones were dredged at this latitude between 14,000 and 5,000 m water depth, and their heavy mineral fraction is rich in biotite and hornblende (Kulm et al.., 1981). A strong apatite reflection in W-214 is also consistent with a continental slope derivation, since authigeflic precipitation of this mineral (as phosphorite) is common in the upper slope environment (Veeh et al. 1973).

Peak Heights vs. Quantitative Petrography

A grain-by-grain petrographic count was performed to determine the absolute abundances of mineral species. As a calibration and cross-check, X-ray diffractograin peak height ratios were plotted against petrographic count ratios for amphibole vs orthopyroxene

(rZ=0.7714)and for orthopyroxene vs. clinopyroxene (r2=0.651) (Fig. 126

8). The correlations are significant considering the complex inter- ference patterns that may be produced by irradiating heterogeneous mineral assemblages, and the compositional mixing and variability that occurs within major mineral groups such as the amphiboles arid pyroxenes. The (310) lattice reflection of amphibole was used in the calibration. The correlation coefficients were increased by averaging two diffractograxn peaks for each of the pyroxene groups: the (610> and (1120) reflections of orthopyroxene, and the (-221) and

(220) reflections of clinopyroxene. By plotting the data on loga- rithmic axes, a ratio of 1:1 occupies a median position along each axis, and ratios of 1:10 and 10:1 are equidistant from this median value.

The amphibole to orthopyroxene ratios are shown in Figure 8a.

Most of the samples cluster according to their geographic location along the trench, and these provincial groupings are spread along the regression line. The amphibole orthopyroxene ratios f or the majority of samples from the South Chile province are near unity or less, reflecting a large input from Quaternary basaltic-andesitic volcanism to the heavy mineral assemblages. Samples from the North

Chile province, the Central Chile province, and the 2°S region of

South Chile exhibit ratios greater than unity. These three regions are also characterized by low lithic content and high quartz and alkali feldspar content in the light mineral fraction (Figs. 6a,

6b). The amphibole :orthopyroxene ratio appears to correlate well with the relative importance 01' plutonic rocks vs. volcanic rocks in the source area, and provides an index of arc dissection. AMPHIBOLE ORTHOPYROXENE ORTHOPYROXENE: CLINOPYROXENE

10 6:1

U) Co o 6: O 3:1 I- I- 2:1 I- I- z 2: z .+ 0 D 0 0 1:1 1:1 +2 *+ U + a- I a. 1° S I:2 0 * I- 0 W 1: 1:3 a. a-

1:6 1:1 I I I I I I I 1:10 1:6 1:2 1:1 2:1 6:1 10:1 1:5 1:3 1:2 1:1 2:1 3:1 6:1 PEAK HEIGHT RATIOS PEAK HEIGHT RATIOS (elo)+(420)(-221)+(220) a1o) :(610)+(420) 2 2 2 * NORTH CHILE * CENTRAL CHILE SOUTH CHILE + 33.OS - San Antonio A o 36.O°8 - Sb Sb Fan saT's- CSIIscSII. Fan + 405°S - AxIi Ch.nn.l * 42.06 - 511.11 Flow

Figure 11-8. Peak height and petrographic count ratios of heavy minerals. Comparison of ratios determined independently by Xray diffraction and optical identification for (A) amphibole vs. orthopyroxene, and (b) orthopyroxene vs. clinopyroxene. 128

The plot of orthopyroxene to cilnopyroxene ratios results in a very poor discrimination of provincial compositions along the trench, and there is a large amount of overlap among the various geographic regions (Fig. Bb). Because the two pyroxenes are highly correlated and derived from a similar provenance, there is never more than a three-fold dominance of one pyroxene group over the other. In general, sands from 33.O°S, 39.7°S, and the North Chile province exhibit ratios greater than unity; sands from 36.9°S and the Central Chile province are less than unity; and sands from

1O.5°S and 2.O0S are near unity. Differences in pyroxene composition are probably related to local variations in the age and chemistry of andesjtic rocks in the source region.

Factor Analysis of Light and Heavy Minerals

The Chile Trench data were subjected to Q-mode factor analysis to explore the interrelationships among samples that are defined by petrologic variables. The variables (mineral and lithic compositions in count percentages, with light and heavy components each normalized to 1OO) are scaled to constant means so.that all minerals, including those of trace quantities, are weighted equally in the analysis. The Q-mode technique describes the envelope of sample variance in terms of a pre-specif led number of principle axes, or factors, in multidimensional space. The factors are orthogonal, non-correlated vectors that represent hypothetical end- member compositions--in this case petrologic assemblages, or ideal petrofacies. The principle axes are then rigidly rotated to achieve 129 the maximum variance among sample loadings--the contributions of each component factor to the total mineral assemblage of each sample. The rotation effectively produces a more equitable distribution of total sample variance among the factors.

A four factor model, accounting for 811% of the total sample variance, produced the most reasonable petrofacies assemblages which are consistent with the petrogenesis of source rocks in the Andean continental arc. The distribution of sample variance is as follows:

Factor I, 31%; Factor II, 17%; Factor III, 25%; Factor IV, 11%.

The factor scores, or the description of each factor in terms of the original variables, are shown in Figure 9. The magnitude of the factor scores are not proportional to the actual quantitative contribution of each mineral species to the total assemblage.

Rather, the factor scores reflect each mineral's ability to uniquely and specifically characterize a particular end-member assemblage, i.e. those minerals with the highest factor scores serve as the best index minerals. For example, although common horribleride is volume- trically the most abundant heavy mineral in Factor III, its factor score is only mediocre because hornblende occurs in a wide variety of igneous and metamorphic environments. Oxyhornblende, on the other hand, occurs almost exclusively as phenocrysts in volcanic rocks; although it rarely exceeds 5% of the heavy mineral fraction in the trench samples, its outstanding factor score in Factor II reflects an excellent index capacity because of its unique association. 130

I 0 z Z9.j 9 I I92,., UI IZfliw 2 1- 3 I., . 2 -.J.J29 141-32 42024 4 P4 04 . .1 00U0eQ 2 UI U11 4 1-

FACTOR I Basic Maqmct.c Arc

FACTOR U Acid Mogmatic Arc

FACTORm Metamorphosed Mogmotic Arc

FACTOR Cratonic Block LIGHTS HEAVIES

Figure 11-9. Chile Trench petrofacies: factoranalysis of petrologic data. Factor scores of five light andeighteen heavy mineral arid lithic species are plotted, emphrsizing those species which are most characteristic and diagnosticof each of the four factors in the petrofacies model. See Table11-1 for estimates of absolute compositions. 131

Composition Model

An extension of the Q-mode factor analysis estimates the corn- positions of the reference axes (factors, or ideal petrofacies) in terms of the original units of measurement (count percentages of minerals and lithics) (Kiovan and Miesch, 1976). The results are given in Table 1, with light and heavy mineral suites each totaling near 100%. The general trends of increasing quartz, increasing alkali feldspar, and decreasing volcanic lithic content, from Factor

I through Factor IV, are noteworthy. The factors appear to be ranked, as given, by their chemical maturity in the surface environment.

It is clear that many of the index minerals with high factor scores are quantitatively minor contributors to the total assemblage. They suffer dilution from the more ubiquitous components-- volcanic lithics, plagioclase, opaque minerals, pyroxenes and arnphiboles--that have accumulated along the Andean margin during nearly 200 m.y. of subduction-related magmatism. However, a sig- nificarit factor score indicates a concentration in excess of the mean value for the margin. Factor IV, in particular, is strongly weighted toward trace minerals whose combined contribution to the heavy mineral suite totals less than ten percent, though these minerals are present in unusual and elevated concentrations. 132

Table 11-1. Factor analysis composition model: estimates of absolute abundances of petrologic components.

FACTORS

I II III

Cl, Quartz 3.2 12.8 28.5 36.7 I- Alkali Feldspar 0.3 9.1 16.8 12.2 I Plagioclase 12.0 26.2 33.1 CD Lithics 47'.8 64.6 27.0 17.8 Micas 0.0 0.0 0.7 0.0

9ö. I I I 9 I

Volcanic lithics 28.7 1LI.7 7.1 6.6 Opaques 5.0 30.4 13.4 25.5 Olivirie 31.2 0.0 0.0 10.k Cllnopyroxene 18.2 12.9 7.8 13.9 Orthopyroxene .1414 10.9 16.5 11.5 OxyhorribLlende 0.0 8.9 0.0 0.0 wCl) Common hornblende 2.7 12.7 20.2 8.9 Blue-green hornblende 0.7 2.2 13.6 6.2 Metamorphic amphibole 0.0 0.8 7.14 6.2 Biotite 0.0 2.6 1.6 1.0 w Chlorite 0.3 0.0 1.9 0.0 I Epidote 0.0 3.0 6.9 2.1 Metamorphic lithics 0.0 0.0 1.0 1.0 Apatite 0.0 0.4 1.8 0.9 Zircon 0.0 0.5 0.8 0.7 Garnet 0.0 0.8 0.3 2.7 Aridalusite 0.0 0.0 0.0 1.3 Tourmaline 0.0 0.0 0.0 0.8 1Q1. 1QU. J1UU.j 91.( J 133

Introduction to Factors

Factor I--the Basic Magmatic Arc petrofacies--is rich in olivine, orthopyroxene, clinopyroxene, plagioclase, and volcanic lithics (Fig. 9). These minerals represent a source area of high-Al basalts and basaltic andesjtes. The source volcanics must be

Pliocene-Quaternary in age because after a few million years this very unstable mineral assemblage will degrade in the surface environment, especially the fresh olivine and pristine volcanic scoria. Basic volcanics with olivine phenocrysts are present only in the active magmatic arc of the South Chile province (Deruelle,

1982; Thorpe and Francis, 1979).

Factor 11--the Acid Magmatic Arc petrofacies--represents a mixed volcanic/plutonic provenance (Fig. 9). Oxyhornblende, biotite, opaque minerals (magnetite), common hornblende, and volcanic lithics indicate an andesitic to rhyolitic volcanic source region. Hornblende, biotite, quartz, alkali feldspar, zircon, and garnet indicate an intermediate to acidic plutonic provenance. This assemblage is perhaps most typical of a "mature" continental arc terrane, being more sialic and differentiated than Factor I. The continental volcanics and volcaniclastic sediments of the upper

Cretaceous and Tertiary, and the plutonic bodies associated with the various Andean orogenies, could erode to yield this suite of minerals. Also, the Plio-Quaternary volcanism of the North Chile province is characterized by quartz andesite, rhyodacite, and rhyolite compositions with common phenocrysts of hornblende and biotite

(Deruelle, 1982; Klerkx et al.,, 1977; Pichier and Zeil, 1972). 134

Factor 111--the Metamorphosed Magmatic Arc petrofacies--is especially rich in chlorite, mica, blue-green hornblende, epidote, metamorphic amphibole (actinolite), apatite, alkali feldspar, quartz, and metamorphic rock fragments (Fig. 9). This assemblage is

typical of low-grade, low P/T metamorphism within the greenschist fades. The ferromagnesian minerals suggest metamorphism of basic

to intermediate igneous rocks; the anomalous alkali feldspar content

is consistent with albitization of originally calcic plagioclase.

Basaltic to andesitic volcanics, metamorphosed under greenschist fades conditions, occur in the basal section of the Andean eugeo-

syncline, in strata of Jurassic to early Cretaceous age (Aguirre et

al., 1978; Levi, 1970). This assemblage is also typical of the

Paleozoic basement in the Coast Range of South Chile (Gonzalez-

Bonoririo and Aguirre, 1970), where it is hypothesized that oceanic

terranes were aecreted to the continental margin and metamorphosed within a Hercynian subduction complex (see Geologic History).

Factor IV--tfle Cratonic Block petrofadies--is dominated by minerals of trace quantities, but their Importance has been scaled

up relative to the other more ubiquitous minerals during the factor

analysis because small changes in accessory minerals may be quite

diagnostic of sedimentary provenance (Fig. 9). Minerals such as

andalusite, garnet, metamorphic amphibole, and metamorphic rock

fragments indicate a dynarnotherrnal metamorphic source of inter-

mediate grade (i.e., regional amphlbolite facies). High factor

scores of tourmaline, garnet, zircon, and quartz are possibly

derived from reworked, multi-cycle quartz arenite sands, such as are

associated with epeirogenic transgressions of cratonic areas. 135

Finally, quartz, tourmaline, and garnet may reflect acidic, late- stage granitic Intrusions. This mineral assemblage is consistent with the mesozonal metamorphism, mature sedimentation, and differentiated magmatism that is associated with ancient continental crust. It is likely derived from the pre-Andean terranes of

Paleozoic and Precambrian age--either directly from the Coast Range outcrops or recycled through the sands of the Andean miogeosynclirie, whose source areas lay in the Eastern Cordillera during Mesozoic time. Baba and Scheidegger (in press) also isolated a quartz- andalusite-garnet assemblage in the river sands of northern Peru, and they proposed a derivation from similar source rocks.

DISCUSSION

Investigators working in the Great Valley forearc basin sequence of California introduced the term "petrofacies" to describe sedimentary deposits that are differentiated on the basis of their detrital mineral arid lithic composition (Mansfield, 1971; Dickinson and Rich, 1972). They found that petrofacies units transcended many local lithofacies boundaries, and were therefore a valuable tool for basin-wide stratigraphic correlation. In tectonically active basins, it is presumed that petrofacies will accurately record the geology of the provenance region, owing to rapid erosion, deposition, and burial processes.

A Q-mode factor analysis of the Chile Trench petrologic data generated four end-member assemblages. These assemblages represent coherent geological associations that are related to subduction 136 processes and the genesis of' the Andean continental arc. We regard these factors as ideal petrofacies, and name them the Basic Magmatic

Arc, Acid Magmatic Arc, Metamorphosed Magmatic Arc, and Cratonic

Block petrofacies. They are ranked, as given, by the degree to which they are differentiated toward sialic continental crust, by the age of the associated provenance region, and by their relative diagenetic stability (the Cratonie Block petrofacies being the most differentiated, the most chemically mature, and ultimately derived from the most ancient terranes). When factor loadings are projected onto each sample and plotted in map view, the geographic distribution of the ideal petrofacies is portrayed.

At this point we must differentiate between ideal petrofacies and actual petrofacies. Actual or naturally occurring petrofacies are typically mixtures of the ideal. Actual petrof'acies may be characterized by the diversity of their petrologic composition, and by the heterogeneity of their geographic distribution. Petrologic diversity describes the variety of source terranes that are represented in the assemblage; this quality is approximated by the equitability in the distribution of variance among the ideal petrofacies within a sample. Heterogeneity of' geographic distribution describes the spatial variability of a petrologic assemblage on the depositional surface; this quality is approximated by the variance of the factor loadings between adjacent samples. 137

Petrofacjes Distribution in the Chile Trench

In the sheet flow basin near 12°S, we find that all of the ideal petrofacles are well represented (Fig. lOa; Note the equitability in the distribution of variance among the factor loadings), implying that a diversity of lithologies in the provenance region have contributed to the petrologic assemblage. Contributing lithologies include extensive exposures of preAndean basement (Factor III) in the Coast Range, and CretaceousTertiary granodiorite/granite plutOns

(Factors II and IV) and Plio-Quaternary basic volcanics (Factor I) in the High Cordillera. At these latitudes Pleistocene glaciation covered much of both the Coastal and High Cordilleras (Paskoff,

1977), so a variety of source terranes were subjected to intense mechanical erosion, and their fragments shed to the offshore during lowered sea level.

Because the Pleistocene ice sheet extended onto the exposed continental shelf, we hypothesize that the glacial terminus was drained by a series of closely spaced submarine gullies on the continental slope. Each gully system might transport a sand composition that is biased toward one or another of the source lithologies. The gully systems likely generate small volume gravity flows that are spatially limited in the trench axis, and which intertongue with sheet flow deposits from neighboring systems. In short, a higher frequency of sediment supply points along the base of the margin will accentuate areal heterogeneity in the petrofacies distribution, and vertical heterogeneity in the petrofacies succession, since there is greater spatial/temporal variability between 138

740 710 76° 75S 72° 37.

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Figure II-lOa. Geographic distribution of Chile Trench petrofacies; factor loadings projected onto core locations. Asterisks denote the type localities of each of the four petrofacies in Figure 11-9. Trench-axis sediment fill is in heavy stipple pattern, trench fans and canyon-mouth deposits are in light stipple, and the axial channel is unshaded. Onshore geology is modified from Ruiz and Corvalan (1968). See Figure Il-lCd for geologic legend. 139

74 73 T2 .c-... 32°

33.

34.

Figure li-lOb. Geographic distribution of Chile Trench petrofacies; factor loadings projected onto core locations. See Figure II-lOa for caption. 140

FACTORS 73' 72° 71° 70° 69° iim r 27°

28°

290

32°

33.

Figure II-lOc. Geographic distribution of Chile Trench petrofacies: factor loadings projected onto core locations. See Figure II-lOa for caption. 141

72' 71' 70' 69' 68' 67;2.

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Figure 11-lOd. Geographic distribution of Chile Trench petrofacies: factor load.ings projected onto core locations. See Figure IL-1L)a for caption. 142 the compositions of individual turbidites. However, some of the areal heterogeneity that is evident in the factor loadings at 1120S is due to hydraulic sorting within individual turbidites as a result of lateral and longitudinal velocity gradients.

Moving north into the charinelized portion of the Chile Trench, we note a general increase in the importance of the Basic Magmatic

Arc petrofacies (of volcanic derivation) at the expense of the Acid

Magmatic Arc and Cratonic Block petrofacies (of plutonic derivation, representing the batholithic roots) (Fig. Wa). This trend is in phase with the climatic gradient, i.e. the decrease in the degree of arc dissection accompanies the decrease in the intensity of glaciation toward the north. At 39.7°S and 1lO.5°S, strong loadings of the Metamorphosed Magmatic Arc petrofacies indicate that the

Paleozoic rocks of the Coast Range continue to provide an important component to the mineral assemblage in the trench. Scree slopes and alluvial fans are evidence that the cool and humid Pleistocene climate caused significant erosion of the summit regions of the

Coast Range at these latitudes (Paskoff, 1977).

A series of cores were taken at 39.7°S near the mouth of the

Callecalle submarine canyon, in a variety of sedimentary environments that include submarine fan, axial channel, and seaward levee environments (Figs. 2, Wa; see Chapter I, this study, for a discussion of these environments and their associated lithofacies).

The petrofacies assemblage at 39705 is less diverse and more spatially homogenous than that of the sheet flow region. The decrease in diversity may be associated with the migration of the

Pleistocene ice sheet to higher elevations in these more northerly 143 latitudes, so that a narrower range of source rock compositions were denuded.Petrofacies homogeneity is presumably developed because the trench is fed by large volume turbidity currents, associated with major submarine canyon systems arid large subaerial drainage basins.As a result, the petrofacies effectively transcends sedimentary environments and lithofacies changes.

Between37°Sand 39°S, existing bathymetric tracklines suggest that the continental slope is cut by a series of more closely spaced submarine canyons.Regions of the trench that are In proximity to two or more canyon mouths might develop a stratigraphy with alternating petrofacies assemblages.The degree to which these petro- fades may be distinguished depends on the distance between canyon heads at the shelf break, and on the amount of petrofacies homogenization that occurs in the shelf and littoral environments, as well as the along-margin variability in the geology of the onshore drainage basins.

A series of cores taken at36.8°Snear the mouth of the Bjo Bio canyon are very much dominated by Factor I loadings at the expense of all other factors; these sediments represent the type occurrence of the Basic Magmatic Arc petrofacles (Figs. 2, lOb).Olivirie--the superior index mineral for this petrofacles---can be traced onshore into the bedload of the Bio Bi.o River, which is anomalously enriched in the mineral (Fig. Ta; Baba and Scrieidegger, in press), and ultimately to its source in high-alumina basalts of the High Cordillera

(Lopez-Escobar et. al.,1981). In their study area near 37.3°S, Lopez-Escobar et al. note that lavas become more basic with decreasing age:olivine phenocrysts are common in Holocene lavas 144

(3-18 vol %), but rare in older, generally quartz-normative rocks of andesite to basaltic andesite composition. If this chronology is true in general for the drainage basin of' the Bio Bio River--that olivine is derived strictly from a Holocene source--it emphasizes the extreme sensitivity of' trench petrofacies, and the swiftness of the system's reaction to changes in the provenance region.

The exclusive dominance of Factor I loadings is surprising, considering thdiversity of geologic terranes in the source region.

Tertiary andesitic volcanics of the high plateau (Factor II, see

Vergara and Munizaga, 19714), Paleozoic greenschist metamorphics of the Coast Range (Factor III), the Paleozoic batholith of the Coast

Range and the andalusite-rich aureole associated with it (Factor IV, see Gorizalez-Bonorino and Aguirre, 1970), and the continent-derived sandstones of the Mesozoic miogeosyncline (Factor IV) might all have made important contributions to the trench petrofacies (Fig. lob).

In addition, Tertiary graben-fill sedimentary sequences, bound and floored by the metamorphic basement of the Coast Range, are uplifted and terraced at the coast just south of the Blo Sb Canyon

(Mordojovih K., 19714; Paskoff, 1977) but the recycling of these ancient terranes is not apparent in the trench. The singularity of provenance type can be explained by (a) the extreme erodibility of unconsolidated, f'ragmental volcanic debris, which effects the preferential stripping of this lithology, and (b) the correspondence of

Quaternary volcanism with the summit regions of the High Cordillera, where Pleistocene glaciation was most severe. It is nevertheless amazing that such an overwhelming petrofacies signature may be derived from such a small proportion of the drainage area. 145

The 310 Bio region is unique in the extreme homogeneity of the

petrofacies distribution, regardless of sedimentary environment or lithofacies. While the axial channel may transport material from

southerly, up-gradient sources, it must be so flooded with material

from the nearby canyon that its petrologic assemblage is indistinguishable from that of the adjacent fan.

Several cores straddle the San Antonio Discontinuity at 33°S;

north of here the trench fill shrinks to one tenth of its volume and active cordilleran volcanism terminates (Figs. 1, 3, lob). It is

clear from the homogeneity of the petrofacies distribution that

these cores share a common source of sediment supply. There is a

latitudinal gradation in the trench from the Basic Maginatic Arc

petrofacies in the Blo Blo region to the Acid Magmatic Arc petro-

fades in the San Antonio region. Andean stratovolcanoes between

33°S and 314°S, exactly that part of the High Cordillera that is

drained by the Maipo River-San Antonio Canyon system, have the high-

est silica content of the South Chile province; these volcanics are

characterized by hornblende phenocrysts as opposed to olivine

phenocrysts farther south. Lopez-Escobar et al. (1977) speculate

that the change in magma chemistry at the terminus of the South

Chile volcanic arc may be related to the change in the angle of the

dipping seismic zone across the tectonic discontinuity at 33S. The

anomalous volcanism has caused a unique petrofacies to develop near

the segment boundary, and it represents the type occurrence of the

Acid Magmatic Arc petrofacies.

Between the San Antonio Discontinuity and 30.5°S, there is a

major change in trench petrofacies that is not well constrained by 146 the sampling density (Fig. lOc).The petrologic assemblage that is derived from the San Antonio canyon system may extend north into the Central Chile province for as much as 200 kilometers. Between 30.5°S and the northern segment boundary at 27.5°S, the Central Chile province is dominated by greenschist minerals of the Metamorphosed Magmatic Arc petrofacies (Fig. lOc); the type occurrence is round near 30.5°S (core F-03).The source terranes are likely composed by the basal section of the Mesozoic eugeosyncline, which outcrops extensively in Central Chile, and by the Paleozoic basement of the Coast Range to a lesser degree.Eugeosynclinal volcanics younger than middle Cretaceous age may account for the subordinate Factor II loadings. Our factor model also shows a significant contribution from the Cratonic Block petrofacies, especially in the northern part of the Central Chile province.We postulate that a recycling of platform sediments from the Andean miogeosyncline produces this signal. Because the strike of the Mesozoic geosynclirle (N-S) is slightly oblique to the present orographic elements (N100E), the miogeosynclinal strata migrate through the continental divide and into the western foothills of the High Cordillera in Central and North Chile. Miogeosyrielirial sediments were derived from a continental highland to the east, which is today represented by pre-Andean terranes in the Eastern Cordilleras of Argentina. The trench petrofacies is quite homogenous f or more than 300 kilometers along the Central Chile province; these sediments are probably derived from a major submarine canyon that heads near La Serena at 300S.The petrofacies of Central Chile remains uniform 147 despite variations in the energy of the depositional environment, as evidenced by cores taken directly in the trench axis and also several hundred meters above the axis on the seaward trench wall.

In North Chile, Quaternary volcanic activity resumes in the continental arc (Fig. 1). However, much of the volcanic debris is trapped in ponded interior drainages of the longitudinal depression and never reaches the coast (Mortimer and Sane, 1975). It is primarily south of 25°S, where a few ephemeral streams reach continuously through the Coast Range and up into the high Andes, that the

Quaternary andesitic volcanies are represented in the factor loadings (Fig. lUd, Factor II). Even there, the volcanic arc clearly does not dominate the petrofacies assemblage as it did throughout much of South Chile (compare Fig. lob).

The mineralogy of the North Chile trench sands is dominated by contributions from the Metamorphosed Magmatic Arc and Cratonic Block petrofacies. This assemblage can be fully accounted for in the

Paleozoic and Mesozoic lithologies of the Coast Range to the exclusion of the High Cordillera. Pluvial conditions affected the

Coast Range of northern Chile during the Pleistocene, and large alluvial fans were built out into the exposed littoral zone. With the return of sea level in Holocene time, the fans began eroding by subaerial and marine processes, providing a source of second-cycle sediment to the offshore (Paskoff, 1977).

The concentration of Factor IV minerals in the lower slope core near 23°S (core W-2I) is outstanding; this core represents the type occurrence of the Cratonic Block petrofacies (Fig. lOd). Several researchers have expressed the opinion, based on evidence from a 148 variety of data, that the Precambrian craton of the southern Peru coast may extend southward beneath the coastal region of northern

Chile (Pichier and Zeil,1972;Dairnayrac et al.,1980;Damm et al.,

1981;Berg et al., 1983).Some workers have cited evidence that this continental crustal block is only a remnant of a once greater rnassif that extended hundreds of kilometers into the Pacific during

Paleozoic time (Miller,1970;Isaacson,1975;Dalmayrac et al.,

1980). Tectonic erosion of this paleocontinent along the lower continental slope, followed by subductiorl and mobilisation of the plucked fragments, has been suggested to account f or continental contamination of Paleozoic and early Mesozoic magmas in northern

Chile (McNutt et al.,1975;Berg et al.,1983). It is thus reasonable that outcrops of ancient, truncated continental crust are exposed on the steep and sediment-starved continental slope, and provide a local sediment source to the North Chile margin. Much of the North Chile province is so extremely arid that large river-canyon systems with well-defined drainage basins have not developed.The avenues of transport across the barren and rugged continental slope are probably many and complex.The trench is not fed by established submarine canyons with a definite and consistent mineralogy, but more likely by sporadic events of restricted provenance that enter the trench from irregular and ill- defined points cr1 the lower slope.Gravity flows generated along this arid margin should be of small volume and skewed toward coarse grain sizes, and therefore of limited areal extent in the trench axis.Thus, the petrofacies of the North Chile province are probably characterized by rather abrupt lateral and vertical 149 changes, or a high degree of heterogeneity in geographic distribution.

Sediment Transport in the Chile Trench

The homogeneity of the petrofacies distribution provides an index to the efficiency of longitudinal transport in the Chile

Trench. For example, the trench petrofacies is very homogenous for more than 300 km in the Central Chile province, suggesting that these sediments were derived from a common source and dispersed along the margin for a distance of this magnitude. Sand transport of hundreds of kilometers along the trench axis is consistent with the lithofacies, depositional morphologies, and seismic stratigraphies that occur in the Chile Trench (Chapter I, this study).

Heterogeneous petrofacies distributions in the North Chile province and in the sheet flow basins of the South Chile province are consistent with more localized trench deposition and limited axial transport. Along-axis transport is hindered in the sheet flow basins because the energy of a turbidity current dissipates rapidly in an unchannelized environment. In North Chile, gravity flows of small volume are generated from the arid Coast Range and the sediment-starved continental slope, so individual gravity flow deposits are spatially confined in the trench basin. Heterogeneity may be enhanced, and axial transport hindered, by a system of more closely spaced sediment supply points along the base of the slope, each drawing from a reduced onshore drainage area and producing gravity flows of lesser volume. 150

Homogenous petrofacies that transgress depositional environments are generally associated with major river-canyon systems that

drain large subaerial basins and generate large volume turbidity

currents. Trench environments that are proximal to major submarine

canyons--including Callecalle, Blo Bio, and San Antonio canyons--are

flooded with a distinctive suite of' minerals from the adjacent main-

land which overwhelms the petrologic signal from up-gradient

sources. The proxiznaj. trench environment is thus an insensitive

indicator of axial transport, though it is most sensitive to changes

in the provenance region, including changes in volcanism, geology,

climate, and morphology. The distal trench environments may provide

a better record of the efficiency of the axial dispersal system, but

we have very little control from these environments.

The petrologic composition of a core on the lowermost continen-

tal slope at 140.505 (core M-12) is essentially noncorrelated with

that of a core only 15 kilometers to the west in the trench basin

(core H-li) (Fig. ba). A similar contrast is observed between a

lower slope core at 23.3°S (core W-2k) arid a trench axis core 20

kilometers seaward (core 11-22)(Figs. 7b, lOd). A very sharp and

pronounced petrofacies boundary coincides with the base of the con-

tinental slope.

The trench petrof'acies are homogenized to some degree by axial

transport in the basin. The slope assemblages are more locally

derived, and perhaps reworked from uplifted trench sequences, fore-

arc basin sequences, or relict shelf deposits or' the margin.

Because of the very short residence time of sediments in the unde-

formed trench basin (300,000 years for a 30km-wide trench at a 151 convergence rate of 10 cm/yr; see Schweller and Kuim, 1978), trench petrofacies should represent the most recent physiography of the adjacent magmatic arc; slope sediments are not constrained by any limiting residence period. We conclude that the petrofacies distribution on the continental slope is more heterogeneous and provincial

than that of the trench, because across-margin transport between major submarine canyons occurs via complex pathways that include the reworking of acoreted or relict assemblages.

Axial transport in the trench is implied even for the small

ponded basin at 23.3°S in the sediment-starved North Chile province.

A this basin represents the absolute maximum depth for the entire

Peru-Chile Trench (8080 m, Fig. 3), axial transport may have

delivered sediment to this region from either sources to the north

or to the south. Indeed, the isolated basins of North Chile may

have once been interconnected to form a coherent longitudinal

dispersal system. A decrease in sediment supply associated with the

Holocene rise in sea level, coupled with continued convergence, has

probably caused their isolation.

Modification of Petrofacies Composition--Provenance to Deposit

The petrologic composition of a sand may deviate from that of

the source rock terrane because the petrofacies assemblage is modi-

fied by chemical weathering in the source area, by selective sorting

during transport and deposition, and by post-depositional diagenetic

alteration (van Andel, 1959; Suttrier, 19V1). The modification of

sand composition by chemical weathering and selective sorting tends 152 to be more advanced along passive margins, where typically low stream gradients and broad continental shelves prolong the residence time of detrital grains in the soil horizon or at the sediment-water interface of the shallow marine environment (Mack, 1984).

Chemical weathering along the Chilean continental arc is insignificant, by comparison, and olivine may form up to 30% of the total heavy mineral assemblage in the trench. This is true even in the

South Chile province where the climate is most humid and the Andes are of lowest relief. The climatic gradient along this portion of the Andean margin affects the trench petrofacies not through the intensity of chemical weathering In the source area, but in the efficiency of sediment trapping in the forearc basin that separates the Coast flange from the High Cordillera, and in the proportion of the drainage basin that is glaciated during low sea level stands.

Both forearc trapping and glaciation may dramatically modify the relative proportions of the source lithologies that contribute to offshore sedimentation.

Selective sorting may occur during deposition in the trench basin between the low- and high-energy portions of a turbidity current. The majority of light and heavy minerals are fairly insensitive to depositional sorting, because of the catastrophic nature of deep-sea sedimentation along a convergent margin. This is especially true in the proximal trench environments, near the mouths of major submarine canyon systems, where gravity flows are typically of large volume and petrofacies effectively transcend lithofacies.

In Central Chile, for example, the petrofacies of the trench axis is similar to that of the seaward trench wall; near the San Antonio, 153

Bio Blo, and Callecalle canyons in South Chile, the petrofacies of the axial channels and fan channels are similar to those of the adjacent levees.

Minerals or lithics with anomalous densities or irregular shapes, such as vessicular volcanic scoria or platy mica, are subject to unnatural concentrations, however. One must be especially careful when such grains form a major component of the petrologic analysis. Selective sorting of fresh, vessicular volcanic lithics, for example, may cause significant deviations from the parent composition on the QFL diagram. In the stratigraphic dimension, the superposition of a high-energy, lithic-poor assemblage and a low-energy, lithic-rich assemblage might wrongly be attributed to variations in the volcanic activity of the source region. If a large component of plagioclase phenocrysts were enclosed in a vessicular glassy rim, causing them to be selectively concentrated, a bias would be imparted to the QmPK diagram as well.

These same arguments apply, in reverse, to non-porous, well-rounded volcanic lithics of magnetite-rich composition. The bias of selec-. tive sorting is especially problematic in ancient rocks where the texture and composition of volcanic lithics is indeterminate due to diagenetic degradation. 154

Diageriesis of Trench Sediments

Cornpositionally immature deep-sea sands, characteristic of

active margins, will suffer diagenetic changes with time and

increasing depth of burial. Typical diagenetic changes include:

(a) chemical dissolution, alteration, or replacement of unstable mineral and lithic framework grains, such as volcanic lithics, fer-

romagnesian minerals, micas, and feldspars; (b) mechanical deforina-

tion and crushing during compaction of ductile components, such as

volcanic lithica and ph.yllosilicates; and (c) cementation through

the growth of authigenjc minerals, such as calcite, quartz, and

phyllosilicates (Dickinson, 1970; Galloway, 19714; G.F.Moore, 1979;

Lopez, 1981). Chemical and mechanical degradation of framework grains, and microcrystalline, inhonlogenous cementation or recrystal-

lization during diagenesis will frequently produce poorly-structured,

fine-grained aggregates that may be Irresolvable

from the original depositiorial matrix (Dickinson, 1970).

It is clear, from the presence of fresh olivine grains and

beautifully preserved volcanic scorla fragments, that the late

Quaternary Chile Trench sediments of this study have undergone neg-

ligable diagenesis in their present depositional environment.

Diagenetia changes have been noted in trench sediments that are

buried to depths of only tens of meters, however, where the initial

stages of grain etching and friable cementation are observed

(Scheidegger et al., 1973; Lopez, 1981).

Diagenesis will likely be accelerated following the shallow

accretion of trench sediments to the lower continental slope during 155

convergence, as a result of: (a) a dramatic increase in the inten-

Sity of low-temperature deformation behind the base of the slope,

including folding, faulting, and cleavage development (Kuim et al.,

in manu); (b) enhanced pore fluid movement associated with the tec-

tonic compaction and dewatering of sediments (Carson, 1977); and (c)

intensified overburden pressures caused by the imbricate stacking of

acoreted sediment packages (von Huene, 1984). The cementation of

subduction complex sandstones, presumably uplifted trench sequences,

is dominated by phyllosilicate minerals (chlorite, sericite,

illite), though quartz overgrowths and locally pervasive carbonate

cements have also been observed (G.F.Moore, 1979; Lopez, 1981

Carson, 1977; Kuim et al., in manu).

It Is instructive to consider the diagenetic and metamorphic

changes that have affected the unstable lithologies of the Andean

eugeosyncline--a composite sequence of volcanic and volcaniclastic

strata that exceeds 20 kilometers In thickness, and represents

nearly 200 m.y. of convergence-related magmatic activity. These

strata are Similar In composition to the immature, arc-derived sedi-

ments that accumulate in the trench basin, and they may offer a

parallel in the progression of alteration mineral assemblages.

However, the eugeosynclina1. sequence accumulated in a subsident

trough within the South American continental block; diagenesis and metamorphism of trench sediments1 once incorporated into a lower

slope subduction complex, will likely be a much more rapid process

for the reasons cited above.

As a source lithology for the Chile Trench sediments, the

Andean eugeosyncline was modeled by the factor analysis as three 156 di5tinet mineral assemblages. These are the Basic Magmatic Arc,

Acid Magmatic Arc, and Metamorphosed Magmatic Arc petrofacies; they are ranked in order of increasing age and diagenetic stability (see

Fig. 9). The Basic Magmatic Arc is characterized by an assemblage rich in olivine, calcic plagioclase, and volcanic lithics. Olivine is exceedingly labile in the surface environment, and its abundance assures a derivation from only the most recent and contemporaneous volcanic activity. As the most unstable minerals are removed t'rom the assemblage, the petrofacies may shift toward the composition of the Acid Magmatic Arc, which is characterized by hydrous ferromagnesian minerals (hornblencie and biotite), minor pyroxene, alkali feldspar, and a trend toward decreasing volcanic lithic content.

According to Levi (1970), pyroxene and oxyhornblende are common in the Tertiary section of the eugeosyncline, though the lower units have reached zeolite-grade metamorphism. These minerals are rare in volcanics of late Cretaceous age, and strata are generally metaiTlor- phosed under prehnite-pumpellyite conditions. Alteration minerals begin to dominate the petrofacies assemblage after greenschist facies metamorphism has been achieved, as in the Jurassic to early

Cretaceous section of the eugeosyncline. By this time, the unstable ferromagnesian minerals of the original volcanic lithologies have altered to chlorite, epidote, actinolite, and mica, and much of the calcic plagioclase has been albitized (Metamorphosed Magmatic Arc).

It is thus conceivable that a fresh petrologic assemblage, derived from some combination of volcanic and plutonic arc terranes, could pass through the entire range of compositions that are idealized by the Basic Magmatic Arc, Acid Magmatic Arc, and Metamorphosed 157

Magmatic Arc petrofacies. It is clear that much of the petrologic signal in a forearc or trench deposit will be lost with age, depend- ing on the degree of diagenetic or metamorphic alteration.When studying the petrofacies f ancient continental arc deposits, one must always be concious of textural clues--the texture of the volcanic lithics, the depositional texture, and recrystallization textures--that may lead to a better understanding of the diagenetic history, hydraulic sorting in the depositional basin, and ultimately to the details of the provenance region.

CONCLUSIONS

1. The petrologic composition of terrigenous sand in the Chile

Trench can be described by four end-member assemblages, generated from a Q-mode factor analysis of light and heavy, mineral and lithic grains:

>Factor I (basic andesite: olivine, pyroxene, plagioclase feldspar, volcanic lithics) is associated strictly with Quaternary volcanism in South Chile, where the continental crust is thinner and more oceanic in composition; it is the least differentiated and most unstable petrologic assemblage encountered.

>Factor II (acid andesite/granodiorite: biotite, hornblende, magnetite, alkali feldspar, volcanic lithics) is derived from extrusive and intrusive lithologies associated with the Mesozoic and

Cenozoic Andean orogeny, and from the Quaternary volcanics of North

Chile and 33°S; it represents a differentiated continental arc petrofacies. 158

>Factor III (greenscilist: chlorite, mica, epidote, blue-green hornblende, actinolite, alkali feldspar) is derived front the basal marine section of the Andean eugeosyrioline, and from oceanic terranes in the Paleozoic basement of the Coast Range; it may be produced by low-grade metamorphism of the Factor I or Factor II assemblage.

>Factor IV (gneiss/granite/platform sediment: tourmaline, andalusite, garnet, zircon, quartz) is associated with the cratonic block of South America, and is especially prevalent off northern Chile where the continental crust is very thick and the margin has been truncated.

The four factors represent ideal petrofacies assemblages that are related to subduction processes and the crustal evolution of the

Andean continental margin. They are named the Basic Magmatic Arc,

Acid Magmatlo Arc, Metamorphosed Magmatic Arc, and Cratonic Block petrofacies, respectively. The factors are ranked by the degree to which they approach the composition of sialic continental crust, and also according to the age of the associated provenance region and their relative diagerietic stability. The type location f or the

Basic Magrnatic Arc petrofacles is on the Blo Blo fan near 37°S, for the Acid Magmatic Arc petrofacies it is near the San Antonio Discon- tinuity at 33°S, for the Metamorphosed Magmatic Arc petrofacies it is in the Central Chile province, and for the Cratonic Block petrofacies it is on the continental slope off northern Chile.

2. While some of the trench sediments approximate the ideal petrofacies assemblages, most sediments are mixtures of the ideal.

Two parameters that aid in the description and diagnosis of 159 naturally occurring petrofacies are the diversity of the mineral! lithic assemblage and the heterogeneity of its geographic distribution. Petrologic diversity refers to the variety of rock types that contribute to the composition of a sediment assemblage, and is represented by the equitability in the distribution of variance among the end-member factors, as they are projected onto individual samples. Heterogeneity of distribution describes the patchiness, provinciality, or local variability of petrologic assemblages in the depositiona]. basin, and is approximated by the degree of non-correlation between assemblages from neighboring sites in the trench.

The petrotacies that are the least diverse are associated with the active continental arc of South Chile PlioceneQuaternary volcanics inundate and dominate the petrofacies at the expense of all other rock types in the source region. In Central Chile, where modern arc volcanism is absent, and in North Chile where volcanic debris from the High Cordillera is trapped in the longitudinal valley, diversity is increased. The most diverse assemblage of all occurs in the sheet flow basin at 2°S in South Chile: Pleistocene glaciers extended to below present-day sea level, subjecting a variety of rock types In both the Coastal and High Cordilleras to intense mechanical denudation.

The petrofacies that are most homogenous In distribution occur near major submarine canyon systems associated with large subaerial drainage basins. The Central and South Chile provinces, north of

41 OS, are good examples. Turbidity currents in this environment are typically large-volume flows, and petrofacies transgress lithofacies 160

boundaries. Heterogeneity is introduced when the trench is fed by more closely spaced sediment supply points, i.e. chutes and gullies of smaller scale than submarine canyons, each of which drains a more limited provenance area.

3. A prominent axial channel has developed in the trench

throughout much of South and Central Chile adjacent to the large submarine canyon systems; it appears capable of transporting sediment for hundreds of kilometers along the margin. In the proximal

trench environment, on trench fans near the canyon mouths, the petrofacies is flooded and homogenized by the local assemblage.

Homogenization of the trench assemblage may be complete between sediment supply points, especially during periods of active down-

axis progradation of the longitudinal dispersal system. The trench assemblage should be a sensitive indicator of' the contemporaneous

configuration of the adjacent magznatic arc (its volcanism, geology, climate, and morphology) because the residence time of' sediments in

the undeformed trench basin is only a few hundred thousand years.

While trench petrofacies are derived primarily from along-

margin transport, slope petrofacies are derived from acrossmargin

transport. The petrofacies of the sediment drape on the continental

slope may be quite unlike that of the trench; the slope petrofacies

is thought to be more heterogeneous in distribution, more provincial

and unmixed. The provenance of continental slope sediments may be

very local, including material reworked from previously accreted

trench sequences, f'orearc basins, or relict shelf deposits.

. We propose that the diagenesis of trench sediments will

accelerate once they are acereted to the lower continental slope. 161

Diagenesi.s of the acoretionary complex is stimulated by (a) an

increase in the intensity of subduction deformation that initiates

folding and cleavage development, (b) an increase in subsurface

circulation associated with overpressured pore fluids, dewatering,

and fracture permeability, and (c) an increase in the effective overburden pressure due to untierthrusting and tectonic imbrication.

As diagenesis proceeds, many unstable framework grains will

degrade to poorly-structured, fine-grained aggregates that may be

irresolvable from the original depositional matrix.Much of the

petrologic signal that we see in modern trench sediments will likely

be obliterated by this process. The apparent diversity of the

assemblage will tend to decrease, and the petrofacies distribution

will tend to become more homogenized as unstable components are

removed. If metamorphism accompanies trench sediment accretion, for

example, an assemblage that was derived from metamorphic basement

may be indistinguishable with one that was derived from a contempo-

rary volcanic terrane but which subsequently recrystallized under

greenschist facies conditions. Careful attention must be paid to

relict depositional and recrystallization textures In order to deci-

pher the diagenetic and metamorphic history of a petrologic

assemblage. 162

CONCLUSIONS

Twenty seven cores and detailed seismic reflection and bathymetry surveys in selected areas of the Chile Trench, along more than 2,000 kilometers of the Andean continental margin, provide the most comprehensive data coverage of' a major trench system associated with a convergent plate boundary. These data are used to constrain sedimentary fades models (lithofacies and petrofacies), to interpret depositional processes, to model trench stratigraphic successions, and to determine source areas in the adjacent continental arc. The Chile Trench represents an end-member model of deep-sea sedimentation along an active continental margin that contrasts with submarine fan sedimentation, which is most purely developed in the continental rise environment of a passive margin. A major objective of this research is to provide criteria, based on the study of a modern trench system, by which ancient trench deposits may be recognized where they are uplifted and exposed on land, and to provide concepts by which the sedimentary fades and stratigraphy may be interpreted.

Several types of depositional bodies exist in the trench sheet flow basins, trench fans, axial channels, ponded basins -- which are integrated to form a coherent, along-margin, sediment dispersal system. Sheet flow basins are correlated with the occurrence of the inland passage between the Coastal and High Cordilleras

(the submergence of the forearc valley) and with the extension of the Pleistocene ice sheet onto the exposed continental shelf south 163 of 4l °S. Between 33°S and 11 O, numerous submarine canyons associated with major onshore river drainages cross the continental slope to provide point sources of sediment to the trench. Trench fans are built at the mouths of the canyon systems, and fan distributary channels become tributary to the axial channel which flows northward along the gravitational gradient. North of 33°S the gradient progressively flattens and the axial channel yields to ponded sheet flow deposition. The gravitational gradient is continually rnodif led by tectonic perterbations of the descending oceanic basement, causing the channel to erode in areas of increased gradient and to deposit in areas of decreased gradient.

The seaward development of the trench fans Is truncated by the axial channel which causes proximal-to-distal fades relationships to develop parallel to the margin, from the canyonmouth to the inter-canyon environment, rather than perpendicular to it (from the canyon-mouth to the abyssal plain, as on passive margin systems).

Several trench fans show evidence of severe erosion and dissection of their down-gradient lobes1 suggesting alternating periods of local fan deposition at the canyon mouth and periods when fan strata are eroded into the axial dispersal system and the proximal fan bodies are by-passed. Erosional episodes may be correlated with the encroachment of the axial channel into the distal inter-canyon environment, and with the active progradation of the axial dispersal system.

The lithofacies of the modern Chile Trench may be directly compared with those described in ancient submarine fan systems.

However, trench systems differ from submarine fan systems in (a) the 164 spatial distribution of lithofacies along the continental margin,

(b) the nature of the depositional bodies which mold lithof'acies relationships and associations, (a) the superposition of lithofacies through time to produce stratigraphic sequences, and (d) the mechanism by which the deposits are preserved in the rock record. The distinction of the trench environment results primarily from the presence of an axial gradient that redistributes sediment parallel to the cordilleran arc, and from the unique structural foundation that frames the strata between the opposed walls of converging lithospheric plates. In a. trench system, stratigraphic successions are produced by the migration of the very surface of the earth through a predictable sequence of sedimentary environments during plate convergence; on a passive fan system, such successions are produced by the migration of sedimentary environments over the surface of the earth.

The composition of the Chile Trench sands ranges from that characteristic of a dissected magmatic arc setting (derivation from exposed batholithic roots) to that of an undissected magmatic arc setting (derivation from supracrustal volcanic cover).Compositions typical of "dissected" maginatic arcs or plutonic source terranes rich in quartz and alkali feldspar, poor in volcanic lithics -- are found in the Central Chile and North Chile provinces, and in the sheet flow basins of the South Chile province, for a variety of reasons. In Central Chile, the plutonic nature of the provenance is correlated with an absence of Quaternary volcanism. Ashore from the sheet flow basins of South Chile, Pleistocene glaciation has carved deep into the batholithic roots of the magrnatic arc in spite of 165 active volcanism. In arid North Chile, the volcanic debris of the

High Cordillera is trapped in the longitudinal forearc basin, allow- irig an exagerrated contribution to the deep-sea from the crystalline rocks of the Coast Range.

Several trench petrofacies are defined which range in composition from the diagenetically immature -- a volcanic lit.hic/olivine/ pyroxene assemblage derived, from the Quaternary volcanism of southern Chile to the diagenetically mature -- a quartz/andalusite/garnet assemblage derived from the ancient preAndean terranes of northern Chile. The petrologic assemblage that is most diverse is found in a sheet flow basin at2°S, because onshore glaciation covered much of the Coastal and High Cordilleras at these southerly latitudes, subjecting a wide variety of source rocks to intense mechanical erosion. The petrologic assemblages that are most homogenous in geographic distribution are associated with major submarine canyon-subaerial river systems, because the petrologic composition of the drainage area is homogenized within large-volume turbidity currents, and petrofacies effectively transcend lithofacies.

The petrologic composition of trench sands should be a sensitive indicator of the contemporary configuration of the adjacent magmatic arc (its volcanism, geology, climate, and morphology) because the residence time of sediments in the undeformed trench basin is only a few hundred thousand years.Chemical weathering in the source area is apparently insignificant because olivine forms a major component of the heavy mineral fraction even where the climate is humid and the cordilleran relief is relatively low. However, hydraulic sorting in the depositional basin may modify the

assemblage by concentrating minerals of anomalous density or shape,

such as vessicular volcanic scoria, platy mica, and magnetite.

Diagenesis will progressively alter the unstable components and obscure much of the original signal; the petrofacies will tend to

become less diverse and more homogenous with time. 167

BIBLIOGRAPHY

Aguirre, L.,R. Charrier, J. Davivdson, A. Mpodozis, S. Rivano, R. Thiele, E. Tidy, M. Vergara, and J.C. Vicente, Andean maginatiarn: Its paleogeographic and structural setting in the central part (30°-35°S) of the southern Andes, Pacific Cool., 8, 1-38, 1974.

Aguirre, L., B. Levi, and R. Offler, Unconformities as mineralogical breaks in the burial netamorphisrn of the Andes, Contrib. Mineral. Petrol., 66, 361-366, 1978.

Aubouin, .3., A.V. Borrello, G. Cecloni, IL Charrier, P. Chotin, J. Frutos, R. Thiele, and J.C. Vicente, Esquisse paleogeographique et structurale des Andes meridionales, Rev. Geogr. Phys. et Geol. Dynarn., 15, 11-72, 1973..

Baba, J., and K.F. Scheidegger, Factors influencing the mineralogy of sands found in the rivers of Peru ana Chile, Geol. Soc. Amer. Bull., in press, 1983.

Bachnian, S.G,,, The Coastal Belt of the Franciscan: Youngest phase of northern California subduction, in Trench-Foreare Geology: Sedimentation and Tectonics on Modern and Ancient Active Plate Margins, edited by J.K. Leggett, pp. 401-418, Geol. Soc. London, Sp. Pub. 10, 1982.

Bachinan, S.G., and J.K. Leggett, Petrology of Middle America trench and trench slope sands, Guerrero margin, Nexico, mit. Rep. Deep Sea Drill. Proj., 66,1429, 1982.

Banerjee, I., Experimental study on the effect of deceleration on the vertical sequence of sedimentary structures in silty sediments, J. Sed. Petrol.,, 47, 771-783, 1977.

Barazangi, M., and B.L. Isacks, Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America, Geology, 4, 686-692, 1976.

Darron, J..A., and C. Keller, 1982, Widespread Miocene deep-sea hiatuses: Coincidence with periods of global cooling, Geology, 10, 577-581, 1982.

Berg, K., C. Breitkreuz, K.-. Damm, 3. Pichowiak, and W. Zeil, 1983, The north Chilean Coast Range--an example for the development of an active continental margin, Geol. Rundsehau, 72, 715-731, 1983.

Berger, W.1-I., Deep-sea sedimentation, in The Geology of Continental Margins, edited by C.A. Burk and C.L. Drake, pp. 213-141, Springer-Verlag, New York, 1974. 168

Boggs, S.,1981$, Quaternary sedimentation in the Japan are-trench system, Geol. Soc. Amer. Bull., 95, 669-685, 19814.

Boothroyd, J.C., Braided streams and alluvial fans, in Terrigenous Clastic Depositional Environments, Tech. Rept. 1l-CRD, edited by M.O. Hayes and T.W. Kana, pp. 17-26, Univ. S. Carolina, 1976.

Borg, 1.1., and D.K. Smith, Calculated X-ray Powder Patterns for Silicate Minerals, Geol. Soc. Amer. Hem. 122, 896 pp., Boulder, Cob., 1969.

Bouma, A.H., Sedixnentology of Some Flysch Deposits. A Graphic Approach to Facies Interpretation, 168 pp.,, Elsevier, Amsterdam, 1962.

Bouxna, A.}1., J.M. Coleman, et al.,, On the Mississippi Fan, Nature, 306, 736-737, 1983.

Caminos, R., C.A. Cingolani, F. Nerve, and E. Linares, Geochronology of the pre-Andean metamorphism and magmatism in the Andean cordillera between latitudes 3Q0 and 36° S, Earth-Sci. Rev., 18, 333-352, 1982.

Carson, B, Tectonically induced deformation of deep sea sediment off Washington and northern Oregon: Mechanical consolidation, Mar. Geol., 24, 289-307, 1977.

Chapple, W.M., and D.W. Forsyth, Earthquakes and bending of plates at trenches, J. Geophys. Res., 84, 7629-6749, 1979.

Charrier, R., and J.C. Vicente, Liminary and geosync].irial Andes: Major orogenic phases and synchronical evolutions of the Central and Magellan sectors of the Argentine-Chilean Andes, Solid Earth Problems Conf., Upper Mantle Proj., vol. 2, pp., 1451-470, Buenos Aires, 1972.

Chase, C.G. Plate kinematics: The Americas, East Africa, and the rest of the world, Earth Planet. 3d. Lett., 78, 355-368, 1978.

Cobbing, E.J., J.M. Ozard, and N.J. Snelling, Reconnaissance geochronology of the crystalline basement rooks of the Coastal Cordillera of southern Peru, Geol. Soc. Amer. Bull., 88, 2141-246, 1977.

Coira, B., J. Davidson, C. Mpodozis, and V. Ramos, Tectonic and magmatic evolution of the Andes of northern Argentina and Chile, Earth-Sci. Rev., 18, 303-332, 1982.

Couch, H., H. Whitsett, B. Huelm, and L. Briceno-Guarupe, Structures of the continental margin of Peru and Chile, Geol. Soc. Amer. Mem., 154, 703-728, 1981. 169

Dalmayrac, B., G. Laubacher, H. Marocco, C. Martinez, and P. Tomasi, La chaine hercynienne d'amerique du suci, structure et evolution d'un orogene intracratonique, Geol. Rundschau,69, 1-21, 1980. Daznm, K.-W., S. Pichowiak, and W. Zeil, The plutonism in the north Chilean Coast Range and its geodynaxnic significance, Geol. Rundschau,70, 10514-1076, 1981. Deruelle, B., Petrology of the Pilo-Quaterriary volcanism of the south-central and meridional Andes, J. Volcanol. Geotherm. Res.,17, 77-l24, 1982. Dickinson, W.R., Interpreting detrital modes of graywacke and. arkose, J. Sed. Petrol.,140, 695-707, 1970. Dickinson, W.R., Compositions of sandstones in circum-Pacific subduction complexes and forearc basins, Amer. Assoc. Petrol. Geol. Bull.,66, 121-137, 1982. Dickinson, W.Fl. and E.I. Rich, Petrologic intervals arid petrofacies in the Great Valley sequence, Sacramento Valley, California, Geol. Soc. Amer. Bull.,83, 3007-30214, 1972. Dickinson, W.R., and C.A. Suezek, Plate tectonics and sandstone compositions, Amer. Assoc. Petrol. Geol. Bull.,63, 216k2182, 1979. Dickinson, W.R., et al., Provenance of North American Phanerozoic sandstones in relation to tectonic setting, Geol. Soc. Amer. Bull., 9, 222-235, 1983. Dickinson, W.R., R.V. Ingersoll, D.S. Cowan, K.P. Helmold, and C.A. Suczek, Provenance of Franciscan graywackes in coastal California, Geol. Soc. Amer. Bull.,93, 95-107, 1982. Dickinson, W.R., and R. Valloni, Plate settings and provenance of sands in modern ocean basins, Geology,8, Ba-86, 1980. Drake, R.E., G. Curtis, and M. Vergara, Potassium-argon dating of igneous activity in the central Chilean Andes--latitude33°S, J. Volcano].. Geotherm. Res., !285-295, 1976. Drake, R.E., M. Vergara, F. Munizaga, and J.C. \Iicente, Geochronology of Mesozoic-Cenozoic magmatism in Central Chile, lat.31°-36°S,Earth-Sd. Rev.,18, 353-363, 1982. Einsele, C., H. Overbeck, H.IJ. Sohwarz, G. Unsold, Mass physical properties, sliding and erodibility of experimentally deposited and differently consolidated clayey muds, Sedimentology, 21, 339-372, 197'4. 170

Enkeboll, R.H., Petrology and provenance of sands and gravels from the Middle America Trench and trench slope, southwestern Mexico and Guatemala, mit. Rep. Deep Sea Drill. Proj., 66, 521-530, 1902.

Flint, R.F., Glacial and Quaternary Geology, 892 pp., John Wiley & Sons, New York, 1971.

Galli-Olivier, C., Climate A primary control of sedimentation in the Peru-Chile Trench, Geol. Soc. Amer. Bull., 80, l849-l852, 1969.

Galloway, W.E., Deposition arid diagenetic alteration of sandstone in northeast Pacific arc-related basins: Implications for graywacke genesis, Geol. Soc. Amer. Bull., 85, 379-390, 1974.

Gonzalez-Bonorino, F., and L. Aguirre, Metamorphic fades of the crystalline basement of Chile, Geol. Rundschau, 59, 979-994.

Gonzalez-Bonorino, F., Metamorphism of the crystalline basement of central Chile., J. Petrol., 12, 149-174, 1971.

Griggs, G.B., and L.D. Kulm, Sedimentation in Cascadia deep-sea channel, Geol. Soc. Amer. Bull., 81, l36l-l384, 1970.

Halpern, M., Geological significance of Rb-Sr isotopic data of northern Chile crystalline rocks of the Andean orogen between latitudes 23° and 27°S, Geol. Soc. Amer. Bull., 89, 522-532, 1 978.

Hem, F.J., and R.G. Walker, The Cambro-Ordovician Cap Enrage Formation, Quebec, Canada: Conglomeratic deposits of a braided submarine channel with terraces, Sedimentology, 29, 309-329, 1 982.

Herron, E.M., Chile Margin near lat 38°S: Evidence for a genetic relationship between continental and marine geologic features or a case of curious coincidences? Geol. Soc. Amer. Mem., 154, 755-760, 1981.

Herve, F.,F. Munizaga, E. Godoy, and L. Aguirre, Late Paleozoic K/Ar ages of blueschists from Pichilemu, central Chile, Earth Planet. Sd. Lett., 23, 261-264, 1974.

Herve, F., E. Godoy, M. del Campo, and J.M. Ojeda, Las metabasitas del basaxnento metamorfico de Chile Central y Austral, Actas 1st Congr. Geol. Chileno, Ft76-F187, Santiago, 1976.

Hesse, R., and S.K. Chough, The northwest Atlantic mid-ocean channel of the Labrador SeaDeposition of parallel laminated levee-muds from the viscous sublayer of low density turbidity currents, Sedimentolog, 27, 697-711, 1980. 171

Hulde, T.W.C., and G.F. Sharman, Fault structure of the descending plate and its influence on the subduotlon process (abstract). SOS, Trans. Amer. Geophys. Union, 59, 1182, 1978.

HsU, K.J., K. Kelts, and J.W. Valentine, Resedimented facies in Ventura Basin, California, and model of longitudinal transport of turbidity currents, Amer. Aasoc. Petrol. Geol. Bull., 6i, 1Q31-105t, 1980.

Hubbert, M.K., and W.W. Rubey, Role of fluid pressure in mechanics of overthrust faulting, Geol. Soc. Amer. Bull., 70, 115-166, 1959.

Imbrie, J., and T.H. van Andel, Vector analysis of heavy mineral data, Geol. Soc. Amer. Bull., 75, 1131-1156, 196l.

Imbrie, J. et al., The orbital theory of Pleistocene climate: Support from a revised chronology of the marine '0 record, in Milankovitch and Climate, Part I, edited by A.L. Berger et al., pp. 269-305, D. Reidel Pub. Co., Dordrecht, Holland, 19814.

Ingersoll, R.V., Petrofacies and provenance of late Mesozoic forearc basin, northern and central California, Amer. Assoc. Petrol. Geol. Bull., 67, 1125-11112, 1983.

Ingersoll, R.V., and C.A. Suczek, Petrology and provenance of Neogene sand from Nicobar and Bengal fans, DSDP sites 211 and 218, J. Sed. Petrol., 119, 1217-1228, 1979.

Isaacson, P.E., Evidence f or a western extracontinental land source during the Devonian period in the central Andes, Geol. Soc. Amer. Bull., 86, 39-46., 1975.

Karig, D.E., and G.F. Sharman, Subduction and accretion in trenches, Geol. Soc. Amer. Bull., 86, 377-389, 1975.

Kelleher, J.A., Rupture zones of large South American earthquakes and some predictions, J. Geophys. Res., 77, 2087-2102, 1972.

Klerkx, J..,S. Deutsch, H. Pithier, and W. Zeil, Strontium isotopic composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the Central and Southern Andes, J. Volcanol. Geotherm. Res., 2,149-71, 1977.

Kiovan, J.E., and J. Imbrie, An algorithm and FORTRAN-IV program for large-scale Q-mode factor analysis and calculation of factor scores, J. Math. Geology, 3, 61-67, 1971.

Kiovan, J.E., and A.T. Miesch, Extended CABFAC and QMODEL computer programs for Q-mode factor analysis of compositional data, Computers and Geosciences, 1, 161-178, 1976. 172

Komar, P.O., Hydraulic jumps in turbidity currents, Geol. Soc. Amer. Bull., 82, i477-iL88, 1971.

Komar, P.D.,, The hydraulic interpretation of turbidites from their grain sizes and sedimentary structures, Sedirnentology, in press, 1984.

Kuim, L.D., and T.M. Thornburg, Acoustic stratigraphy of the Peru-Chile Trench, 20° to t3°S latitude (abstract), Geol. Soc. Amer. Ann. Mtg., p. 537, 1982.

Kuim, L.D., R. von Heune, et al. (Eds.), Initial Reports of the Deep Sea Drilling Project, Vol. 18, 1077 pp., U.S. Government Printing Office, Washington, 1973.

Kuim, L.D., W.J. Schweller, A. Molina-Cruz, and V.J. Rosato, Lithologic evidence f or convergence of the Nazoa Plate with the South American continent, mit. Rep. Deep Sea Drill. Proj., 3A1, 795-801, 1976.

Kuim, L.D., W.J. Sehweller, and A. Masias, A preliminary analysis of' the subduction processes along the Andean continental margin, 6° to 35°S, in Island Arca, Deep Sea Trenches and Back-Arc

Basins, Maurice Ewing Series, Vol. I, edited by . Taiwani and W.C. Pitman, III, pp. 285-301, Amer. Geophys. Union, Washington, 1977.

Kuim, L..D., R.A. Prince, W. French, S. Johnson, and A. Mastas, Crustal structure and tectonics of the central Peru margin and trench, Geol. Soc. Amer. Mem., l54, '45-468, 1981.

Kuim, et al., ALVIN observations of the subduction zone off Oregon, in manuscript.

Lahsen, A., Upper Cenozoic volcanism and tectonism in the Andes of' northern Chile, Earth-Science Rev., 18, 285-302, 1982.

Laniz, R.V., R.E. Stevens, and M.B. Norman, Staining of plagioclase feldspar and other minerals, U.S. Geol. Surv. Pror. Paper 501B, pp. 152-153, 1964.

Ledbetter, M.T.., Fluctuations of Antarctic bottom water velocity in the Vema Channel during the last 160,000 year, Mar. Geol., 33, 71-89, 1979.

Leggett, J.K., The sedimentological evolution of a lower Paleozoic accretionary fore-arc in the Southern Uplands of Scotland, Sedimentology, 27, L101-U1T, 1980.

Levi, B.., Burial metamorhpic episodes in the Andean geosyncline, Central Chile, Geol. Rundschau, 59, 99-1O13, 1970. 173

Levi, B., Eastward shift of Mesozoic and early Tertiary volcanic centers in the Coast Range of Central Chile, Geol. Soc. Amer. Bull., 81!, 3901-3910, 1973.

Lomnitz, C., On Andean structure, J. Geophys. Res., 67, 351-363, 1962.

Lonsdale, P., Abyssal circulation of the southeastern Pacific and some geological implicatons, J. Geophy. Res., 81, 1163-1176, 1976.

Lopez, C., Petrography and diagenesis of sands and sandstones, Deep Sea Drilling Project Leg 66, mit. Rep. Deep Sea Drill. Proj., 66, 505-521, 1982.

Lopez-Escobar, L., F.A. Frey, and M. Vergara, Andesites and high-alumina basalts from the Central-South Chile High Andes: Geochemical evidence bearing on their petrogenesis, Contrib. Mineral. Petrol., 63, 199-228, 1977.

Lopez-Escobar, L., M. Vergara, and F.A. Frey, Petrology and geochemistry of lavas from Antuco volcano, a basaltic volcano of the southern Andes (37025 'S), J. Volcanol. Geotherm. Res., 11, 329-352, 1981.

Lowe, D.R., Grain flow and grain flow deposits, J. Seth Petrol., '46, 118-199, 1976.

Mack, G.H., Exceptions to the relationship between plate tectonics and sandstone composition, J. Sed. Petrol., 5'4, 212-220, 198'4.

MacKinnon, T.C., and D.G. Howell, Turbidite facies in an ancient aubduction complex--the Torlesse terrane of New Zealand, Geo-Marine Letters, 3, 211-216, 198'l.

Mammerickx, J., and S.M. Smith, Bathymetry of the southeast Pacifo, Geol. Soc. Amer. Map and Chart Series MC-26, Boulder, Cob., 1978.

Maxnmerickx, J., E. Herron, and L. Dorman, Evidence for two fossil spreading ridges in the southeast Pacific, Geol. Soc. Amer. Bull., 91, 263-271, 1980.

Mansfield, C..F., Stratigraphic variations in sandstone petrology of the Great Valley Sequence in the southern Coast Ranges west of Coalinga, California (abstract), Geol. Soc. Amer. Abstr. with Frog., 3,157, 1971.

Maynard, J..B., R. Valboni, and H.-S. Yu, Composition of modern deep-sea sands from arc-related basins, in Trench-Forearc Geology: Sedimentation and Tectonics on Modern and Ancient Active Plate Margins, edited by J.K. Leggett, pp. 551-562, Geol. Soc. London, Sp. Pub. 10, 1982. 174

McBride, S.L.,, J.C. Caelles, A.H. Clark, E. Farrar, Paleozoic radiometric age provinces in the Andean basement, latitudes 25-30°S, Earth Planet. Sd. Lett., 29, 373-383, 1976.

McKerrow, W.S., and L.R.M. Cocks, A lower Paleozoic trench-fill sequence, New World Island, Newfoundland, Geol. Soc. Amer. Bull., 89, 1121-1132, 1978.

MoMillen, K..J., and T,R. Haines, Late Quaternary sediments of the southern Mexico margin, mit. Rep. Deep Sea Drill. Proj., 66, 1I37-4I3, 1982.

MeMillen, K.J.,,R.}1. Enkeboll, .J.C. Moore, T.H. Shipley, and J.W. Ladd, Sedimentation in different tectonic environments of the Middle America Trench, southern Mexico and Guatemala, in Trench-Forearo Geology: Sedimentation and Tectonics on Moder and Ancient Active Plate Margins, edited by J.K. Leggett, pp. 107-120, Geol. Soc. London, Sp. Pub. 10, 1982.

McNutt, R.H., J.H. Crocket, A.H. Clark, J.C. Caelles, E. Farrar, S.J. Haynes, and M. Zentilli, Initial 87Sr/86Sr ratios of plutonic and volcanic rocks of the Central Andes between latitudes 26° and 29°S, Earth Planet. Sd. Lett., 27, 305-313, 1975.

Middleton, G.V., and M.A. Hampton, Subaqueous sediment transport and deposition by sediment gravity flows, in Marine Sediment Transport and Environmental Management, edited by D.J. Stanley and D.J.P. Swift, pp. 198-218, John Wiley & Sons, New York, 1976.

Miller, H., 1970, Das problem des hypothetischen "pazifisehen kontinents" gesehen von der chilenlschen PazifikkUuste, Geol. Rundschau, 59, 927-938, 1970.

Minster, J.B., and T.H. Jordan, Present-day plate motions, .1. Geophys. Res., 83, 5331-53514, 1978.

Moore, G.F., Petrography of subduction zone sandstones from Nias Island, Indonesia, J. Sed. Petrol.,49, 71-814, 1979.

Moore, G.F., H.G. Biliman, P.E. Hehanussa, and D.E. Karig, Sedimentology and paleobathymetry of Neogene trench-slope deposits, Nias Island, Indonesia, J. Geol., 88, 161-180, 1980.

Moore, J.C., Complex deformation of Cretaceous trench deposits, southwestern Alaska, Geol. Soc. Amer. Bull., 8'4, 2005-2020, 1973.

Moore, J.C., Selective subduction, Geology, 3, 530532, 1975. 175

Moore, J.C., and A. Allwardt, Progressive deformation of a Tertiary trench slope, Kodiak Islands, Alaska, J. Geophys. Res., 85, l711-1I756, 1980.

Moore, J.C., B. Biju-Duval, et al., 1982, Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A, Geol. Soc. Amer. Bull., 93, 1065-1077, 1982.

Mordojovich, K., Geology of a part of the Pacific margin of Chile, in The Geology of Continental Margins, edited by C.A. Burk and C.L. Drake, pp. 591-598, Springer-Verlag, New York, 19711.

Mortimer, C., and N. Sane, Cenozoic studies in northernmost Chile, Geol. Rundsehau, 6I, 395-1120, 1975.

Mufoz Cristi, J., Chile, Geol. Soc. Amer. Mem., 65, 191-2114, 1956.

Mufioz Cristi,3., Geologia de Chile, 209 pp., Editorial Andres Bello, Santiago, 1973.

Mutti, E., Distinctive thin-bedded turbidite facies and related depositional environments in the Eocene Heeho Group, Sedimentology, 214,107-131., 1977.

Mutti, E., and F. Ricci Lucchi, Le torbiditi dell'Appennino settentrionale: Introduzione all'analisi di facies, Metn. Soc. Geol. Ital. 11, 161-199, 1972 (Engi. Transi., lift. Geol. Rev., 20, 125-165, 1978).

Nelson, C.H., and L.D. Kulm, Submarine fans and deep-sea channels, in Turbidites and Deep Water Sedimentation, edited by G.V. Middleton and A.H. Bouma, pp. 39-78, Short Course Notes, Soc. Econ. Paleont. Mm. Pacific Section, Los Angeles, 1973.

Ness, G.E., and L.D. Kulm, Origin and development of Surveyor Deep-Sea Channel, Geol. Soc. Amer. Bull., 8U, 333933511, 1973.

Nilsen, T.H., and G.G. Zuffa, The ChugacliTerrane, a Cretaceous trench-fill deposit, southern Alaska,in Trench-Forearc Geology: Sedimentation and Tectonicson Modern and Ancien Active Plate Margins, edited by J.K. Geol. Soc. London, Sp. Pub. 10, 1982.

Normark, W.R., Growth patterns of deep-sea fans, Amer. Assoc. Petrol. Geol. Bull., 514, 2170-2195, 1970.

Normark, W.R., E. Mut.ti, and A.H. Bouma, Problems in turbidite research; A need for COMFAN, Geo-Marine Letters, 3, 53-56, 19814. 176

Ocola, L.D., R.P. Meyer, and L.T. Aldrich, Gross crustal structure under Peru-Bolivia Altiplano, Earthquake Notes, 142,33-148, 1971.

Palacios M., C., and R. Oyarzun M., Relationship between depth to Benioff zone and K and Sr concentrations in volcanic rocks of Chile., Geology, 3, 595-596, 1975.

Paskoff, H.P., Quaternary of Chile: The state of research, Quat. Res., 8, 2-31, 1977.

Pescatore, T., The Irpinids: A mode]. of tectonically controlled fan and base-of-slope sedimentation in southern Italy, in Sedimentation in Submarine Canyons, Fans and Trenches, edited by D.J. Stanley and G. Kelling, pp. 325-339, Dowden, Hutchinson & Ross, Stroudsburg, Penn., 1978.

Pithier, H., and W. Zeil, 1972, The Cenozoic rhyolite-andesite association of the Chilean Andes, Bull. Volcanology, 35, 11214-1452, 1972.

Piper, D.J.W., Turbidite muds and silts on deep-sea fans and abyssal plains, in Sedimentation in Submarine Canyons, Fans and Trenches, edited by D.J. Stanley and G. Kelling, pp. 163-176, Dowden, Hutchinson & Ross, Stroudsburg, Penn., 1978.

Piper, D.J.W,, H. von Huene, and JR. Duncan, Late Quaternary sedimentation in the active eastern Aleutian Trench, Geology, 1, 19-22, 1973.

Pitman, W.C., III, ILL. Larson, and EM. Herron, Magnetic lineations of the oceans, Geol. Soc. Amer. Map, Boulder, Cole., 19714.

Prasad, S., and R. Hesse, Provenance of detrital sediments from the Middle America Trench transect off Guatemala, Deep Sea Drilling Project Leg 67, mit. Rep. Deep Sea Drill. Proj., 67, 507-513, 1982.

Prince, R.A., and L.D. Kuhn, Crustal rupture and the initiation of imbricate thrusting in the Peru-Chile Trench, Geol. Soc. Amer. Bull., 86, 1639-1653, 1975.

Prince, R.A., et al., Part I, Bathymetry of the Peru-Chile continental margin and trench, Geol. Soc. Amer. Map and Chart Series MC-314, Boulder, Cob., 1980.

Ricci Lucehi, F., and E. Valmori, Basin-wide turbidites in a Miocene, over-supplied deep-sea plain: A geometrical analysis, Sedimentology, 27, 2141-270, 1980.

Huiz F., C. and J. Corvalan D., Mapa Geologico de Chile, scale 1:1,000,000, Instituto de Investigaciones Geologicas, Santiago, 1968. 177

Sanders, J.E., Primary sedimentary structures formed by turbidity currents and related resedimentation mechanisms, Soc. Econ. Paleontol. Mineral. Sp. Pub. 12, 192-329, 1965.

Scheidegger, K.F., L.D. Kuim, and E.J. Runge, Sediment sources and dispersal patterns of Oregon continental shelf sands, J. Sediment. Petrol., 141,1112-1120, 1971.

Scheidegger, K.F., L.D. Kuim, and D.J.W. Piper, Heavy mineralogy of unconsolidated sands in Northeastern Pacific sediments: Leg 18, Deep Sea Drilling Project, mit. Rep. Deep Sea Drill. Proj., 18, 877-888, 1973.

Scholl, D.W., aid M.S. Marlow, Sedimentary sequences in modern Pacific trenches and the deformed circum-Pacific eugeosyncline, Soc. Econ. Paleontol. Mineral. Sp. Pub. 19, 193-211, 19714.

Scholl, D.W., MN. Christensen, R. von Huene, and M.S. Marlow, Peru-Chile trench sediments and sea-floor spreading, Geol. Soc. Amer. Bull., 81, 1339-1360, 1970.

Scholl, D.W., R. von Huene, T.L. Val].ier and D.G. Howell, Sedimentary masses and concepts about tectonic processes at underthrust ocean margins, Geology, 8, 5614-568, 1980.

Schweller, W.J., and L.D. Kuhn, Depositional patterns and channelized sedimentation in active eastern Pacific trenches, in Sedimentation in Submarine Canyons, Fans and Trenches, edited by D.J. Stanley and G. Kelling, pp. 311-3214, Dowden, Hutchinson & Ross, Stroudsburg, Pennsylvania, 1978.

Schweller, W.J., L.D. Kulm, and R.A. Prince, Tectonics, structure, and sedimentary framework of the Peru-Chile Trench, Geol. Soc. Amer. Mern.1514, 323-350, 1981.

Seely, D.R., P.R. Vail, and G.G. Walton, Trench slope model, in The Geology of Continental Margins, edited by C.A. Burk and C.L. Drake, pp. 2149-260, Springer-Verlag, New York, 19714.

Shanmugam, G., and R.J. Moiola, Eustatuc control of turbidites and winnowed turbidites, Geology, 10, 231-235, 1982.

Simkin, T., L. Siebert, L. McClelland, D. Bridge, C. Newhall, and J.H. Latter, Volcanoes of the World, Smithsonian Inst., 232 p., Hutchinson Ross Pubi. Co., Stroudsburg, Penn., 1981.

Stanley, D.J., and A. Maldonado, Depositional models f or fine-grairied sediment in the western Hellenic Trench, Eastern Mediterranean, Sedimentology, 28, 273-290, 1981.

Stanley, D.J., H.D. Palmer, and R.F. Dill, Coarse sediment transport by mass flow and turbidity current processes and downslope 178

transformations in Anriot Sandstone canyon-fan valley systems, in Sedimentation in Submarine Canyons, Fans and Trenches, edited by D.J. Stanley and G. Kelling, pp. 85-115, Dowden, Hutchinaon & Ross, Stroudsburg, Penn., 1978.

Stauder, W., Mechanism and spatial distribution of Chilean earthquakes with relation to subduction of the oceanic plate, J. Geophys. Res., 78, 5033-5061, 1973.

Stevenson, A.J., D.W. Scholl, and T.L. Valuer, Tectonic and geologic implications of the Zodiac Fan, Aleutian Abyssal Plain, northeast Pacific, Geol. Soc. Amer. Bull., 9, 25927j, 1983.

Surlyk, F., and J.M. Hurst, Evolution of the early Paleozoic deep-water basin of north Greenland--Aulacogen or narrow ocean? Geology, 11, 77-81, 1983.

Suttner, L.J., Sedimentary petrographic provinces: An evaluation, Soc. Econ. Paleontol. Mineral. Sp. Pub. 21, 75_814, 197k.

Thorpe, R.S.., and P.W. Francis, Variations in Andean andesite compositions and their petrogenetic significance, Tectonophysics, 57, 53-70, 1979.

Underwood, M.8., and D.E. Karig, Role of submarine canyons in trench and trench-slope sedimentation, Geology, 8, 1l32436, 1980.

Underwood, M.B., 5.8. Bachman, and W.J. Schweller, Sedimentary processes and fades associations within trench and trench-slope settings, in Quaternary Depositional Environments on the Pacific Continental Margin, edited by 4.E. Field, A.H. Bouma, and I. Colburn, pp. 211-229, Soc. Econ. Paleont. Mm. Pacific Section, Los Angeles, 1980.

Valloni, R., and Maynard, J.B.,, Detrital modes of recent deep-sea sands and their relation to tectonic setting: A first approximation, Sedimentology, 28, 75-83, 1981. van Andel, T.H., Reflections on the interpretation of heavy mineral analyses, J. Sed. Petrol., 29, 153-163, 1959. van Andel, T.H., G.R. Heath, and T.C. Moore, Cenozoic History and Paleoceanography of the Central Equatorial Pacific Ocean, Geol. Soc. Amer. Hem. 1143,13I pp., Boulder, Cob.., 1975.

Veeh, H.H., W.C. Burnett, and A. Soutar, Contemporary phosphorite on the continental margin off Peru, Science, 181, 8145-8147, 1973.

Vergara, M., and F. Munizaga, Age and evolution of the Upper Cenozoic andesitic volcanism in central-south ChIle, Geol. Soc. Amer. Bull., 85, 603-606, 19714. 179 von Huene, R., Modern trench sediments, in The Geology of Continental Margins, edited by C.A. Burk and CL. Drake, pp. 207-211, Springer-Verlag, New York, 197'l. von Huene, H., Structural diversity along modern convergent margins and the role of overpressured pore fluids in subduction zones, Bull. Soc. Geol. France, 26, 11-23, 198'L. von Huene, H., J. Aubouin, et al., Leg 67: The Deep Sea Drilling Project Mid-America Trench transect off Guatemala, Geol. Soc. Amer. Bull., 91, 121-'32, 1980.

Walker, R.G., Deep-water sandstone fades and ancient submarine fans: Models of exploration for stratigraphic traps, Amer. Assoc. Petrol. Geol. Bull., 62, 932-966, 1978.

Watkins, et al., Accretion, underplating, subduction, and tectonic evolution, Middle America Trench, Southern Mexico: Results from DSDP Leg 66, Oceanologica Acta, 1!, 213-22k!, 1981.

Yerino, L.N., and J.B. Maynard, Petrography of modern marine sands from the Peru-Chile trench and adjacent areas, Sedimentology, 31, 83-89, 198k!.

Zeil, W., Geologie von Chile, 233 pp.. Gebruder Borntraeger, Berlin, 1961!.

Zeil, N., The Andes: A Geological Revue, 260 pp., Beitrage zur regionaleri geologie der erde, v. 13, Borntraeger, Berlin, 1979. APPENDIX FACTOR ANALYSIS CF CHILE TRENCH HEAVY AND LIGHT MINERALS-CONST. IIEANS 200Z NCR

RAW DATA

10 NO. SAMPLENAME 1 2 3 4 5 6 7 8 9 VLC-RX OPAQUE OLIVIN CLI-PA HYPERS EN$TAT CPIN-H5 8CR-HO PIT-AMP lIST 6.000 11.800 3.0 1 24.007 8.000 8.000 0.300 3.500 0.800 0.300 24.900 2 22.094 10.000 21.000 0.000 8000 10.900 1.600 17.700 7.100 7.100 0.7 3 01.035 8.000 22.000 1.300 15.800 19.600 2.500 10.800 4,600 4.000 2.3 4 01.242 12.000 11.000 0.300 19.600 21.100 1.800 11,100 7.700 4.600 2.8 S El.573 0.000 15.000 1.200 5.000 2.400 1.000 17.200 12.000 12.900 4.1 6 02.159 17.000 16.000. 1.200 6.100 4.100 0.000 17.800 11.000 8.800 . 1.0 5. 2 7 03.010 10.000 29.000 0.700 6.300 3.700 0.900 15.700 10.000 500 2. 8 03.415 17.000 29.000 0,300 6.000 2.500 1.000 13.000 7.000 6.300 1.2 9 f3.013 11.000 24.000 1.000 3.800 2.900 1.300 17.500 11.200 10.300 .0.6 9.800 1.000 1.800 9.3 1.0 20.294 6,000 38.000 1.000 9.300 25.000 1.700 1.300 3.3 11 ft.263 22.000 29.000 3.000 23.400 15.400 1.100 4.500 1.300 3.300 2.000 1.000 3.8 1.2 73.097 22.000 27.000 4.600 15.600 13.500 1.500 0.900 0.4 13 17.015 26,000 8.000 26.500 15.300 8.000 1.100 4.100 1.900 0.600 0.0 14 17310 26.000 9.000 26.700 19.000 9.800 2.400 5.600 1.200 0.400 0.000 0.0 15 29.139 32.000 7.000 30.300 21.400 6.300 2.000 1.000 0,400 0.2 16 19.313 30.000 11.000 27.000 17.700 9,500 3.800 4.400 2,000 0.900 0.0 17 18.067 27.000 7.000 33.400 16.200 6.600 3.500 4.200 0.600 0.400 0.0 19 18.392 40.000 6.000 27.500 14.300 6.300 3.500 1.500 0.200 0. 19 15.116 19.000 11.000 22.500 15.500 16.100 3.300 6.100 2.400 0.700 2 9.100 3.300 1.900 0,2 20 1.5.392 23.000 9.000 15.200 13.800 14.000 4.900 22 14.255 16.000 15.000 28.600 16.300 14.000 2.500 9.700 2.900 1.000 0.4 22 13.026 17.000 10.000 14.500 1.4.300 1.8.400 3.300 11.000 3.000 2.600 0.3 1. 0.2 23 13.112 13.000 8.000 12.500 1.3.000 19.400 4.700 15.200 5.500 500 1.900 0.4 24 16.091 18.000 11.000 13.200 14.100 lB.000 3.200 8.700 4.300 1.400 0.2 25 16.148 18.000 13.000 10.200 13.400 17.900 2.000 10.200 2.400

. 5 28 11.153 20.000 11.000 13.300 14.700 13.100 4.300 9.300 6.800 2.300 0. 16. 1.0 27 21.345 26.000 5.000 12.500 20.700 12.700 5.500 10.700 400 5.700 0.400 7.6 28 12.373 21.000 17.000 13.500 11.000 B.500 2.300 10.100 2800 2.700 29 08.190 21.000 12.000 9.000 13.800 2.900 4.000 22.200 2.100 4.8 1.7 30 07.217 7.000 10.000 13.500 19.300 12.400 4.600 16.100 9.000 1.400 0.8 31 04.202 10.000 9.000 9.100 14.200 8.000 3,500 19.500 11.800 4.800 4.. 32 10.091 24.000 14,000 7.900 12.300 4,400 3.100 17.500 1.700 0,600 0.6 33 05.131 7.000 12.000 1.100 21.200 9.200 1.400 20.600 19.200 6.100 1,000 14.000 11.300 2.100 0.8 34 05.311 5.000 26.000 . 9.200 13.200 13.600 181

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