Stratigraphy and Structure of the Mendenhall Gneiss Rough Dr

Stratigraphy and Structure of the Mendenhall Gneiss, South-Central San Gabriel Mountains, California

By Jeffrey T. DeLand

Undergraduate Senior Thesis presented to the faculty of the Geological Sciences Deparetment, California State Polytechnic University, Pomona, California

May 22, 2003

Abstract

STRATIGRAPHY AND STRUCTURE OF THE MENDENHALL GNEISS

By Jeffrey T. DeLand Department of Geological Sciences California State Polytechnic University, Pomona

An undergraduate thesis presented on the Stratigraphy and Structure of the Mendenhall Gneiss including structural and lithologic data colleted in remote regions of the Angeles National Forest, southern California. Geologic field mapping of basement rocks in the south central San Gabnel Mountains has yielded data enabling correlation of the largest continuous section of metamorphic rock with displaced rock bodies located on opposing sides of strike-slip fault zones. Palinspastic reconstructions provide insights that can be used for correlation with mountain ranges such as the Chocolate and Orocopia Mountains located more than 200 km south, and on the north side of the San Andreas Fault. New data supports tighter constraints on piercing points along the San Gabriel Fault with 218 ± 3 Ma Triassic Mt. Lowe intrusive rocks in contact with 1700 Ma Paleoproterozoic Mendenhall Gneiss. Large and small scale structural features provide information which can be used to aid in reconstruction of late Cenozoic brittle deformation events, as well as help in the determination of which land mass (Antarctica, Australia, Siberia or China) rifted from the western Laurentian margin during the 750 Ma break up of Rodinia.

ii TABLE OF CONTENTS

Abstract……………………………………………………………………………………..ii

List of Figures………………………………………………………………………………iv

Introduction…………………………………………………………………………………1

Previous Studies……………………………………………………………………………5

Rock Units and Field Relations Paleoproterozoic Fine Grained Banded Gneiss………………………………………9 Paleoproterozoic Granite Augen Gneiss ...... 15 (Mz?) or (pC?) Foliated Diorite or Hornblende Diorite...... 17 Triassic Plutonic and Intrusive Rocks...... 19 Cretaceous Plutonic and Intrusive Rocks...... 22

Structural Analysis Plot Sets 1-8…………………………………………………………………...26-32 Methods…………………………………………………………………………...24 Foliation…………………………………………………………………………..33 Lineation……………………………………………………………………….....34 Faults……………………………………………………………………………...35

Regional Implications / Interpretations / Significance……………………………...…….36

Conclusions……………………………………………………………………………….47

Future Studies……………………………………………………………………………..49

Acknowledgments………………………………………………………………………...50

Bibliography…………………………………………………………………………...... 51

Plates Plate 1 - Geologic Map of the Study Area………………………………………..52 Plate 2 – Cross Section A-A’…………………………………………………..…53 Plate 3 – Cross Section B-B’……………………………………………………..54 Plate 4 – Cross Section C-C’……………………………………………………..55 Plate 5 – Cross Section D-D’……………………………………………………..56

Appendices Structural Data…………………………………………………………………57-58

iii LIST OF FIGURES

1. Geologic Map of the San Gabriel Mountains…………………………………………….3 2. Location Map of the Eastern Transverse Ranges showing Study Area ...... 3 3. Simplified Geologic Map of Southern California showing Study Area...... 4 4. Metamorphic Facies Diagram showing Type Metamorphism...... 6 5. Index Map of Central San Gabriel Mountains showing Study Area...... 7 6. Photograph of Mendenhall Gneiss showing texture...... 8 7. Photograph of Mendenhall Gneiss showing foliation and fault...... 10 8. Photograph of Mendenhall Gneiss showing folding...... 11 9. Photograph of Mendenhall Gneiss showing folding...... 12 10. Geologic Map showing localities of Mendenhall Gneiss………………………………13 11. Photograph of granite augen gneiss showing foliation…………………………………16 12. Photograph of gneissic diorite showing cross cutting relationships ...... 17 13. Photograph of gneissic diorite xenolilth in Cretaceous Granite...... l8 14. Photograph of Mt. Lowe intrusive rocks with diorite dike……………………………..19 15. Photograph of gneissic diorite in Mt. Lowe…………………………………………….20 16. Distribution of Dating Localities for Triassic Mt. Lowe...... 20 17. Concordia Diagram showing age of Josephine Mountain rocks...... 23 18. Stereographic Plot of Mendenhall Gneiss...... 33 19. Stereographic Plot showing metamorphic lineation...... 34 20. Middle Miocene palinspastic reconstruction of San Gabriel Mtns...... 39 22. Paleoproterozoic reconstruction of Laurentia (Siberia Connection) ...... 45 23. Neoproterozoic reconstruction of Laurentia (Cathaysia)...... 46 24. Diagram showing possible connection of China and Laurentia (Cathysia) ...... 47

iv I. Introduction

The Mendenhall Gneiss was first studied while early California geologists began mapping an adjacent anorthosite - gabbro - syenite intrusive sequence during early exploration of the western San Gabriel Mountains (Miller, 1934, 1946), but was not studied in any detail until Oakeshott (1958) described the type locality around

Mendenhall Peak (Barth, et al., 1995). This type locality has been the focus of many studies, (Barth, et al., 2001, see also figure 1) while other localities of the Mendenhall

Gneiss remain largely unexplored. The purpose of this project is to map the stratigraphy and structure of one of these locations on the north side of the San Gabriel Fault. Using

brunton compasses and topographic basemaps, data was taken on rock foliations and

lineations, fault orientations along with slip direction lineation when available, as well as

axial planar surfaces to construct a basement rock geologic map. The research area

includes the largest unfaulted, continuous section of Paleoproterozoic metamorphic rock

in the San Gabriel Mountains that can be compared and correlated with other areas.

Late Cenozoic strike slip faulting along the San Andreas, San Jacinto and San

Gabriel Faults have given way to the increasing study of structural relationships for the

purpose of correlation. Various lithologic units may offer piercing points along certain

faults which when palinspastic reconstructions are made show distinct patterns that can

be useful in determination of the amount of movement on lateral faults as well as

formulating the tectonic and/or depositional history of a region (Ehlig, 1981; Nourse,

2002, see figure 2). Detailed mapping of the San Gabriel Mountains basement terrain

may also offer evidence as to which present-day continent existed on the western

continental margin of Laurentia prior to Neoproterozoic rifting of the supercontinent

1 Rodinia. The San Gabriel Mountains offer a distinct challenge to the geologist in that

multiple rock units of varying ages may be associated along multiple fault systems, and

record many separate tectonic events.

The present-day San Gabriel Mountains are believed to have been formed by Late

Cenozoic thrust faulting along the south boundaries, for example the Sierra Madre and

Cucamonga fault systems. These reverse faulting events may have been caused by the

left lateral transform movement that offset the San Gabriel fault in the “Big Bend” region

(Ehlig, 1981). This type of scenario combines transform movement with compression of the adjacent tectonic plates to form a rotational structural environment known as transpression and transrotation. Subsequent erosion and brittle deformation of the basement terrain, along with 160 – 240 km of movement laterally by the San Andreas

Fault (Matti and Morton, 1993; Dillon and Ehlig, 1993; Powell, 1993), have shaped the crystalline rocks into what we see today.

Figure 1 shows a location map including the major lithologic units and faults in the study area. The study area is marked in a red and black rectangle, and it can be seen on the Chileo Flats USGS 7.5 minute topographic quadrangle. It is located along

Angeles Crest Highway (Highway 2) in the Angeles National Forest, just north of the San

Gabriel Fault in the central San Gabriel Mountains. Figure 2 shows the location of the

San Gabriel Mountains relative to the San Andreas Fault and San Bernardino Mountains; also possible correlative units are located in mountain ranges throughout Southern

California (see figure 3).

Figure 1 Geologic Map of the San Gabriel Mountains modified from Ehlig (1981) showing approximate location of study area. Inset Map shows terranes of the San Gabriel Mountains; abbreviations are SGT = San Gabriel terrane; CT = Cucamonga terrane; SAT San Antonio terrane; PS = Pelona Schist. Abbreviations for Cretaceous Plutons in San Gabriel terrane are JMI = Josephine Mountain intrusion; VMS = Vetter Mountain stock; WMB = Waterman Mountain/Mt. Wilson batholith.

Figure 2 Location map of the central and eastern Transverse Ranges of southern California, showing outcrops of the Mesoproterozoic San Gabriel anorthosite complex (pink) and the Mendenhall and Augustine gneisses (black). Study area marked in red rectangle. Taken from (Barth et al., 2001).

Chocolate Mountains schist. Modified from Ehlig (1993). - Orocopia - Simplified geologic map of Southern California showing present day location San Gabriel Mountains, major strike slip fault Figure 3 zones, as well the distribution of Pelona

4 II. Previous Studies

Many ambitious researchers have provided a vast amount of data on the

Mendenhall Gneiss itself, the other surrounding metamorphic rocks, local faults, and the intrusive rocks native to the study area. As for particular samples taken in the study area, at this time, the author is not aware of any analysis other than thin section investigation to have taken place with samples taken directly in the area of study. Due to the high relief of the mountains, poison oak abundance, seasonal fire danger, temperature extremes, scarcity of easily traveled roads, and dense vegetation mapping was mostly done on main roads, or fire roads. The only published geologic map is the USGS 1:250,000 Los

Angeles sheet mapped by Tom Dibblee (Dibblee, 1969) which does not have the small scale detail that is required for lithologic subdivision and correlation. In fact, on the LA sheet, in this particular area of study there is only one single unit colored brown which is labeled undifferentiated metamorphic rock. Perry Ehlig (1981) began to analyze the tectonic history having to do with the central and eastern San Gabriel Mountains in which he recognized the Precambrian gneiss-amphibolite complex located on the upper plate of the Vincent Thrust. He also characterized the rocks adjacent to the South Branch of the

San Gabriel Fault as well as mapping much of the high country in the eastern and central parts of the range (Nourse, 2002). In relation to this study, he also proposed a 22 km offset of the eastern contact of Mendenhall gneiss with the Mt. Lowe intrusion along the

San Gabriel Fault.

Leon Silver (Silver, et al., 1963; Silver, 1971) gathered information on the ages and correlation of basement units including the ca 1700 Ma Mendenhall Gneiss, the 1200

±15 Ma anorthosite – syenite - gabbro intrusive complex, which lies along strike of the

5 San Gabriel Fault from the study area, the 220 ± 10 Ma Mt. Lowe - Parker Mountain granodiorite, as well as the occurrences of augen gneisses using U/Pb dating techniques.

He began to correlate the crystalline rocks in the San Gabriel Mountains with rocks that lie 200 – 250 km south, and on the opposite side of the San Andreas Fault in the

Orocopia and Chocolate Mountains (Nourse, 2002; see also figures 2 and 3). Using isotopic, geochemical, and geochronological tools, other scientists including Andrew

Barth et al., (1995, 2001) were able to characterize the different petrologic identities of the Mt. Lowe intrusion as well as obtain a more precise age of 218 ± 3 Ma. Barth classified the metamorphic and plutonic history in the western San Gabriel Mountains, including pyroxene and feldspar solvus geothermometry in the Mendenhall Gneiss which yielded metamorphic crystallization temperatures and pressures that are 900° - 950°C, and 0.6 GPa, respectively, granulite facies metamorphism (Barth, et al., 2001; see also figure 4).

Figure 4 Metamorphic facies diagram showing temperatures and pressures of metamorphosed rocks. Black rectangle indicates type Mendenhall Gneiss granulite facies Metamorphism at 6kb and ~950C. Figure courtesy of Professor David Jessey; California Polytechnic University Pomona, Department of Geological Sciences.

s of; Ehlig (1958), Jennings and Strand (1969), Morton (1973), Bordugo Spittler (1986). Portion in Index map of the central San Gabriel Mountains, showing geographic features such as mountain peaks, major rivers, freeways, color coded is the general location of study area for this paper. Modified from Nourse (2002). Figure 5 topographic quadrangle locations, study area orange

Figure 6 A Mendenhall Gneiss exposure in Lady Bug Canyon polished by stream action, notice vertical dip and small scale compositional banding alternating from quartzofelspathic to biotite rich gneiss. Student sitting in photo is approximately 6 feet tall, 3 feet tall sitting.

8 This work showed that felsic gneisses with a probable detrital zircon component as old as1800 Ma and granite augen gneiss 1680 ± 40 Ma were metamorphosed during emplacement of the 1196 ± 5 Ma anorthosite – syenite – gabbro complex.

III. Rock Units and Field Relations

Many different types of rocks have been found in the area of study. The section below gives rock definitions, map classifications, field occurrence, regional significance, as well as a tectonic background and geologic history of each rock unit. Photographs and figures will be utilized whenever available.

Paleoproterozoic Fine Grained Banded Gneisses (Map units p? qfgn, p? gn, p? bgn , p? g +bgn, p? a, colored brownish red on Plates)

The oldest unit on the map is the Paleoproterozoic fine grained gneisses (p?gn, see figure 6) which range in composition from a light colored quartzofelspathic gneiss

(quartz and feldspar rich; p?qfgn), a biotite rich gneiss (p?bgn), to a dark colored fine grained foliated amphibolite (p?a). These rock units along with interlayered bodies of granitic augen gneiss are collectively called the Mendenhall Gneiss, or in some cases are referred to as the San Gabriel Gneiss (see figures 6, 7, 8). The Mendenhall Gneiss lies structurally on the upper plate of the Vincent Thrust and is the country rock for younger plutonic intrusions. It is characterized by millimeter to 20 centimeter scale banding throughout the unit, and relatively simple mineral assemblages. Heating that caused ductile deformation has resulted in tight to isoclinal folding (see figures 7 and 8), while outcrops have been compressed and shortened along with boudenage that stretches parallel to the foliation, and biotite rich units occasionally are garnet bearing (Barth, et

9 al., 2001; Nourse, 2002). Dating zircon cores from the type Mendenhall Gneiss (figure

10) give very discordant ages between ~1.7 and ~1.2 Ga (Barth, et al., 2001). This

discordancy is due to the 1.19 Ga intrusion of the nearby anorthosite – syenite – gabbro

assemblage and subsequent metamorphic events that led to an amphibolite facies

metamorphism. The extreme heat and low pressure re-sets or “zones” the zircons used for age dating such that 1.2 Ga rims surround older cores.

Figure 7 Photograph of Mendenhall Gneiss with right – lateral strike slip fault cutting the outcrop. Notice characteristic fine banding of the Mendenhall Gneiss, along with alternating light and dark layers. Rock hammer is approx 14 cm long.

In looking at a possible protolith for this rock, it is important to think about the

depositional, tectonic, and thermal environments that this rock unit has been subject to in

its 1.7 billion years. In Paleoproterozoic time, on the western edge of the continent

10 known as Rodinia, there was a depositional environment that was fairly complex. Before the right lateral strike slip motion of the San Andreas Fault carried these rocks some 250 km northward, they originated somewhere near the present – day Salton Sea. At that time and in that area, the collision of the Mojavia and Yavapai tectonic plates brought about the Ivanpah Orogeny that took place 1700 – 1710 million years ago (Barth, et al.,

2000; Wooden and Miller 1991). This orogeny created a vast amount of volcanic sediments from the fore – arc, as well as a large quantity of terrestrial sediments running off of the Laurentian Craton that collected in basins located to the southwest of the

Yavapai Province.

Figure 8 Photograph taken of the Mendenhall Gneiss showing fine banding as well as isoclinal folding. Outcrop dipping steeply into the page, photo taken looking down. Rock unit truncated in middle by right lateral fault. Pencil is approx 10 cm long.

Figure 9 Photograph of Mendenhall Gneiss showing inclusions of foliated diorite, isoclinal folding and fine scale banding associated with the unit. Notice how the diorite is also isoclinally folded. Pencil for scale is approximately 10cm.

Figure 10 Simplified geologic map of the Mendenhall Peak area, western San Gabriel Mountains, with data from P.L. Ehlig (unpublished mapping 1958), Carter (1980), and Barth, et al., (1995). Circles are localities sampled for geochronology, triangles and squares are mafic granulite and felsic gneiss localities, respectively, used for geothermometry (Barth, et al., 1995).

Although no sedimentary mineral assemblages are present in the outcrops studied, the wide scatter of zircon ages suggests a detrital origin for the Mendenhall Gneiss (Barth, et al., 2001). Any pre-existing depositional fabric was soon to be cooked as the heat of the orogenic belt itself gave the Mendenhall Gneiss its first thermal event and destroyed any relict bedding, sedimentary structures, if indeed it was a sedimentary protolith, and any igneous flow foliation it might have had if the protolith was volcanic. The upper intercept of the U/Pb dating of zircons is believed to indicate the date at which an igneous protolith was originally crystallized, at 1789 Ma (Barth, et al., 2001). Detritus shed from this protolith into a basin, possibly an aulacogen, which may have been metamorphosed during the late Paleoproterozoic, and then metamorphosed again at 1196 Ma during formation of the anorthosite – syenite – gabbro complex. Some discrepancy has risen concerning another granulite facies metamorphic event that occurred about 1440 million years ago which, according to Silver (1963), is the cause of the discordant age in zircons

13 dated from a granulite pegmatite (Ehlig, 1981). But as Barth states in his 2001 paper in the Journal of Geology, the lack of these ages on other types of rocks may suggest that these rocks were not formed on the edge of the Laurentian Craton, but rather were put there by some continental collision event and left behind following the breakup of

Rodinia.

The next thermal event was the emplacement of the anorthosite – syenite – gabbro complex at 1196 Ma that intruded adjacent to the rock body (Barth, et al., 1995,

2001). The areas that have been dated (see figure 10) radiometrically show the zircon dating clock reset due to the high temperatures that are experienced during metamorphism. Although nobody has tried dating samples taken from my study area, it seems that the gneisses in this research area are in a better condition for dating due to the fact that this area resides farther away from the heating event that occurred ~ 1.2 Ga, hence the rocks farther away from the thermal event show less deformation. No large bodies of this intrusive complex were mapped in the study area, although some dikes that have a composition and texture representative of this intrusion have been seen in some localities.

The area experienced a long period of solitude in which the rock unit exhibits no clues as to a thermal or tectonic event taking place until the emplacement of the Mt.

Lowe intrusive complex in late Triassic (Ehlig, 1981; Barth, et al., 1991). This intrusive event is followed by the intrusion of the Mt. Wilson and Mt. Waterman plutons in late

Cretaceous time, as well as the Josephine Mountain intrusion (Barth, et al., 1995). These events emplaced large amounts of plutonic rocks in the area, probably causing thermal overprint. Subsequent erosion and brittle deformation events displaced the rocks from

14 their origin and shape them into steep cliff faces, landslide debris, and the general alluvium around the study area.

Paleoproterozoic Augen Gneiss (Map unit pCagn, pCbagn, colored red on Plates)

Throughout the San Gabriel Mountains occurs a medium to coarse grained biotite augen gneiss. This rock is one of the most distinctive rocks in the San Gabriel

Mountains. Augen gneiss occurs as concordant intrusions which share the same foliation and trends as the fine grained gneisses. It is characterized by a coarse grained leucocratic type, as well as a more abundant darker – more biotite rich version. The leucocratic version is characteristic of having augens up to 8 cm long, of which make up around 60% of the total rock volume (Nourse, 2001). The more common, biotite rich version commonly contains 0.5 to 2 cm long augens composed of potassium feldspar.

Similarities between the coarse grained augen gneiss and the finer grained gneisses found in the study area relate thermal events that have recrystallized the rock mass as a whole.

Small scale isoclinal folding seen in outcrop and large map scale structures are continuous throughout both units as shown on the geologic map and cross sections (refer to Plates 1-5). Structural similarities shared between these two rock units may be attributed to synchronous post emplacement thermal events that have deformed the separate units simultaneously.

Dating zircons in the rock gives ages of 1670 +/- 20 Ma (Silver, 1971) in rocks taken from the San Gabriel Mountains, as well as north of the San Andreas Fault in the

Orocopia and Chocolate Mountains. A layer contained within the Mendenhall gneiss contain zircons which indicate un upper intercept age of 1679 +/- 22 Ma (Barth, et al.,

15 2001) in the zoned cores, while the outer rims indicate an age of 1172 +/- 22 Ma ( Barth,

et al., 2001) which corresponds with the intrusion of the anorthosite – syenite – gabbro

complex.

Figure 11 An outcrop of Paleoproterozoic augen gneiss along the West Fork San Gabriel River is admired by fellow students Shawn Wilkins, and Seth Brodie.

The distinctive

texture of this rock gives

some idea as to what it

may originally have been.

It appears that sometime

shortly after the

accumulation of the fine

grained Mendenhall

gneiss protoliths, an

intrusive event

predominantly composed

of porphyritic granite and/or porphyritic quartz monzonite occurred (Silver 1971). The dates given for such an event are revealed in the inner zones of the dated zircons from augen gneiss. The essentially porphyritic granitic rock was subsequently deformed along with the surrounding rocks in the sequence of heating and brittle deformation events. In the

16 Mendenhall Peak area this deformational fabric is intruded by the 1196 ± 5 Ma

anorthosite – syenite – gabbro complex.

Mz (?) or pC (?) gneissic or foliated diorite or quartz dirorite (Map units bhqd, bhdi, hdi, hbdi, colored purple on Plates)

Figure 12 Gneissic diorite in contact with Triassic Mt. Lowe intrusive rocks, and cross cutting granitic dikes. Rock hammer approx 14 cm long.

An enigmatic unit, of which not much is known, appears in great quantity in the study area (see figures 12 and 13). This unit is a foliated diorite, ranging from a highly foliated, biotite rich, almost schistose texture, to a coarse grained foliated hornblende diorite or quartz diorite. Age of this unit is not known, nor has it been dated or mapped in any significant quantity. Field relationships suggest that the emplacement of this unit occurred after the formation of the augen gneiss, but before the Triassic intrusive events, which leaves quite a bit of time for speculation. Xenoliths of this foliated diorite have been seen in outcrops of Triassic and Cretaceous intrusions which demonstrate a late

17 Triassic emplacement age. Although a roughly hornblende rich mafic intrusive series

accompanies the Triassic intrusive events (Nourse, 2002), field relations and samples

indicate no relation, while no subsequent geochemical studies have been done. General

weathering of this unit appears to be very high, and the look of the rock shows that it

does not seem to be in the same condition of younger units.

Figure 13 Photograph showing a gneissic diorite xenolith fully enclosed within a Cretaceous monzogranite. Rock hammer is approximately 14 cm long.

18 Triassic Plutonic and Intrusive Rocks

(Map units TRdi, TRpbqm, TRlbqm, TRhqmzd, colored light blue on Plates)

Early Mesozoic time offered a variety of thermal intrusive events in the study area. The most major of these incidents is referred to as the 218 ± 3 Ma Mt. Lowe intrusion which intrudes the area with the Mendenhall Gneiss as the country rocks. Many studies have been done on this rock unit for use in correlation with rocks of similar age and composition in the mountains that lie to the south, and on the opposite side of the San

Andreas Fault (Ehlig, 1981; Barth, et al., 1991; Nourse, 2002). The vertical contact at the north east edge of the study area between the Mt. Lowe intrusive rocks and the

Mendenhall Gneiss projects south east into the West Fork San Gabriel River (below

Cogswell Reservoir) and meets up with the San Gabriel Fault. Here is what Ehlig used as his classic piercing point on the north branch of the San Gabriel Fault.

Figure 14 Mt. Lowe (TRhqmzd) intrusive rock outcrop in lower Shortcut Canyon with a layer of Triassic diorite (TRdi) cutting across. Rock hammer is approx 14 cm long.

Figure 15 Xenolith of diorite (TRdi) in Mt. Lowe (TRhqmzd) intrusive rocks. Photo taken in lower Shortcut Canyon. Rock hammer is approximately 14 cm long.

Figure 16 Graphic shows distribution and radiometric data for Triassic plutonic and volcanic rocks in the Southwestern U.S. Solid circles are radiometrically dated localities with ages in Ma. All dates by U/Pb zircon geochronology. Inverted V symbols are outcrops of Triassic and Triassic (?) volcanic rocks. Double solid lines mark inferences to the independence dike swarm. Figure from Barth et al., 1990.

20 The Triassic arc (see figure 16) follows a roughly north-northwestern trend corresponding to the back arc volcanism and plutonism of the era (Barth, et al., 1991).

The Mt. Lowe intrusion forms a compositionally zoned pluton ranging in composition due to fractional crystallization, in which the pluton crystallized from the bottom up

(Ehlig, 1981). It is broken down into many different zones according to main mineral assemblages which are either present or lacking in certain localities. As a whole, the intrusion is characterized by high feldspar content, ranging from 60 – 95%, and on average, low quartz content, around 10% (Ehlig, 1981). The stratigraphy of this pluton

from the bottom – up consists of a medium grained hornblende diorite on the bottom,

grading to a coarser grained quartz diorite with hornblende phenocrysts in a

predominantly sodic feldspar matrix. Further upward the pluton zones into a coarser

“dalmation” phase with larger hornblende phenocrysts. Around 1.5 km above the base of

the pluton begins a zone in which the rock begins to bear garnets. These garnets appear

as crystals as much as 2 cm across as well as fill in cracks that appear to be mending

seams during residual fluid crystallization. The final stage of crystallation occurs in the

upper zone of the intrusion. It is characterized by conspicuous Orthoclase phenocrysts

that have been referred to as “pigeon eggs” due to their roughly egg-shape as well as

similarities in size (Ehlig, 1981).

Magma generation for the emplacement of these plutons is generally indicative of

being derived from the partial melt of pre-existing crustal rocks due to high silica content

in the rocks themselves (Barth, et al., 1995). Analysis of the Mt. Lowe Intrusion has yielded an isotopic signature which corresponds with a parent magma of roughly high- aluminum basaltic composition (Barth, et al., 1995). This data supports the conclusion

21 that these magmas were derived by a combination of partial melt of subducting balsaltic

crust, as well as hints at a high pressure partial melting event of the lower continental

crustal material (Barth, et al., 1995; Silver 1971).

Subsequent to emplacement of the pluton, these rocks have been subjected to

various thermal and deformation events. The general foliation of the pluton remains

somewhat parallel to the bottom, which indicates an igneous flow foliation (Ehlig, 1981).

The rock unit has been metamorphosed near upper amphibolite facies near granitic rocks

of the central San Gabriel Mountains, while also subjected to lower amphibolite facies

near the western part of the range near the Soledad Basin (Ehlig, 1981). These

metamorphic events have enhanced the original foliation as well as replaced some of the

hornblende by epidote in some localities. Radiometric dating techniques employed on

this rock unit have yielded ages of 220 ± 10 Ma using U/Pb zircon methods, as well as

208 ± 7 Ma using whole – rock methods (Ehlig, 1981; Silver, 1971).

Cretaceous Plutonic and Intrusive Rocks (Map units Kbmgr, Klbmgr, colored light green on Plates)

Intrusive events in the Late Mesozoic are characterized by granitic sills that extend into the study area. These intrusions project eastward from the Late Cretaceous

Josephine Mountain Intrusion (Barth, et al., 1995). They regularly occur as concordant

sills that intrude parallel to the local foliation in the metamorphic rocks of the

Mendenhall Gneiss, as well as the Mt. Lowe intrusive rocks. Small occurrences of the

Mt. Wilson batholith crop up as a diorite to quartz diorite found in the southern part of

the study area, south of the San Gabriel Fault. These rocks show very little to no

foliation in outcrop, and are usually in very good shape due to a good resistance of

weathering. More common in the study area are sills related to the Josephine Mountain

22 Intrusion. This rock suite is a mostly calc-alkaline series showing very little to no

foliation in outcrop. Truncation of the rock unit on the southern side of the study area limits study possibilities, but gives a potential piercing point along the San Gabriel Fault.

The rock units in the study area that correspond to this intrusion are a porphyritic biotite monzogranite (Kbmgr), as well as a more leucocratic biotite monzogranite (Klbmgr).

Granites from the intrusion of the Josephine Mountain complex yield U/Pb zircon ages of Late Cretaceous origin from 88 ± 3 Ma, to the discordant age of 78 ± 8 Ma

(Nourse 2002, Barth et al., 1995). Zircon analysis shows some discordance due to contamination from the gneisses in which it intrudes. These rocks also crystallized

Figure 17 Concordia diagram of zircon from a tonolite sample from the Josephine Mountain Intrusion Taken from Barth et al., 1995.

under oxidizing conditions, while it contains an assemblage of quartz, biotite, almandine

garnet, muscovite, and magnetite in the same sub assemblage (Barth, et al., 1995).

Cretaceous granitic rocks are believed to have come from partial melting of crustal rocks

that pre-existed in the area of formation. The fine grained equivalents of the phaneritic

23 rocks include basalt to basaltic andesite, to a high silica rhyolite (Barth, et al., 1995).

Differences in composition of adjacent plutons have been attributed to the mixing of the parent rocks with the magmas, along with fractional crystallization (Barth, et al., 1995).

Quaternary sediments (Map units Qal, Qcv, Qls, all colored yellow on Plates)

Brittle deformation, general uplift of the mountains, and exposure to the elements has caused deposition of various units marked on the map in yellow. Quaternary alluvium, map unit Qal, is restricted to active stream channels. Quaternary colluvium, map unit Qcv, is the result of the steep terrane of the San Gabriel Mountains, and occurs as cobble to boulder size angular fragments of rock bodies located on steep slopes. Rapid uplift of the area has caused landslides to occur, as well as formed alluvial terraces on the south side of the San Gabriel Fault. The landslides are marked as Qls, and occur as large block slides that displace conformable rock units, and alluvial terraces are denoted as

Qoa.

IV. Structural Analysis

Methods Structural analysis of the study area is accomplished using statistical

methods to identify trends in the various rock units, as well as the map area as a whole.

Stereographic projections can represent a 3-dimensional orientation in space, in 2 dimensions on paper, in this case using equal area stereonets, and a stereographic plotting computer program called GEOrient. The program allows many different types of plots to be analyzed different ways, as well as produces some good quality graphics. Cross sections are utilized for depicting what may be happening at depth, calculating unit thickness, and give visual representations of structural relationships (refer to Plates 1-5).

24 The following plot sets (1 through 8) represent the data taken and recorded during field investigation of the study area.

Plot Set 1

1 2

3 4

Stereographic projections of the biotite rich member of the Mendenhall Gneiss. (1) Planes to 83 foliations taken throughout the study area. (2) Poles to planes showing two best fit orientations related to the two clusters of poles, as well as fold axis. (3) Contour plot showing point density, best fit orientations and fold axis. (4) Contour, and density dot plot showing point density, best fit planes and fold axis. Best fit plane orientations (dotted lines); S74°E, 80° SW; N59°W, 72° NE. Trend and plunge of fold axis; (arrow) S69°E 31°.

Plot Set 2

1 2

3 4

Stereographic projections of the quartzofelspathic member of the Mendenhall Gneiss. (1) Planes to foliations of 116 data points taken throughout the field area. (2) Poles to foliations showing 2 best fit representative planes and trend and plunge of fold axis. (3) Contour plot color coded to show point density, showing 2 representative planes, and fold axis. (4) Contour and color coded dot plot showing point density, 2 best fit planes, and fold axis. Best fit planes (dotted lines); S58°E, 82° SW; N52°W, 82° NE. Trend and plunge of fold axis (arrow); S56°E 24°.

Plot Set 3

1 2

3 4

Stereographic projections of the amphibolite unit in the Mendenhall Gneiss. (1) Planes to 22 foliations, (2) poles to those 22 planes, two best fit planes and fold axis. (3) Contour plot colored to show point density. (4) Contour and dot density plot to show point density, two mean planes to foliation and fold axis. Best fit plane orientations (dotted lines); S65°E, 82° SW; N81°W, 77° NE. Trend and plunge of fold axis (arrow); N71°W 39°.

Plot Set 4

1 2

3 4

Stereographic nets plotted for foliations of the biotite augen gneiss. (1) Plot showing planes to 56 foliations, (2) Plot showing poles to same 56 foliations, best fit planes to the two pole clusters, and fold axis. (3) Contour plot showing color coded pole density, best fit planes and fold axis. (4) Contour and color coded dot density plot showing two best fit planes and fold axis. Best fit plane orientations (dotted lines); S65°E, 69° SW, N75°W, 83° NE. Trend and plunge of fold axis (arrow); N72°W 22°.

Plot Set 5

1 2

3 4

Stereographic plots of the foliated diorite unit exposed in the study area. (1) Planes to 16 metamorphic foliations of the foliated diorite unit, (2) poles to the 16 foliations taken, best fit planes and fold axis. (3) Contour plot showing color coded point density, best fit planes and fold axis. (4) Color coded contour dot plot showing point density, best fit planes and fold axis. Best fit plane orientations (dotted lines); S70°E, 50° SW; S82W°, 69° NW. Trend and plunge of fold axis (arrow); N90°W, 24°.

Plot Set 6

1 2

3 4

Stereographic plots representing the Mt. Lowe intrusive complex. (1) Planes to 52 metamorphic foliations taken in the study area, (2) Poles to those 52 planes, as well as a best fit mean plane. (3) Contour plot representing a color coded pole density with location of the best fit plane. (4) Color coded contour dot plot showing point density as well as best fit plane. Best fit fold axis plane orientation; S40°E, 47° SW.

Plot Set 7

1 2

3 4

Stereographic plots representing fault trends measured in the study area. (1) Planes to 30 fault orientations, (2) Poles to the same 30 measurements, as well as best fit planes from the 3 pole clusters, (3) Contour plot color coded to show density and 3 best fit planes, (4) Contour dot plot color coded to show density and 5 best fit planes, (5) Planes to faults showing lineation, triangles denote lineation. Best fit plane orientations;S52°W, 85°NE; S79°E, 84°SW; S13°,85°SW.

Plot Set 8

1 2

3 4

Stereographic projections showing foliation with lineation for lithologies in the study area. (1) Planes to foliation and lineation of data collected on the quartzofelspathic unit in the Mendenhall Gneiss; (2) Planes to foliation and lineation of the biotite rich member of the Mendenhall Gneiss; (3) Planes to foliation and lineation of the coarse grained granite augen gneiss; (4) Planes to foliation and lineation of the Mesozoic intrusive rocks measured in the study area. Curves show fault planes, triangles denote trend and plunge of striated rocks.

Figure 18 Stereographic projection of metamorphic rock units (277 points) which make up the Mendenhall Gneiss. Includes all data collected on biotite rich quartzofelspathic, granite augen, and amphibolite units gneiss units. Plot shows color coded point density plot, two best fit planes (dashed curves), fold axis (arrow), and interlimb angle (shorter arrow). (Best fit plane orientations; S66°E 84° SW, N64°W 79° NE. Fold axis trend and plunge S66°E 8°. Inter limb angle ~10°-15°)

Foliation

Plot sets 1 through 6 represent the results of measurements on metamorphic foliation. Notice how some of the plots have multiple foliation pole clusters. These are

the general trends of the individual rock units in accordance to their respective

stereographic plots. Lines with arrows denote axis trend and plunge of the best fit fold axes, assuming that the foliations have been cylindrically folded during post-

metamorphic deformation, which correspond in direction to the red and blue symbols

denoted on the geologic map (see Plate 1). The fold axes are large scale structures that extend continuously throughout the Mendenhall Gneiss. Fold trends are roughly parallel, which is expressed in the stereographic projections (see plot sets 1-4 and figure 18), and can be seen on the geologic map (see Plate 1). Foliation trends within the main

33 Mendenhall Gneiss unit (see figure 18) appear to be very similar in orientation. The two main pole clusters give way to a nearly 180° separation in metamorphic foliation.

A tectonic event seems to have affected the area of study after foliation development, as these features are seen in all Paleoproterozoic lithologic units. Gently plunging antiform and synform structures indicate a compressional event in which the main stress direction was oriented from the NNE and SSW. Given the steepness of the fold limbs, and an inter limb angle of 10°-15°, this event must have shortened these rocks significantly (see figures 8 and 9, refer to Plates 2-5). Cross section analysis gives a consistent percentage shortening of ~40%, which indicates that these rocks have been shortened ~40% relative to their initial length. Measurements taken of the small-scale isoclinal fold hinges show a population of orientations which are closely similar to large scale structural features seen on the geologic map and cross sections (refer to Plates 1-5).

Figure 19 Stereographic plot revealing poles to lineation that are color coded to show point density. Data plotted on the quartzofelspathic, biotite rich, augen, and amphibolite gneiss units in the study area.

Lineation

Metamorphic lineation is

common among most rock units

exposed in the study area. This

fabric is caused by shearing of the

rock mass at depth while the rock is very hot and under intense pressure. Tectonic stress is then applied to the rock unit and

34 instead of deforming in a brittle state as it would on the surface, the rock tends to smear

and form a feature called lineation. This feature is helpful in identifying stress regimes

that may have occurred in the past. The main feature of the study area is the fact that the

lineations have steep plunges that are almost down-dip (although some are oblique).

With one main pole cluster and two adjacent lesser dense pole clusters show that if you

rotate the folded foliations back to horizontal you get a NNE – SSW original trend of

lineation (see plot set 8 and figure 19). This idea makes it difficult to determine whether

these lineations predated the folding or formed during the folding.

Plot set 8 shows characteristic lineation in all rock units in which the structure

was found. They all share the same general lineation trend which indicates that at some time in the past, an event occurred which (in localized areas) sheared these rocks in a ductile state. Appearance of these lineation features indicates that these rocks were subjected to a stress event when the rock was at depth and in a relatively ductile state.

This heating and shearing of the rocks forms lineation features with S and C planes that show direction and magnitude of shear, note that these features occur in almost every rock unit which suggests that this event affected all rock units before the intrusion of the

Cretaceous rocks. Data points to formation of lineation to have taken place synchronous with the foliation or tight folding event.

Faults

Brittle deformation events occur in the Cenozoic that cut all units in the area. As shown by plot set 8 these faults have two distinct trends. One trend is correlative to that of the San Gabriel Fault. Its S70°W orientation is shown in one of the best fit planes shown in the stereographic projection. This analysis suggests that the stress regime in the

35 area that caused the rupture of the San Gabriel fault had a maximum stress direction

coming in from the northeast and the southwest. Using lineation of these faults which

contain hematite and chlorite staining on fault surfaces indicate a nearly pure strike slip

orientation with very little normal or reverse faulting component. The other two best fit

planes represent the conjugate set of faults that occur as smaller stress faults. These faults offset all units as well with a right lateral component that also has little or no dip

slip component.

This data gives way to ideas about the San Gabriel Fault system as a whole (see

figures 1 and 3). This fault system is a largely southwest striking right lateral strike slip

fault zone which cuts through the San Gabriel Mountains. This fault indicates some of

the first movements along a stress regime that ultimately created the San Andreas Fault

system. Movement along this fault zone cut the Mendenhall Gneiss in a few areas,

displacing some of the outcrops more than 30 km from their original location.

Palinspastic reconstructions of the area show this by correlating the rock units and

recovering the slip on these faults to reveal what these rock units may have looked like at

certain times in the geologic past (see figure 20).

V. Regional Implications/Interpretations/Significance

The folded map scale stratigraphy shows unit thickness in cross section (see main

plate) as well as interaction between the different rock units. Cross section analysis

shows that unit thicknesses vary depending on location, and thickness is dependant upon

the amount of intrusive rocks that may have displaced the country rocks. The

quartzofeslpathic and biotite rich units are mapped together and give a combined

thickness of ~160m (525 ft) to ~750+m (2500ft), on average the unit thickness is ~215m

36 (700ft). Granite augen gneiss varies drastically depending upon which area the cross section is located with thicknesses ranging from as small as a few meters, to about 300m

(1000ft), with an average thickness of ~120m (400ft). The foliated diorite unit also has a varying thickness from a few meters to about 330m (1100ft), with an average thickness of 50m (170ft) (refer to Plates 1-5).

As the units of the study area seem to be deformed as a whole, the relationships between these rocks can be used to correlate through palinspastic reconstructions, fault movements and rates to other areas (see figure 20). Both the large scale folds, as well as small scale features such as isoclinal folding and lineation data can be used for detailed comparison to rocks that are located on opposite sides of fault zones. Antiform and synform structures come into contact with fault zones which cut the area and can be used for comparison to correlative units either on the south side of the San Gabriel Fault or on the north side of the San Andreas Fault. In looking at the palinspastic reconstruction in figure 20, you can see possible correlation with rock units located in the Chocolate and

Orocopia Mountains. These rock units display similar characteristics in comparison to the Mendenhall Gneiss located in the San Gabriel Mountains. Other structures such as the Vincent Thrust also make the data more convincing as to what rocks correlate with others located on opposite sides of fault zones.

The northwest – southeast trend indicative to the San Gabriel fault can also be seen roughly parallel to the map scale structures. Similarity in trends of the brittle and ductile deformation events may or may not be a coincidence. Chronology suggests that the events which compressed the Mendenhall Gneiss ductally do not correspond directly with brittle deformation events occurring in the Cenozoic. Orientations of both ductile

37 and brittle deformation stresses are similar in that the maximum stress directions come in

from the northeast and southwest. This can mean two different scenarios; these ideas

hypothesize around stresses that caused the ductile deformation and brittle deformation

events. The creation of structures in the study area may have occurred simultaneously, or

nearly simultaneously, giving gross foliation to the rock mass while the uplifting, and

cooling rocks began to act brittle. Another idea is that these two events took place totally

independent of each other, and the similarity in trends is attributed to the fact that rocks

tend to break at their weakest points. The weakest points in this particular rock body may

have been on the axis of these map scale features. Map scale structures were created in

the first event, folding the rock into its current orientation, while a second event, a more

recent event, occurred subsequent to tectonic uplift of the area. This uplifted area had

cooled and was subject to brittle deformation events caused by stresses similar to what is happening today.

The many ductile and brittle deformation events that have taken place throughout the history of the Mendenhall Gneiss cause problems because one event may mask another. Field relations taken from other areas in which the Mendenhall Gneiss is present indicate formation of isoclinal folding to have occurred at or near the same time as a partial melting event; evidence is given by metamorphic mineral assemblages throughout the rock unit which have replaced its original crystal structure (Barth, et al., 1995). These mineral assemblages record multiple deformation events, having occurred at the time in which the protolith first formed, intrusion of the anorthosite – syenite – gabbro complex,

38 slip fault zones. Notice the location and 38 km along the south branch of he San Gabriel Fault. Taken from Nourse Middle Miocene palinspastic reconstruction of the San Gabriel Mountains along major strikd

Figure 20 of rock units and their relationships to the respective faults. This particular model restores 240 km slip along San Andres Fault zone, 22 of lateral slip along the north branch San Gabriel Fault 2002.

39 plutonic events in the Mesozoic, and others that may have taken place any time in

between (Barth, et al., 1995). Many periods of deformation may be masked by events

that have taken place after that time. When heat and pressure is applied to a body of

rock, changes occur within the rock that as long as no other chemical component is added

to the rock mass, will only alter the physical composition of the rock. The chemical

composition of the rock mass as a whole does not change, only the occurrence of

different minerals.

Plot set 6 shows metamorphic foliation as shown in the Mt. Lowe intrusive

complex. The Triassic foliations in the Mt. Lowe rocks show comparatively the same

general trends as the older Paleoproterozoic rocks, but only in one direction. These rocks

do not show the two separate orientations that reveal fold axis orientation which suggests

that these rock bodies are not as deformed in this area as the older rocks. An event of

deformation must have occurred before the emplacement of the Cretaceous rocks that

affected the Triassic and older rocks. As these Triassic and Cretaceous rocks were

intruded into the host rocks, they were emplaced parallel to the local foliation into areas

in which were the easiest to intrude. This is shown in the cross sections (see Plates 2-5)

as intrusive dikes and sills that have orientated themselves parallel to foliation.

These data also give opportunities to address questions relating to which

Neoproterozoic rocks were connected to the western margin of Laurentia before the 750

Ma rifting event. Aside from the Achaean terrane located near the center of the

Laurentian craton, the oldest rocks exposed on the western Cordillera occur today in the

San Gabriel Mountains and the San Bernardino Mountains (Barth, et al., 2000, 2001).

These rocks include the Mendenhall Gneiss, and data suggests that since no older rocks

40 are seen to the west of these units after restoration of Cenozoic faulting, these rocks mark part of the western margin of Laurentia. Many hypotheses have been given about how to correlate lithologic units on the present southwestern United States to another continent which would have resided adjacent to these rocks by looking at the depositional, igneous, tectonic and metamorphic events that shaped the area (Unrug, 1997). Different theories have been developed to explain the complexity of such an undertaking. The SWEAT,

AUSWUS (see figure 21), Siberia connection (see figure 22), and Cathaysia hypotheses explain Proterozoic rifting, and state different models as to which continent appears to have resided prior to separation. Throughout the last few years, researchers have collected large amounts of data to either support or disprove these theories.

One of the earliest such hypothesis for this late Precambrian fit of continents, named SWEAT for Southwestern United States-East Antarctica, was proposed by E.M.

Moores (1991). He suggested that the rocks located in the southwestern U.S. correlated best with those of eastern Antarctica, and rocks located in the northwest U.S. correlate with rocks found in Australia (see figure 21). He reasoned that the evidence for such a correlation is based on the age and stratigraphy of two thick sedimentary rock sequences along the western margin of the continental United States. These rocks range in locality from Canada and Montana, all of the way down to southwestern California (Moores,

1991). He stated that the late pre- Cambrian Gondwana rocks prior to rifting can be related to similar rock units located on Antarctica. Implications of this hypothesis include trends of the Greenville province orogenies continue onto rocks that have been found in east Antarctica, orogenies occurring to the north of Greenville belts extend onto southeastern Australia, other orogenies that become truncated before the continental

41 margin do not show up in correlated units, and evidence of similar rocks that occur in the

Yavapai, Mazatzal provinces and the Belt Purcell groups can be seen in Antarctica

(Moores, 1991). These implications are very thought provoking and speculative to say

the least, but offered the first insight into what may have happened in a time that seemed

to have been erased in the rock record until this point.

Karl Karlstrom (1999) expanded on this work by placing Australia adjacent to

southwestern Laurentia, and placing Antarctica further to the south (see figure 20). This

proposal is called AUSWUS for Australia-Southwest U.S. He provides evidence of this

hypothesis by citing examples of how the SWEAT proposal does not meet certain criteria

to correlate, and argues that the AUSWUS hypothesis provides an explanation of

geologic and paleomagnetic data that is better constrained. Relationships of rock units

exposed in southwest U.S. and Australia appear to be similar in age, composition and

tectonic setting. These units display the same mid-crustal shortening events, according to

composition may have formed from juvenile volcanic arc assemblages, and display

correlative matches between major ore assemblages (Karlstrom, 1999). Paleoproterozoic

and Mesoproterozoic rock units exposed in Australia may correlate to rocks such as the

Mendenhall Gneiss and the granite augen gneiss found in the study area, and may display

similar structural characteristics.

Subsequent to the proposition of the ideas stated previously, James W. Sears and

Raymond A. Price (2000) revised one of their hypothesis related to the placement of the

Siberian platform adjacent to southwestern Laurentia (see figure 21). This idea negates

the whole concept of having Australia or Antarctica located where they were previously

ustralian

Figure 21 Reconstruction of Laurentia for 1.7 to 0.8 Ga showing both locations of the Australian continent supported by the SWEAT and AUSWUS hypotheses., and locations of tectonic provinces. Location of the A continent and rock correlations are based on older rock units. Figure modified from Karlstrom, Williams, McLelland, Geissman, and Ahall,

43 thought to be before the break up of Laurentia, and the idea that Siberia was located on the northeastern margin of Laurentia (see figure 23). Using isotope analysis they initially related Sr/Sr isotope ratios in Siberia to those of similar ratios located in western

Laurentia, and by using general rock trends were able to correlate these two continents.

This hypothesis also uses some of the same techniques observed in other ideas to validate the assumption that it was in fact Siberia and not Antarctica or Australia. The Belt-

Churchill basin is a linear feature exposed in western Laurentia and when it is placed adjacent to the Taimyr trough, a long depressional feature located in northern Siberia that displays a poorly understood Mesoproterozoic stratigraphic section, it forms one continuous truncated basin that was rifted apart (Sears and Price, 2000). This feature is an accumulation of sediment that may have been transported from one side of the continent to the other and back again before the area rifted apart. If this is indeed what happened, the sediment transported to the area could be a possible protolith for the

Mendenhall Gneiss.

Another idea proposed in the 1990’s by Zheng-Xiang Li states that he may have found a connection between the Yangtze block of South China and the western margin of

Laurentia (Li, 1995, 2002). Using SHRIMP (sensitive high-resolution ion microbe) analysis, he was able to conclude that a Grenvillian continental collision between south

China and western Laurentia happened by relating metamorphic events that affected both the Yangtze and Cathaysia blocks and by looking at locations of sedimentary basins on the Yangtze block that may have come from the Cathaysia block (Li, 1995, 2002; see figure 24).

Figure 22 Paleoproterozoic reconstruction of Laurentia showing locations of rifted margins and tectonic provinces related to Sears and Price’s Siberian Connection model. Shows trends associated with correlation of separate continents. Modified from Sears and Price, 2000.

Figure 23 Neoproterozoic reconstruction of supercontinent Rodinia showing locations of rifted continents. This is one of the first figures showing location of continents. Does not relate to the SWEAT, AUSWUS, or Siberian connection models. A-F=Albany-Fraser mobile belt, M=Musgrave block, TL=Tasman line, TT=Thelon-Taltson line, BB=Belt basin. Modified from Hoffman, 1991.

46 Figure 24 Diagram showing the possible position of the Yangtze block adjacent to western Laurentia. Y=Yangtze block, T=Western Tazmania, B=Belt Basin, BC=British Colombia, M=Mackenzie Mt., NC=NW Canada. Modified from Li, 2002.

Using the appearance of 1800 Ma

basement rocks and Archean protoliths,

the correlation of the Cathaysia block and

the Yavapai and Mazatzal blocks can be

made. When this puzzle is all put together, located between the eastern Antarctica block and the western margin of

Laurentia is a separate small continent composed of the Yangtze and Cathaysia blocks

(Li, 2002; see figures 23, 24).

These hypothesis all offer differing ideas about what may have happened 750 Ma ago. Using basically the same techniques of analysis, many conclusions are formulated but can not be positively proven. Detailed mapping projects similar to this paper offer large amounts of data that can be used to help solve problems like these. Much more data needs to be collected before any ideas can be absolutely proven.

VI. Conclusions

Geologic mapping of the Mendenhall Gneiss and surrounding rocks have illustrated correlation possibilities associated with many different events. Detailed mapping of structures in the Mendenhall Gneiss can be correlated with lithologic units located on opposite sides of Cenozoic fault zones located throughout Southern California.

Restoration and palinspastic reconstructions of these faults show characteristics that are

47 very similar, and present on both sides. Areas in the San Gabriel Mountains on the north side of the San Gabriel fault contain rocks that correlate with mountain ranges located around 250 km to the south and on the north side of the San Andreas fault, in the

Chocolate and Orocopia Mountains. These rock units display the same characteristics and generally have the same geochemistry in both localities. Although protoliths and environment of deposition remain in doubt at this time due to multiple brittle and ductile deformation events, subsequent dating and chronological evidence offer insight into the history of this rock unit.

Events that shaped the history of this rock are being established in order to relate these units to past continental collision and rifting events that have taken place in the

Proterozoic. Essentially, since these gneisses are the oldest rocks exposed in western

U.S.A., detailed mapping is a necessity in order to resolve correlation problems with the rifting of western Laurentia. Determination of the mystery continent which resided there has been a problem that many geologists have tried to resolve, citing evidence based upon speculative as well as concrete ideas. As the techniques for correlating lithologic units become more and more complex, it will all come down to projects like these to close up any doubts and misgivings about conclusions. The more data that is collected in an area of study, the more the geologic community can speculate about the history of events that shaped the planet as we see it today.

The Mendenhall Gneiss, as it is seen in the area of study, has been dated fairly well at ~1.7 Ga, and has been exposed to a multitude of deformation events (Barth et al.,

2001). Showing very small scale foliation, large map-scale folding structures and multiple intrusion events, this rock shows a very complicated history which makes

48 studying the unit a difficult task. The rocks exposed demonstrate features that enable the correlation of the largest continuous section of metamorphic rock in the San Gabriel

Mountains to other areas located on opposite sides of large strike-slip faults. Mapping of the area by taking rock orientations whenever available gives hundreds of data points in which to characterize its behavior, and predict what may be happening at depth. Studies of this type are necessary in order to define characteristics used in correlation and reconstruction of the south-central San Gabriel Mountains and its correlative units located in Southern California, and around the globe.

VII. Future Studies

Possible future studies in the area not mapped may help to define characteristics of certain rock units in more detail. One idea may be to map canyons and other roads located to the north of Angeles Crest Highway to clarify the occurrence of the granite augen gneiss that outcrops in many localities in the study area. Mapping of Ladybug

Canyon has yielded evidence that more of this rock type exists to the north that may help in defining structural relationships related to the environment of emplacement of its protolith, and data that may help define events that characterize the eye catching augen gneiss. Other rock types may be seen on the north side of the map that has not yet been mapped in any significant quantity. The Triassic and Cretaceous intrusive rocks that occur in the areas already mapped may be in this unmapped area in significant quantity.

These rocks do not show up in any published geologic map of the area, and may be helpful in determining dates of deformation events. The gneisses exposed in the study area also may outcrop in this area and also need to be studied in detail because rocks in

49 this area are located farther away from the 1190 Ma intrusive anorthosite-syenite-gabbro

complex, and may show less deformation that would help in dating zircons in the rock.

Generally, more mapping in unmapped areas is necessary to help constrain rock types for

the purpose of correlating the Mendenhall Gneiss to rocks located on other continents,

and Southern California, the more data collected, the better the results.

VIII. Acknowledgments

A very big thank you goes to Professor Jon Nourse for his patience and guidance

on this long arduous journey, without him, none of this would have been possible. The

American Association of State Geologists (AASG) provided a grant for $3300.00 that made the start of this mapping project possible, I thank the AASG Student Mentorship

Program for enabling me to obtain guidance and experience in field mapping. I would also like to thank those incorrigible fellow students that helped with field mapping, data collection and advice. They include; Terry Watkins, Shawn Wilkins, Meredith Staley,

Seth Brodie, Jennifer Beal, Julie Parra and Cami Anderson. Thank you all for your help and support. I would also like to thank my parents for paying for this whole college endeavor, without them; also, this would not have been possible.

50 BIBLIOGRAPHY

Barth, Andrew P., et al., Joseph L. Wooden, Drew S. Coleman, 2001 “Shrimp-RG U-Pb Zircon Geochronology of Mesoproterozoic Metamorphism and Plutonism in the Southwestern United States.” The Journal of Geology vol. 109, p 319-325. Barth, Andrew P., et al., Joseph L. Wooden, R.M. Tosda, Jean Morrison, D.L. Dawson and B.M. Henry, 1995 “Origin of gneisses in the aureole of the San Gabriel anorthosite complex and implications for the Proterozoic crustal evolution of southern California.” Tectonics vol. 14, no. 3, p. 736-750. Barth, Andrew P., et al., J.L. Wooden, R.M. Tosdal, J. Morrison, 1995 “Crustal contamination in the petrogenisis of a calc-alkalic rock series: Josephine Mountain Intrusion, California” GSA Bulletin vol. 107; no. 2; p 201-212. Barth, Andrew P., et al., 1997 “Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin.” Tectonics vol. 16, no. 2, p. 290-304 Dalziel, Ian W.D., 1991 “Pacific margins of Laurentia and East Antarctica – Australia as a conjugate rift pair: Evidence and Implications for an Eocambrian supercontinent.” Geology vol. 19, p. 598-601. Dibblee, T.W., Jr., 1998, Geologic map of the Mt. Wilson/Azusa quadrangles, southern California, Map No. DF 67, Dibblee Geological Foundation. Ehlig, Perry, 1981 “Origin and Tectonic History of the San Gabriel Mountains.” p. 258-266. Silver, Leon T., 1971 “Problems with crystalline rocks of the Transverse Ranges.” Geological Society of America abstracts with programs, Cordilleran Section, Riverside, CA., p. 193-194. Karlstrom, Karl E., Michael L. Williams, Stephen S. Harlan, James McLelland, John W. Geissman, Karl-Inge Ahall, 1999 “Refining Rodinia: Geologic Evidence for the Australia- Western U.S. connection in the Proterozoic.” GSA Today vol. 9, no. 10, p. 1-6. Li, Zheng-Xiang, Linghua Zhang, Christopher McA. Powell, 1995, “South China in Rodinia: Part of the missing link between Australia-East Antarctica and Laurentia?” Geology vol. 23, no. 5, p. 407-410. Li, Zheng-Xiang, Xian-hua Li, Hanwen Zhou, Peter D. Kinny, 2002, “Grenvillian continental collision in South China: New SHRIMP U-Pb zircon results and implications for the configuration of Rodinia.” Geology vol. 30, no. 2, p. 163-166. Moores, E.M., 1991 “Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis.” Geology vol. 19, p. 425-428. Nourse, Jonathan A. 2002 “Middle Miocene reconstruction of the central and eastern San Gabriel Mountains, southern California, with implications for evolution of the San Gabriel fault and Los Angleles basin.” Geological Society of America Special Paper 365, p. 161- 185. Sears, James W., Raymond A. Price, 2000 “New look and the Siberian connection: No SWEAT.” Geology vol. 28, no. 5., p. 423-426. Unrug, Raphael, “Rodinia to Gondwana: The Geodynamic Map of Gondwana Supercontinent Assembly.” GSA Today vol. 7, no. 1, p. 1-4.

56 Amphibolite Triassic Biotite Gneiss Quartzofelspathic Gneiss Augen Gneiss Gneiss Intrusive S41E,79,NW S71W,54,SE S85E,71,SW S51E,65,SW N13E,65,NW S46E,34,SW S59E,71,SW S21W,73,NW S60E,15,SW S34E,64,SW N15E,74,SE S22E,55,SW S35E,85,SW N88E,48,SE S35E,49,SW N88E,46,SE N60W,72,SW N74W,53,SW S66E,45,SW S71E,79,SW N84W,75,SW S54E,41,SW S49E,83,SW N37W,72,SW S41E,84,NE N24W,83,NE N54W,69,SW N39E,39,SE S26E,61,SW N53W,79,SW S77E,59,SW N31W,54,SW S39E,64,SW N90W,37,SW S75E,73,SW N56W,80,NE S65E,85,SW S06W,54,NW S65E,90,SW N81E,44,SE S52E,77,SW S61E,62,SW S70E,60,SW N70W,81,SW S56E,90,SW S41W,79,NW S44E,55,NE S24E,61,SW S54E,65,NE N88E,76,SE S80W,85,SE S36E,29,SW N65W,90,NE S35E,43,SW S81E,64,SW S82E,87,SW N49W,81,SW N64W,81,NE S73E,80,SW S67E,66,SW S40E,90,SW N66W,52,SW N04E,70,SE S38W,85,SE N39W,51,NE S39E,43,SW S55E,80,NE S47E,75,NE N10E,65,SE S30E,76,SW N81E,55,SE S77E,41,SW S58E,85,SW N59W,79,NE N39W,81,NE S40E,75,NE N32E,54,SE N75W,49,SW S76E,62,NE N52E,48,SE N84E,56,SE S59E,71,SW S83E,46,SW S75E,37,SW S80E,59,NE N69E,49,SE N74E,65,SE S05E,85,SW S21E,66,SW N74W,46,SW S79E,76,NE N85E,85,SE S59E,80,NE S35E,85,SW N63W,79,NE S05E,51,SW S84E,90,SW S50E,83,SW N80W,90,NE S29E,49,SW S72E,76,SW S00E,40,W N61W,43,NE N80E,46,SE S39E,86,SW S11E,58,SW N85W,84,NE N06E,32,NW N64W,80,SW N64E,54,SE S52E,54,SW S29E,69,SW S86W,79,NW S12W,26,NW N79W,78,NE N54E,62,SE S34E,51,SW S20E,39,SW N70W,46,SW S46W,29,NW N50W,75,NE S79W,41,SE N60W,66,SW S60E,53,SW S71W,53,SE S31W,30,NW N71W,81,SW S434E,74,SW S76E,59,SW S49E,65,SW S70E,46,SW S90W,24,SE S84E,79,SW S63E,44,SW S86E,49,SW N71W,79,SW S72W,35,NW Foliated Diorite S62E,85,SW N86W,52,SW S54E,59,SW N72E,45,SE N54E,34,NW S26E,52,SW N70W,90,SW S85W,60,NW N90W,32,SW N90W,44,SW S36W,36,NW S85E,80,SW N66W,72,NE S90E,59,NE N56E,30,SE N77W,60,SW S20E,71,SW S60E,76,NE N75W,81,NE N59W,54,NE S84E,40,SW S85E,85,SW S22E,62,SW S80E,84,SW N59W,77,SW S76W,39,NW S66E,45,SW S11E,85,SW S26W,17,SW S74E,49,SW N46,W,49,SW N82E,19,NW S41E,84,NE S46E,75,SW S47W,29,NW S69E,56,SW S40E,41,SW N73W,65,NE S77E,57,SW S65E,82,SW N32W,36,SW S69E,46,SW S54W,44,SE S64E,76,SW S71E,56,SW S65E,56,SW S55E,61,SW S84W,79,NW N71E,75,SE S48E,64,SW S65E,85,SW N59W,82,SW N52E,46,SE S70W,84,NW N90W,66,SW S19E,55,SW S70E,60,SW S74E,76,NE N16W,16,NE N90W,61,NE S77E,80,SW S68E,75,NE S54E,65,NE N60E,85,SE S29E,51,SW S31W,71,NW S78E,86,SW S64E,52,NE S81E,64,SW N76E,71,SE S82E,34,SW N15W,74,SW S81E,77,NE S42E,45,NE S40E,90,SW N85E,75,SE S56E,48,SW N49W,60,NE N39W,50,NE S47E,61,NE S55E,86,NE N65E,73,SE S41E,61,SW S80W,63,NW N52W,55,NE N66W,79,SW S51E,75,NE N87W,86,NE N38E,25,NW N50W,45,NE N90E,75,NW N73W,90,SW S76E,61,SW N59W,60,NE S10E,43,SW N80W,90,SW S44E,34,SW N68W,81,SE S60E,90,SW S50E,39,SW S29E,55,SW S57E,68,NE N11W,59,NE S59E,90,SW S27E,29,SW S61E,40,SW S31E,48,NE S74W,80,SE S59E,90,SW N27W,29,SW S59E,90,SW N50W,48,NE S41W,59,SE S62W,60,SE S09E,48,SW S49E,70,SW N50W,48,NE S76E,43,SW S59E,76,NE S15E,46,SW S29E,79,SW S59E,69,NE N46W,84,NE S23E,34,SW S23E,34,SW S49E,88,SW S54E,76,NE S73W,59,SE N44E,39,SE N51E,51,SE S55E,54,NE S58E,78,NE N63E,18,SE N90W,S4,SW S49E,65,SW S07E,61,SW S26E,74,NE S58E,85,SW S70E,73,NE S70E,66,NE S39E,82,SW S75E,80,NE S50E,39,NE N74W,81,NE S60E,78,SW N81W,63,NE S76E,62,NE N73W,79,NE Appended Structural Data

57 Triassic Faults Biotite Gneiss Quartzofelspathic Gneiss Augen Gneiss Intrusive S80W,71,SE N25W,63,NE N81W,80,NE S86E,72,NE S54E,69,SW S61E,64,NE S77E,81,NE S10E,71,SW N61W,43,NE N88W,64,NE S27E,53,SW S59E,65,NE S49E,84,SW S20E,40,SW N64W,80,SW S76E,60,NE S74W,61,NW N16W,90,SW N78E,80,NW S61E,88,NE N76W,24,NE S71E,62,NE N39W,39,NE N84E,52,NW N79W,78,NE S84E,90,SW N71W,81,SW N90W,44,NE S82W,76,SE N61W,83,NE N51W,90,SW N77W,90,NE N62W,77,NE S84W,72,SE N50W,75,NE N52W,85,NE N41E,81,SE N83W,80,NE S78W,82,SE N76W,79,NE N00W,75,SW N78E,84,NW S29W,89,NW S76E,76,SW S24E,88,SW S63E,61,NE S55W,62,NW S36E,61,NE S59W,71,NW N61W,63,NE N46E,60,NW S67E,61,NE S82E,88,SW N85W,75,NE N31E,88,NW S60E,85,SW S82E,69,NE S85W,85,NW S80E,80,SW N87W,75,NE S74E,75,SW S85E,90,NE S26E,64,NE S56E,69,NE N90E,52,SE N62W,85,NE N31E,87,SE N85W,86,NE S46W,79,NW N62W,90,NE S04W,82,NW N62W,90,NE S35W,90,NW N76W,87,SW N32E,90,NW N65W,90,NE N36E,90,NW N39W,63,NE S40E,71,NE N44W,84,SW N79W,87,NE N79W,24,NE N60E,81,SE S81W,28,NW S65E,75,NE S52E,89,SW

Appended Structural Data Continued