STRUCTURAL AND KINEMATIC EVOLUTION OF THE MIDDLE CRUST DURING LATE CRETACEOUS EXTENSION IN WESTERN NEW ZEALAND
A Thesis Presented
by
Williams C. Simonson
to
The Faculty of the Graduate College
of
The University of Vermont
In Partial Fulfillment of the Requirements for the Degree of Master of Science Specializing in Geology
October, 2003 Accepted by the Faculty of the Graduate College, The University of Vermont, in partial fulfillment of the requirements for the degree of Master of Science, specializing in Geology.
Thesis Examination Committee:
Advisor
Keith A. Klepeis, Ph.D.
Barry Doolan, Ph.D
Tracy Rushmer, Ph.D.
Chairperson
Donna Rizzo, Ph.D.
Special Assistant, David S. Dummit, Ph.D. to the Provost for Graduate Eductation
Date: August 28, 2003 CITATION
Material in this thesis will be submitted for publication in the geologic and geophysical journal, Tectonics, pending the arrival of geochronologic data, in the following form:
Simonson, W.C., and Klepeis, K.A., (in prep.), Structural evolution of the middle crust during exhumation by continental extension: The Paparoa Metamorphic Core Complex, South Island, New Zealand. Tectonics.
ii ACKNOWLEDGEMENTS
This thesis would not exist without the help of many. In no particular order I am grateful to:
My parents, Pat and Ric Simonson, who have been educating me for my entire life. Thank you so much for everything that you have given me (TPS, AC). I promise to repay you with love and understanding.
My teachers, far too numerous to list, who have challenged and pushed me. Thank you for your time and energy.
Keith Klepeis, my mentor, who has taken me so many places I never expected to see. Keith, under your guidance I achieved things I was sure were never possible…drawing the WHC x- section, giving a talk at GSA, writing a paper…Thank you is clearly not enough. I'll get you back somehow.
The geology faculty at the University of Vermont, for supporting me and advising me on how to improve my story.
The geology faculty at Amherst College, who put me in the position to do what I have done for the last two years.
The National Science Foundation, for funding my travel to New Zealand
Mrs. Catherine Suiter, for funding the Suiter Award, which covered the cost of my travel to GSA in Denver, CO.
Dave, Gabi, Denise, and Ian, for allowing me to share your toys and your work space.
Steve Marcotte, for taking so many measurements in the field and taking part in many, many discussions about all things structural.
Kenny Oldrid, for being Kenny Oldrid during the summer of 2003
Dave West at Middlebury College, for allowing me to use his brand new Spectroscope.
UVM Geology students, thanks for the fun times and moral support.
J. Renee, for your love.
The Institute for Geological and Nuclear Sciences, in Dunedin, NZ, for kind hospitality and permission to use the rock saw.
The University of Vermont, for funding my GTF stipend and paying for my summer research in 2002
Pat Frank, for helping with all those little details
iii TABLE OF CONTENTS
CITATION……………………………………………………………………………….ii
ACKNOWLEDGEMENTS…………………………………………………………….iii
LIST OF FIGURES……………………………………………………………………viii
CHAPTER 1: Introduction…………………………………………………………….…1
1.1 STATEMENT OF PURPOSE AND SIGNIFICANCE………………………….…1
1.2 APPROACH………………………………………………………………………...2
1.2.1 Study area selection ……………………………………………………………...2
1.2.2 Field Techniques……………………………………………………………...…..3
1.2.3 Laboratory Techniques …………………………………………………………..3
1.3 THESIS OUTLINE…………………………………………………………………..4
CHAPTER 2: Literature Review…………………………………………………………6
2.1 INTRODUCTION…………………………………………………………………..6
2.2 CONTINENTAL EXTENSION………………………………………………….…6
2.3 METAMORPHIC CORE COMPLEXES…………………………………………10
2.4 EVOLUTION OF DETACHMENT ZONES AND LOWER PLATE ROCKS…..12
2.5 WESTERN NEW ZEALAND AS AN EXTENSIONAL SETTTING…………...15
2.6 CASE STUDIES OF SYN, POST-OROGENIC EXTENSION…………...……...25
CHAPTER 3: Manuscript prepared for submission to Tectonics: "Structural evolution of the middle crust by exhumation during continental extension: The Paparoa Metamorphic Core Complex, South Island, New Zealand"………………………………………………………………………………….26 3.1 INTRODUCTION……………………………………………………………….…27
3.2 PREVIOUS WORK………………………………………………………………..28
3.2.1 Regional geologic and structural relationships………………………………...28
3.2.2 Paparoa metamorphic core complex…………………………………………...32
3.2.3 Lower plate cooling history…………………………………………………….42
3.3 DUCTILE DEFORMATION IN THE LOWER PLATE………………………….35
3.3.1 Zone of mylonitic fabrics……………………………………………………….35
3.3.2 Central core zone……………………………………………………………….39
3.3.3 Ductile deformation on the northern border of the core……………………….41
3.4 BRITTLE AND SEMI-BRITTLE DEFORMATION……………………………..44
3.4.1 Brittle faults…………………………………………………………….………45
3.4.2 Semi-brittle faults……………………………………………………………….50
3.5 KINEMATIC ANALYSIS………………………………………………………...50
3.5.1 Kinematic analysis of mylonitic rocks………………………………………….50
3.5.2 Kinematic analysis of ductile fabrics outside the mylonitic zone………………57
3.5.3 Kinematic analysis of fault-slip data…………………………………………...58
3.6 CORRELATION OF STRUCTURES AND DEFORMATION SEQUENCE…...60
3.7 GEOCHRONOLOGY……………………………………………………………..61
3.7.1 Sample locations………………………………………………………………..61
3.7.2 U-Pb Ages………………………………………………………………………62
3.8 DISCUSSION……………………………………………………………………..62
3.8.1 Three-dimensional strain and kinematic evolution of the middle crust………...62 3.8.2 Extensional mechanisms………………………………………………………..65
3.9 CONCLUSIONS…………………………………………………………………..67
CHAPTER 4: Strain and Kinematic Analysis………………………………………….69
4.1 INTRODUCTION…………………………………………………………………69
4.2 STRAIN……………………………………………………………………………69
4.2.1 Background……………………………………………………………………..69
4.2.2 Methods…………………………………………………………………………72
4.2.3 Results…………………………………………………………………………..74
4.3 KINEMATIC ANALYSES………………………………………………………..83
4.3.1 Background and methods……………………………………………………….83
4.3.2 Kinematic vorticity analysis…………………………………………………….86
4.3.3 Shear-sense determinations …………………………………………………....89
4.3.4 Results of kinematic vorticity analysis………………………………………….92
4.4 SUMMARY AND CONCLUSIONS……………………………………………...93
CHAPTER 5: THESIS SUMMARY AND CONCLUSIONS…………………………95
5.1 SUMMARY AND CONCLUSIONS……………………………………………….95
5.2 SUGGESTIONS FOR FUTURE WORK……………………………………………97
COMPREHENSIVE BIBLIOGRAPHY……………………………………………...99
APPENDICES…………………………………………………………………………115
APPENDIX A: KINEMATIC INDICATORS………………………………………..116 APPENDIX B: IGNS PETLAB DATABASE SPREASHEET………………………128
APPENDIX C: STRUCTURAL DATA……………………………………………..137 LIST OF FIGURES
Page #
Figure 2.1. End member models showing predicted styles of deformation produced by continental extension………………………………………………………..8
Figure 2.2. Metamorphic core complex model after Lister & Davis (1989) and Reynolds et al., (1990)…………………………………………………………………...14
Figure 2.3. Paparoa Metamorphic Core Complex after Tulloch & Kimbrough (1989). Map includes data compiled from Bowen (1964), Hume (1977), Nathan (1978) and White (1987) Abbreviations in caps refer to structural transects and sample suites; CF - Cape Foulwind, CH - Charleston, WHC - White Horse Creek, PS - Pike Stream, BR - Buller River, BG - Buckland Granite…………………………………………………………………………………....17
Figure 2.4. Thermal history for the syntectonic Buckland Granite (lower plate intrusion) in the northern Paparoa Range. U/Pb data from Muir et al. (1994), Musc Rb/Sr ages from Graham & White (1990), Apatite FT data from Seward (1989), Ar/Ar ages from Spell et al. (2000)……………………………………………………………..21
Figure 3.1. Paparoa Metamorphic Core Complex after Tulloch & Kimbrough (1989). Map includes data compiled from Bowen (1964), Hume (1977), Nathan (1978) and White (1987) Abbreviations in caps refer to structural transects and sample suites; CF - Cape Foulwind, CH - Charleston, WHC - White Horse Creek, PS - Pike Stream, BR - Buller River, BG - Buckland Granite…………………………………………………………………………………....29
Figure 3.2. White Horse Creek structural transect. a.) Map of structural data; b.) Cross- section showing the folded mylonitic front and WHC strain gradient ; c.)-e.) Outcrop scale field sketches of F2,F3 folds; f.)-i.) Lower hemisphere, equal-area projections of structural data; j.) Block diagram illustrating the geometry of F2-F4 in the highest strain zone………………………………………………………………………………………36
Figure 3.3. Charleston structural transect. a.-c.) Lower hemisphere projection of fabric data; a.) site CH1, b.) CH2, c.) CH3-7. Circles - poles to foliation; triangles - lineations………………………………………………………………………..………..40
Figure 3.4. Buller River structural transect. Contact relationships after Tulloch & Kimbrough (1989)……………………………………………………………………….42
Figure 3.5. Cape Foulwind structural transect. a.) - d.) Lower hemisphere projections of fabric data, circles - poles to foliation; triangles - mineral lineations.
viii a.) CF1,4 b.) CF3 c.) CF2, d.) CF5-6. Horizontal scale for cross-sections is b.) equal to the vertical scale….…………………………………………………………43
Figure 3.6. Brittle fault geometries for the northern domain. a.) View looking SSW at NE and SW dipping faults, site CH1 b.) Looking down on what may have been the Slip surface of a low-angle normal fault, now dissected by steep, smaller faults. c.)-f.) Fault planes and straie for data sets CF 1 regional and CF 1 block, respectively. d. -h.) X and Z axis contours (see text)…………………………………………………………46
Figure 3.7. Fault-fan style of normal faulting observed in the central core zone at site CH7 a.) Faults and straie b.) Z axis contour c.) X axis contour d.) Sketch of framed area in photograph………………………………………………………………………47
Figure 3.8. Vertical profile of site WHC1 showing upright, open (F3) folding and conjugate brittle faulting. a.) Poles to foliation (circles) and mineral lineations b.) Fault planes and straie c.),d.) X and Z axis contours…………………………………………..48
Figure 3.9. Outcrop and thin section scale kinematic indicators. a,b.) Mylonitic fabric photographed in the field. S-C fabric and large beta-type feldspar clast directly below pencil indicate dextral motion, parallel to the pencil (lineation); c.) S-C fabric, imbricated, antithetically microfaulted feldspar in the center of field of view, sigma-type clast in the upper left-hand corner indicate dextral motion.Photograph taken under crossed polars with the gypsum plate inserted; d,f.) Plagioclase feldspar porphyroclasts sampled from site WHC3. Large beta-type clast in the upper third of "d."; sigma-type and delta-type clasts in the upper third of "f."; g,h.) S-C fabrics developed in the Foulwind Granite at sites CF2 and CF3. These fabrics are interpreted as related to semi-brittle faulting. "g." indicates sinistral motion as C planes are oriented NE-SW and S planes are oriented NW-SE. Large beta-type clast in center of field of view. "h." displays well developed dextral S-C fabric…………………………………………………………….51
Figure 3.10. Kinematic vorticity analysis. a.) Field photo showing plagioclase porphyrocasts b.) Feldspar Rf/phi data (see text) c.)-e.) Hyperbolic nets showing clast distribution, plots rotated parallel to foliation as seen in (a.) f.) Beta clast in plane light g.) Sigma clast in plane light g.) Delta clast in plane light……….…………………………55
Figure 3.11. Kinematic fault plane solutions for brittle faults affecting lower plate rocks. Solutions are based on the scatter of X and Z axes (squares and circles, respectively)…59
Figure 3.12. Schematic block diagrams illustrating the structural evolution, (D1 - D5), of the metamorphic core during exhumation. a.) D1-D2; b.) D3; c.) D4-D5. Bold arrows indicate the orientations of stretching and extension axes remained nearly parallel during deformation and exhumation of the middle crust...……………………………...64
ix Figure 4.1. Principle planes determined in accordance with the orientations of fabric elements………………………………………………………………………………….73
Figure 4.2. Three-dimensional strain results from the quartz analysis. Lower hemisphere plots show the orientation of the principle ellipsoid axes (X,Y,and Z) determined using the program Strain 3D and Rf/φ data from three perpendicular planes….………………75
Figure 4.3. Three-dimensional strain results from the feldspar analysis. Lower hemisphere plots show the orientation of the principle ellipsoid axes (X,Y,and Z) determined using the program Strain 3D and Rf/φ data from three perpendicular planes…………………………………………………………………………………….78
Figure 4.4. a.) Flinn plot describing illustrating the three dimensional strain geometry determined from strain analysis; b.) Shapes of feldspar grains measured in three- dimensional feldspar strain analysis……………………………………………………..80
x CHAPTER 1: Introduction
1.1 STATEMENT OF PURPOSE AND SIGNIFICANCE
After the general acceptance of plate tectonics, the focus of many studies became the thermal and mechanical evolution of continental crust as it was affected by changing plate boundary conditions. Contractional structures, such as fold and thrust belts and vertically thickening ductile shear zones, have been observed in regions where collisional tectonics is thought to have increased (sometimes by more than a factor of two) the thickness of the Earth’s crust. Extensional structures, including high and low angle normal faults, detachment faults, metamorphic core complexes and vertically thinning ductile shear zones have been observed in regions which have experienced extension and crustal thinning.
While crustal thickening is nearly always attributed to plate collision, a consensus regarding the dominant mechanism of continental extension (post-orogenic extension) in thickened orogens, however, has yet to be reached. A variety of models have been proposed in the recent literature pointing to a range of potential causes for large-scale lithospheric extension. Popular models invoke gravitational instability (Burchfiel et al.,
1992), crustal thickening (Coney & Harms, 1984), and thermal weakening due to partial melting (Vanderhaeghe & Teyssier, 1997) as extensional mechanisms. The purpose of this study is to make a contribution to the debate, as we add data from a region of continental crust extended during the late Cretaceous breakup of the Gondwana supercontinent. We show, here, that extensional deformation and crustal thinning may
1 take place without significant weakening of the middle and lower crust, and that it may be driven dominantly by a change in regional plate boundary dynamics.
Among the specific questions addressed in this thesis are: (1) How was deformation partitioned among mid-crustal rocks during the late Cretaceous extensional event? (2)
What was the kinematic evolution of the middle crust as it passed through the brittle- ductile transition zone? (3) What were the dominant mechanisms for extension in the region? Addressing these issues is important for developing a complete understanding of continental lithospheric evolution during extension.
1.2 APPROACH
1.2.1 Study area selection
Previous work on the South Island's "West Coast" by numerous workers
(Shelley, 1969; 1972; Nathan, 1978; Tulloch and Kimbrough, 1989; Tulloch and Palmer,
1990; Spell et al., 2000) allows this study to address specific questions regarding the structural history of the region and place them in the context of a system with well understood boundary conditions. Tulloch and Kimbrough’s (1989) conception of the
“Paparoa Metamorphic Core Complex” provides an excellent starting point for this research, as these workers describe and locate the regional characteristics that define the system. However, some questions about the structural and kinematic evolution of deformation in the plutonic and metamorphic core of the complex remain. Thanks largely to previous mapping, we were able to select specific transects along the coast and inland within the creeks where core deformation could be measured and documented. Structural
2 transects are named in accordance with their geographical position; Pike Stream (PS),
White Horse Creek (WHC), Charleston (CH), Cape Foulwind (CF), Buller River (BR)
(Figure 2.3). These abbreviations are also used for sample identification.
1.2.2 Field techniques
Field work is arguably the most critical component to a study such as this. The most critical objectives while in the field are: (1) Collecting enough structural data over a large enough area to understand the structural and kinematic evolution of the system.
Structural data includes sketches, form cross-sections, photographs and measurements of foliation, lineation, fold axes, fold axial planes, fault planes, slickenlines, orientation of
S-C fabrics and other kinematic indicators observed in the field. (2) Collecting enough samples over a large enough area to document changes in microtexture and understand microscale kinematics and deformation mechanisms. Samples collected in the field are oriented so geometrical elements may be understood in real space.
1.2.3 Laboratory techniques
Laboratory analyses generally fall into two categories, those involving field data
(including structural measurements, sketches and sections) and those involving samples
(including strain analysis, kinematics, and microtextural analysis). Field data are generally processed immediately after thin section preparation, and involve the organization of regional structural data into specific forms (i.e. lower hemisphere equal- area projections for planar and linear data). Form cross-sections drawn in the field are digitized using Adobe IllustratorTM. Regional cross sections are drawn, digitized and
3 keyed to strip maps. Fault and slickenline populations are plotted and kinematic solutions generated using the program “Faultkin v.4.1” written by Rick Allmenginger
(www.geo.cornell.edu/geology/faculty/RWA/maintext.html)(Allmendinger et al.,
1989). Clast orientation (Rf/φ for kinematic vorticity analysis) are plotted using hyperbolic nets. (See Chapter 4 for a description of methods)
A total of 40 oriented samples were collected in the field. Each was cut into cubes using a diamond-studded rock saw, such that thin section faces are parallel to the XZ plane (parallel to lineation, perpendicular to foliation, Figure 4.1). On some samples thin sections were made for the YZ (perpendicular to lineation, foliation) and XY (parallel to foliation) planes as well. Thin sections were prepared by Spectrum Petrographics, inc. at a moderate cost. Thin section analysis included documentation of microstructure, bulk kinematics, and three-dimensional strain. (See Chapter 4 for a complete discussion)
1.3 THESIS OUTLINE
This thesis is written following the University of Vermont Graduate College guidelines for a journal article thesis. Chapter 1 provides a general introduction to the study. Here, the purpose, approach and methods are outlined and the significance of studying extension in western New Zealand as it relates to contemporary issues in the fields of Tectonics and Structural Geology are presented.
Chapter 2 provides a review of recent literature that addresses both topical and regional issues. Topics reviewed include mechanisms and styles of continental extension, metamorphic core complexes and geometry of mylonitic zones. The studies discussed in
4 this portion of the chapter include some of the key models tested by the results of this study. Additionally, a review of literature discussing the elements and nature of mid-late
Cretaceous extension in western New Zealand are presented, so the reader may clearly understand what was known prior to the undertaking of this study. Finally, characteristics of previously studied collapsed orogenic belts are presented and their extensional mechanisms are discussed.
Chapter 3 contains a manuscript prepared for submission to the journal Tectonics, pending the arrival of geochronologic data from the University of Arizona. The paper focuses on the structural and kinematic evolution of the mid-crustal core of the Paparoa metamorphic core complex. The orientation and evolution of ductile and brittle structures throughout the core are presented, as are rigorous kinematic analyses including kinematic vorticity. A discussion of the results focuses on the dominant processes accommodating extension at a variety of scales.
Chapter 4 presents the methods, results, and interpretations for strain and kinematic analyses. While kinematic analyses are presented as part of Chapter 3, they are expanded upon here and include a full discussion of background and assumptions. A three- dimensional strain analysis conducted on both quartz and feldspar grain shapes is presented here, complete with a discussion of background and assumptions as well.
Chapter 5 is a summary of the most important conclusions drawn from this study, and includes a section on suggestions for future work.
5 CHAPTER 2: Literature Review
2.1 INTRODUCTION
After the general acceptance of plate tectonics, the focus of many studies became the evolution of continental crust as it became involved in a wide variety of tectonic settings (see below for examples). Of particular interest became the mechanisms and manifestations of continental extensional tectonics in the Earth’s crust. Geologists began investigating areas of active extension, such as in the Red Sea and in east Africa, as well as areas where extensional deformation affected older rocks in the record such as collapsed mountain belts and passive continental margins. Significant attention was paid to the effects (thermal, structural, and chemical among others) the construction and collapse of mountain belts exerted on continental lithosphere. In general, these studies may be divided into two domains: those examining deformation associated with the contractional phase of orogenesis, i.e. mountain building, and those examining the extensional phase, or orogenic collapse. This thesis is concerned with the “structural fingerprint” left on the continental crust below present day New Zealand as it became involved in a major plate boundary reorganization and transition from crustal contraction to crustal extension. This chapter provides a brief history of thinking on the mechanisms by which continental crust structurally reaponds to extension under both ductile and brittle P-T conditions.
2.2 CONTINENTAL EXTENSION
6 Early workers in continental extensional terranes argued extension was accommodated by large-scale pure shear and igneous dilation of the lithosphere (Proffet
1977; Wright and Troxel 1969, 1973; Anderson 1971; Armstrong 1968, 1972) (Figure
2.1). According to these models, the crust is thermally weakened by magmatic activity, then flattened and deformed dominantly by coaxial strain. Stretching of the lithosphere in this manner was supported by McKenzie (1978) in his study of normal fault bounded sedimentary basins. McKenzie suggests sedimentary basins produced by stretching and slow cooling of the lithosphere are likely to be found in two geological environments: rifted margins and back arc basins.
An alternative to the pure shear stretching model was proposed by Brian
Wernicke (1985) known as “uniform-sense normal simple shear” of continental crust
(Figure 2.1). The basis of this model was the discovery of many low-angle faults in the
Basin and Range province with large-scale (tens to hundreds of kilometers), of normal offset (Wernicke, 1981). Wernicke suggested these low-angle extensional faults may be rooted deep in the lithosphere as ductile shear zones forming a continuous, curved dislocation through tens of kilometers of crust. His hypothesis was a mechanism for generating very large amounts of extension without generating excessive relief. Wernicke eloquently writes in his concluding remarks, “Thus, one cannot conclude that the mantle lithosphere and lower crust penetratively flatten directly beneath discontinuous upper- crustal normal fault systems… (and) There is no means of theoretically predicting what the strain field of a compositionally heterogeneous, extending lithosphere will be, and it is therefore worthwhile to entertain any hypothesis that is consistent with observations
7 that bear directly on strain geometry.” (Wernicke, 1985 p. 122). Wernicke’s words nicely sum up why there is still healthy debate surrounding the style of extensionally deformed continental crust.
Shortly after Wernicke’s low-angle normal fault hypothesis was published in the journal Nature, Wernicke and Burchfiel (1982) published a review of structures, mainly brittle structures, found to accommodate extension in the Earth’s crust. They decided to establish two classes of structures: rotational and non-rotational. Rotational structures are those in which extension is accommodated by rotation of geological features, namely bedding planes. Wernicke and Burchfiel (1982) argue that two fault geometries are possible for this class, planar and listric (curved). Extension by motion on listric, rotational faults generally produces a series of imbricate, normal fault bounded slices.
Syntectonic sedimentary “growth basins” are also commonly associated with this type of faulting. Non-rotational extension does not require curved fault planes, thus all faults associated with this mechanism are planar. Wernicke and Burchfiel (1982) show field examples of both classes of extensional structure, exposed in the Basin and Range province and interpreted from seismic profiles.
A number of key conclusions were drawn by Wernicke and Burchfiel (1982) regarding modes of extension. (1) Extended and actively extending terranes usually accommodate tensile stresses by a combination of rotational and non-rotational structures, and thus by a combination of curved and planar, high-angle and low-angle normal faults. From this analysis, followed the burden of the field geologist to prove that faults belonged to one class or the other, as incorrect diagnosis leads to large errors in
9 calculated values of extension. (2) Large-scale extension is accommodated by large displacements on planar, low-angle normal faults (non-rotational) and contemporaneous rotation of imbricate fault blocks bound by listric and planar faults.
In April, 1985 the Geological Society of London held a conference on continental extensional tectonics. One of the most interesting papers to come out of this event was written by J.A. Jackson (1987) who discusses the nature of brittle faults directly related to seismic activity in extensional regimes. Jackson’s data set includes first motion analyses of earthquakes in extensional settings around the globe including Greece, Turkey, western USA, NE China, Yunnan and East Africa. He concludes that the “overwhelming majority” of seismically induced normal faults nucleate in the depth range 6-15 km, and dip between 30 and 60°. Jackson also indicates the faults in his study, where exposure or seismic lines are good enough, appear to be approximately planar from the surface through the upper crust.
Jackson’s work calls into question, but does not refute, the claims made by
Wernicke and Burchfiel (1982) that large scale extension is accommodated by slip on low angle (10-20°), planar normal faults. The theories may be reconciled, if slip on low- angle faults could be proven to be aseismic, or if planar low-angle normal faults exposed at the surface today represent the exhumation of shallow dipping, ductile shear zones below the nucleation depth of most extension induced earthquakes. A third hypothesis suggested by Davis (1987) is that some low angle normal faults may have been uplifted by large scale, range bounding steep normal faults.
2.3 METAMORPHIC CORE COMPLEXES
10 A decade before Wernicke presented his model for Basin and Range evolution, a team of geologists in western North America were discovering a new structural/tectonic province: the metamorphic core complex (e.g. Armstrong, 1972; Coney, 1973a,b, 1976,
1978, 1979; Davis and Coney, 1979; Crittenden et al., 1980). In general, metamorphic core complexes are terranes characterized by a heavily eroded, brittley deformed upper plate separated from a generally younger ductiley deformed lower plate by a shallow dipping detachment fault. The upper plate generally consists of low grade to unmetamorphosed cover rocks deformed by populations of planar, “domino” style and listric normal faults (Crittenden et. al., 1980). These normal faults always splay into the underlying detachment or “decollement”. Graben and half-graben structures are common in the upper plate, as are fault bounded sedimentary basins (Crittenden et. al., 1980).
Lower plate rocks, sometimes referred to as “basement” or “core” rocks, comprise deformed plutonic and metamorphic rocks of varying age and protolith (Crittenden et. al.,
1980). Metamorphic grade in lower plate rocks is distinctly higher than in the upper plate; kyanite, sillimanite, and andalusite bearing assemblages are not uncommon
(Reesor, 1970; Armstrong, 1968; Compton and others, 1977; Tulloch and Kimbrough,
1989). Recent work in the footwall of the Snake Range decollement in eastern Nevada and western Utah indicates stable mineral assemblages such as: quartz+muscovite+biotite+garnet+plagioclase+staurolite±kyanite±tourmaline±apatite, suggesting peak metamorphic pressures of 810 ± 70 MPa (30 ± 3 km) and temperatures >
600° C (Lewis et al., 1999). Lower plate rocks are characterized by localized zones of high ductile strain, where core rocks become mylonitized and generally display an arched
11 regional foliation pattern where maximum dips on the flanks of arches rarely exceeds 20-
30°. Lineations generally plunge shallowly, parallel to inferred motion of upper plate transport and locally sub-parallel to the axes of regional arching. Ductile deformation in lower plate rocks is overprinted by brecciation and other brittle structures with increasing proximity to the detachment surface. Ductile fabrics are truncated, abruptly, in some cases by the detachment surface owing to late stage movement on the decollement
(Coney, 1974). Granitic plutons of varying composition and size intrude the lower plate at different stages during deformation. These plutons are useful for both deformation studies and thermal studies of evolving core complexes.
2.4 EVOLUTION OF DETACHMENT ZONES AND LOWER PLATE ROCKS
While most metamorphic core complexes identified in the field display the general characteristics described in the preceding section, significant variety exists when examining detailed styles of deformation in both upper plate and lower plate rocks. Of particular interest to this study are the evolution of lower plate structures recording the multistage development of core/footwall exhumation.
In 1986, Gregory Davis, Gordon Lister and Stephen Reynolds, attempted to interpret many of the observations found in metamorphic core complexes by proposing a model for the structural evolution of the Whipple and South mountains in California and
Arizona (Davis et al., 1986). Their model suggests a five stage development beginning with (1) the formation of a sub-horizontal zone of mylonitic fabrics of variable thickness
(3.5 km in the Whipples and less than 200 m thick in the South Mountains) within lower plate rocks. This event may rotate any previously formed structures into parallelism with
12 mylonitic fabrics and structures. Mylonitic fabrics may also cut older fabrics discordantly. (2) A later generation of mylonitic fabrics forming narrow (0.1-10 cm thick) zones of high shear strain cuts and/or rotates early formed fabrics (including mylonites) at high angles. (3) A zone of chloritic breccia up to 300 m thick (in the Whipples) forms as a result of shearing, brecciation, and retrograde alteration of kinematically inactive mylonites and non-mylonitic lower plate rocks. Pseudotachylite veins may splay off from subhorizontal breccia surfaces. Davis and others (1986) suggest this may be the result of seismically related fault melts. (4) Formation of throughgoing, discrete, low-angle detachment faults underlain by thin zones (< 1 m) of microbreccia or ultracataclasite. (5)
The last stage involves shallow level detachment faulting and the formation of fault gouge in upper plate rocks directly above the detachment. This model is useful for describing the evolution of lower plate rocks in a general sense, however the geometry of ductile structures such as the “mylonitic front” and polyphase folds are not addressed here.
Mylonitic front The term “mylonitic front” was introduced by Davis and others (1980) to describe the structure of lower plate rocks affected by penetrative foliations and lineations.
Mylonites are characterized by ductility contrasts between mineral constituents, namely quartz and feldspar, where the former behaves plastically through grain boundary migration and dynamic recrystallization and the latter behaves brittley through microfracture. In the Whipple and Buckskin Mountains of southeastern California and western Arizona, other minerals shown to flow ductiley include micas and some amphiboles while brittle deformation mechanisms occur in some amphiboles, epidote,
13 and sphene (Davis et al., 1980). Lithological units that become mylonitized include both basement rocks and well as syntectonically injected granitic plutons (Davis et al., 1980).
As a result, mylonites may differ in hand sample texture, grain size, and composition.
Davis (1980) emphasizes that the mylonitic front is a well-defined, mappable structural boundary between mylonitic and non-mylonitic rocks, and that it is not a discrete fault surface.
Based on field observations from the South Mountains, Santa Catalina, and
Harcuvar core complexes in southern Arizona, Reynolds and Lister (1990) proposed a model for the structural geometry and kinematic evolution of the mylonitic front. In each of these core complexes, the mylonitic front is folded, characterized by a gently dipping limb sub-parallel to the overlying detachment and a steeper, back-dipping limb (Figure
2.2). Kinematic indicators present in the fabric on both limbs reveal that rocks preserving sense of shear information were crystallized prior to folding of the mylonitic front. For example normal, top to the NE fabrics on the gently dipping limb are accompanied by reverse, top to the NE fabrics in the back dipping zone. Additionally, mylonitic fabrics in the back-dipping zone are cut by steeper, antithetic shear zones characterized by retrograde mineral assemblages. Reynolds and Lister (1990) attempt to interpret these observations by evaluating a series of potential models for the structural and kinematic evolution of the mylonitic front.
2.5 WESTERN NEW ZEALAND AS AN EXTENSIONAL SETTING
Prior to the undertaking of this study, it was recognized that northwest Westland
(Figure 2.3) had experienced regional continental extension in the Late Cretaceous
15 (Tulloch and Kimbrough, 1989; Tulloch and Palmer, 1990; Bishop, 1992; Spell et al.,
2000). By the time Tulloch and Kimbrough (1989) presented their core complex model for extensional deformation, the local geology was well mapped and age data suggesting a major metamorphic/cooling event in the late Cretaceous has already been proposed for the lower crust exposed in Fiordland (Gibson and Ireland, 1988). Northwest Westland contains the following elements characteristic of an extended continental terrane:
Structures
1. Two, opposite dipping, low angle (10-15°) normal faults, or detachment faults of the type described by Wernicke (1981)(Figure 2.3). These were identified by Tulloch and
Kimbrough (1989), who suggested on the basis of new age data determined for the
Charleston Metamorphic Group, that the faults were the dominant structures in a symmetrical metamorphic core complex. In the hanging walls of both faults rests the
Greenland Group, a sedimentary sequence characterized by greywackes and argillites and containing Ordovician graptolite fossils (Laird, 1972; Nathan, 1978). Ductile deformation in the footwall of these faults suggests motion was top to the NE in the northern half of the core (exposed in Buller Gorge) and top to the SW in the southern half of the core
(exposed in Pike Stream). A maximum estimate for displacement on the northern fault
(Ohika detachment of Tulloch and Palmer, 1990) using a 14-18 km emplacement depth for the Foulwind Granite (Spell et al., 2000) and assuming a continuous, planar slip surface is 60-80 km.
16
2. Offshore and onshore, WNW-ESE trending, normal fault bound basins occupied by up to 5 km thick sequences of Mid-Cretaceous, nonmarine breccia-conglomerates and fluvial sedimentary rocks of the Pororari group (Nathan et al., 1986; Bishop, 1992).
Deformed granitic clasts in the Hawk’s Crag Breccia (northern Paparoa range) studied by
Tulloch and Palmer (1990) are believed to be derived from Early Cretaceous source plutons. Syn-extensional sedimentary deposits are separated from younger rift deposits by a major regional unconformity of early Haumurian age (Nathan et al., 1986). The age of the unconformity is just younger than the age of the oldest ocean crust in the Tasman sea (Weissel & Hayes, 1977) causing some workers to favor a breakup unconformity model marking the transition from rift to drift (Falvey, 1974; Braun & Beaumont, 1989;
Bishop, 1992).
Magmatism
Igneous activity, both intrusive and extrusive, played a significant role in the events leading up to rifting of the Pacific margin of Gondwana (Tulloch and Kimbrough,
1989; Bishop, 1991; Waight et al., 1998a; Waight et al., 1998b). Subduction related magmatism associated with the contractional phase of orogenesis produced the Western
Fiordland Orthogneiss (WFO) in Fiordland and the Separation Point Suite (SPS) in
Westland. These rocks have ages in the range of ∼125-114 Ma, and are characterized geochemically by distinctive alkali-calcic, sodic compositions, high Sr content and high
Sr/Y ratios (Muir et al., 1995). This geochemical signature is described as “adakitic”, and is thought to represent melting of a mafic crustal underplate of garnet amphibolite or
18 eclogite composition beneath a thickened arc (Muir at al, 1995). The SPS is well east of
the PMCC and does not bear a penetrative ductile fabric.
Early to Middle Cretaceous plutonism in the western and southern regions of
Westland is dominated by the I-S type Rahu Suite (Tulloch, 1983; Tulloch &
Braithwaite, 1986). Rahu Suite plutons have crystallization ages as old as 114 Ma
(Tulloch and Kimbrough, 1989). These plutons are generally granitic in composition and
display evidence for crustal contamination (Graham & White, 1990). Plutons such as the
Buckland Granite (Figure 2.3) in the northern Paparoa Range display evidence for syn-
extensional deformation, uplift and cooling during a ∼30 m.y. period beginning ∼112-110
Ma (Tulloch and Kimbrough, 1989; Tulloch and Palmer, 1990; Spell et al., 2000). Mid
Cretaceous magmatism is dominated by the Hohonu Suite, dated at ∼114-110 Ma
(Waight et al., 1998a). Hohonu Suite plutons are characterized by amphibolite-biotite tonalite to muscovite-biotite monzogranite compositions and are typically calc-alkaline
(Waight et al., 1998). The emplacement of the Hohonu Suite overlaps in time with the end of subduction related magmatism (SPS) and the onset of crustal extension (Waight et al., 1998a).
Mid-Cretaceous dikes have been described in numerous localities in Westland
(Nathan, 1978; Tulloch and Kimbrough, 1989; Bishop, 1992; Waight et al., 1998b).
These dikes are dominantly alkaline and have been described as lamprophyres. The
Hohonu Dike Swarm in the Hohonu Range, dated at ∼84 Ma (Waight et al., 1998b), has been interpreted as synchronous with extension as it coincides with the generation of the first ocean crust in the Tasman Sea. Bishop (1992) compiled orientation data for 393
19 dikes thought to be Cretaceous in age and found a bimodal distribution, where the
dominant trend is 120° (WNW-ESE) and the secondary trend is 020° (NNE-SSW). The
dominant trend measured in the study is consistent with a NE-SW extension direction.
Thermochronologic evolution
Isotopic studies performed on samples from plutons that intruded the lower plate
during deformation allow for lower plate cooling histories to be reconstructed (Tulloch
and Palmer, 1990; Muir et al., 1994, 1995, 1997; Ireland and Gibson, 1998; Spell et al.,
2000). A 109.6±1.7 Ma crystallization age for the Buckland Granite by Muir et al.,
(1995) and a 111.0±0.8 Ma monazite Th/Pb age was determined by Ireland and Gibson
(1998). Spell et al., (2000) show the Buckland Granite was unroofed immediately after
emplacement, cooling at a rate of ∼35°-60°C/Myr until ∼92 Ma, when the rates slowed
considerably to an average of 8°C/Myr. A similar cooling history was reconstructed by
Spell et al., (2000) for the Okari granitic gneiss in the southern Paparoa range. These
samples indicate rates of ∼50°C/Myr between ∼110 and 90, then slower cooling at a rate of ∼5°C/Myr through ∼80 Ma.
Tulloch and Palmer (1990) presented a variety of isotopic results obtained from clasts in the Hawk’s Crag Breccia , a member of the Pororari Group in the northern
Paparoa range. The petrology of the clasts led Tulloch and Palmer to propose the
Buckland Granite, the Blackwater Pluton and possibly the Steele Pluton as sources.
Muscovite K-Ar age data suggest the source was uplifted through the 350°C isotherm at
20
∼112 Ma. This age is slightly older than the ages determined by Spell et al., (2000).
Using an age for the Hawk’s Crag Breccia of ∼104 Ma (Raine, 1984; Nathan et al.,
1986) Tulloch and Palmer (1990) suggest uplift to the surface (a 10 km vertical distance
assuming a geothermal gradient of 35°C/km) was accomplished in less than ∼7 Ma,
suggesting a minimum uplift rate of 1.4 mm/yr (or 1.1 mm/yr at a gradient of 45°C/km).
Sericite ages obtained from hydrothermally altered samples close to the Ohika and Pike
detachment faults indicate ages of ∼98 Ma and ∼85 Ma respectively. Tulloch and Palmer
(1990) interpret these results as evidence the Pike detachment remained active longer
than the Ohika detachment, and suggest the ∼85 Ma age links motion on the Pike
detachment to continental rifting and the opening of the Tasman Sea.
The reconstructed thermal histories determined by previous workers are
consistent with an initial period of rapid cooling followed by protracted cooling rates
preceding continental rifting at ∼84 Ma. This trajectory is common to other metamorphic core complexes, however overall cooling rates tend to vary. For example, the South
Mountains metamorphic core complex in Arizona cooled from ∼700°C to ∼100°C in less than 5 million years, equating to an average rate of 192 ±74° C/Myr (Livaccari, 1995).
The PMCC displays a cooling history that lasted 4 to 5 times longer. The Shuswap
Metamorphic Core Complex in southern British Columbia, has a thermal history akin to the PMCC. A rate of 50°C/Myr persisted for the first ∼8 Ma, followed by slower cooling at 16°C/Myr for the next ∼12 Ma (Vanderhaeghe et al., 2002). While initial cooling rates for the Shuswap and the PMCC are nearly identical, the second phase of cooling differs by a factor of two. In sum, the PMCC thermal history displays the characteristic core
22 complex trajectory of initial rapid cooling followed by slower rates, however, overall the
PMCC is an example of a core complex that cooled relatively slowly.
2.6 CASE STUDIES OF POST-OROGENIC EXTENSION
Extensional deformation of continental crust may occur in a wide range of tectonic settings: continental rift valleys, back arc basins, collapsed orogenic belts, or regions of anamolously high heat flow. For the purposes of this study extensional deformation may be grouped into two categories: (1) deformation produced by a period of continental extension that followed a period of contraction and crustal thickening; (2) deformation produced during continental extension that did not follow a period of contraction and thickening. This study is interested in the styles of deformation produced during the former, as western New Zealand records a phase of contractional deformation contemporaneous with subduction and terrane accretion during a ∼15 Ma period beginning around ∼129 Ma (Hollis et al., 2003). In this section, styles of extensional deformation in other collapsed mountain belts, namely the Himalaya, the Canadian
Cordillera, and the Scandinavian Caledonides are presented for comparison with the data presented in Chapter 3.
Many theories have been proposed for how mountain belts accommodate extension. The signature of the tectonic shift from contraction to extension, the focus of this study, is linked to the mechanism or cause of collapse. Coney and Harms (1984), suggest that the collapse of the Canadian Cordillera was caused by compressional crustal thickening and associated plutonism. This study is based on palinspatic reconstructions of continental crust along decollment surfaces, which reveal the presence of a 60 km thick
23 crustal welt in the hinterland. P-T conditions at the base of the welt were sufficient to produce crustal melts. According to this study, injection of these magmas effectively reduced intraplate stress and accommodated extension by gravitational collapse.
Burchfiel et al., (1992) and Edwards et al., (1996) present evidence for syn- orogenic extension in the high Himalaya. E-W striking, low-angle detachment faults in the upper crust were found by these workers to be contemporaneous with and parallel to contractional faults and shear zones in the lower crust. Burchfiel’s (1992) model attributes syn-orogenic extension to the gravitational instability of overthickened crust.
Edwards et al. (1996) build on Burchfiel’s work by suggesting that extension in the
Himalaya, so far, has occurred in two distinct periods separated by folding, major plutonism and uplift.
Vanderhaeghe and Teyssier (1997; 2001b; 2002) emphasize partial melting, thermal weakening, and buoyancy forces as factors which strongly influence the shift from contraction to extension. These workers favor a model in which the generation of a partially molten layer during mountain building controls the evolution of the crustal column. They suggest the presence of a partially molten layer leads to mechanical decoupling of the upper and lower plate during subduction, a result which allows orogens to flow independently. The effects of partial melting during mountain building have been studied in the Tibetan Plateau, the Shuswap range in the Canadian Cordillera, the French
Massif, and in the Greek isles (Vanderhaeghe and Teyssier, 1997; 2001).
The Scandinavian Caledonides formed as a result of Late Silurian-Early Devonian collision between Baltica and Laurentia during what is known as the Scandian orogeny
24 (Stephens & Gee, 1985). The resultant architecture of the belt is a series of stacked nappes, where the farthest traveled exotic terranes represent the uppermost allochthons
(Roberts, 1988). The thickness of the Caledonide Orogen is thought to have reached 100-
120 km, as peak metamorphic conditions are indicated by microdiamond and coesite bearing eclogites in the Western Gneiss Region of western Norway (Smith, 1984;
Dobrzhinetskaya et al., 1995). The dominant mechanism for extension in the region is still under debate, however many studies have identified extensional shear zones that nucleated at nappe contacts (Norton, 1986; Fossen, 1992, Fossen & Dunlap, 1998;
Osmundensen et al., 1998; Osmundsen et al., 2003). In this case, pre-existing anisotropies in the architecture of the mountain belt exerted a strong influence on the style of extensional deformation.
25 CHAPTER 3: Tectonics manuscript
STRUCTURAL EVOLUTION OF THE MIDDLE CRUST DURING EXHUMATION BY CONTINENTAL EXTENSION: THE PAPAROA METAMORPHIC CORE COMPLEX, SOUTH ISLAND, NEW ZEALAND
ABSTRACT During the period ∼112-84 Ma, regional extension affected the Pacific margin of the Gondwana supercontinent. This event produced the Paparoa Metamorphic Core Complex in Westland (northwest South Island) New Zealand, where mid-crustal rocks in the core have been exhumed by NE and SW dipping, low-angle detachment faults. Core rocks, exposed in discontinuous sections along the NW coast of the South Island, record a history of at least five events (D1-D5) of ductile, semi-brittle and brittle deformation. We used these exposures to determine the structural and kinematic evolution of the middle crust during Late Cretaceous regional extension. Ductile deformation (D1-D4) is distributed heterogeneously throughout the core. In the north, foliations developed in deformed granitoids are regionally folded about shallow E plunging axes; mineral lineations generally plunge ENE. In the south, the most intense ductile deformation is focused and exposed along a 5 km coastal section near White Horse Creek (WHC). Multiple generations of folds, foliations and lineations developed in the Charleston Metamorphic Group gneisses are interfolded with a thin (< 50 m thick) section of protomylonites, mylonites and ultramylonites. At least three generations of coaxial NE-SW-plunging folds (F2,F3,F4) are developed within the main mylonitic zone. A strain gradient within the WHC transect is defined by an increase in fold tightness and fold axis rotation (F2,F3) towards the direction of maximum extension. Kinematic vorticity (Wk) analysis using systems of rotated plagioclase feldspar porphyroclasts, indicates the main mylonitic zone was characterized by thinning in the vertical (XZ) plane, and by an environment of general shear where simple shear dominated over pure shear (0.74 < Wk < 0.78). Sense of shear analysis on planes parallel to gently plunging mineral lineations and perpendicular to foliation suggests normal motion, top down to the SW and NE. Based on the orientation of D1-D4 structures, the WHC strain gradient and Wk results, a three-dimensional environment of deformation characterized by constriction (X-extension; Z,Y-shortening) is proposed. D5 deformation produced a series of NE, SE, and SW dipping semi-brittle and brittle faults that cut all previously formed ductile structures. Kinematic analysis of fault-slip data indicates strains produced during faulting were characterized by vertical shortening and sub- horizontal (NE-SW) extension. These results suggest the directions of maximum extension (X) and shortening (Z), as well as sense of shear stayed constant during the transition from ductile to brittle deformation in the middle crust.
26 3.1 INTRODUCTION
Previous work in extensional terranes has shown that low-angle detachment faults and the formation of metamorphic core complexes are common products of continental extension (e.g. Crittenden et al., 1980; Wernicke, 1981; Lister and Davis, 1989).
Detachment faults are capable of exhuming deep (>20 km) crustal levels rapidly
(>2mm/yr) without requiring extreme topographic relief because of their low-angle orientations (e.g. Wernincke, 1981; Lewis et al., 1999). Studies have shown ductile deformation in the footwall of detachments and brittle deformation in the hanging wall are temporally and kinematically related (e.g. Lister and Davis, 1989; Tulloch and
Palmer, 1990; Bishop, 1992). However, determining how structural and kinematic patterns change as rocks move from mid-crustal levels to upper crustal levels, is an important process that has received less attention. This aspect is less well known, partially because in most exposures of deformed middle crust the inadequate memory of rocks leads to a lack of preservation of structural changes and strain history during rapid uplift and exhumation.
In this paper, we present the structural and kinematic evolution of the mid-crustal core of the Paparoa Metamorphic Core Complex (PMCC) in Westland, New Zealand, where structures record the transition from ductile to brittle deformation during the period
∼112-84 Ma (Tulloch and Kimbrough, 1989). The prolonged cooling history (>25 M.y.) recorded by lower plate rocks (Tulloch and Palmer, 1990; Spell et al., 2000), as compared to some other core complexes (e.g. South Mountains MCC, ∼5 M.y.) may account for the preserved structural changes (Livaccari et al., 1995) (Figure 2.4). We build on previous
27 studies of crustal structure in the core of the PMCC (Shelley, 1970; Shelley, 1972;
Tulloch and Kimbrough, 1989) and show continental extension resulted in the formation of a narrow (< 50 m thick) mylonitic shear zone characterized by vertical shortening and sub-horizontal (NE-SW) stretching. As the ductile middle crust was exhumed into the brittle upper crust, stress was relieved by slip on systems of conjugate, brittle normal faults. We use fault-slip data to develop kinematic solutions describing strain geometry during brittle deformation, and develop a kinematic model for the progressive deformation of exhumed middle crust by comparing these results to the geometry and kinematics of ductile deformation. This results of this study add a critical component, the structural and kinematic evolution of the middle crust, to published models that document the extensional system that dominated crustal evolution in New Zealand during the Late
Cretaceous.
3.2 PREVIOUS WORK
Field observations, isotopic analyses, geophysical data and plate reconstructions indicate that northern Westland experienced a period of regional continental extension during the period ∼112-84 Ma. This section summarizes the results of this previous work.
3.2.1 Regional geologic and structural relationships
Rocks north of the Alpine Fault on New Zealand’s South Island comprise fault- bounded terranes of early Paleozoic to Mesozoic age belonging to both the Eastern and
Western provinces (Landis and Coombs, 1967). Eastern and Western province rocks are
28 separated by the Median Tectonic Zone (MTZ), a region of plutonism and deformation that probably formed in situ, adjacent to the Western province (Mortimer et al., 1999).
This study is predominantly concerned with extensional structures affecting Western province rocks, most specifically those belonging to the Buller terrane (Cooper, 1979).
The dominant lithologies within the Buller terrane are quartzite, quartz rich turbidites and black shales of the Greenland group (Figure 3.1). Graptolite bearing sedimentary rocks indicate a Cambrian-Ordovician age of deposition. The paleo-Pacific, passive continental margin of Gondwana has been suggested as the environment of deposition.
Buller terrane rocks were intruded by two main pulses of granitic magmatism, one in the Devonian which produced the S-type Riwaka complex and one in the Cretaceous which produced the I-S type Rahu suite (Tulloch, 1983; Tulloch and Brathwaite, 1986;
Tulloch, 1988). The Foulwind granite, exposed along the Tasman coast south of Cape
Foulwind, may be related to the Devonian pulse, as Muir reported a U/Pb ion probe zirocn age of 327 +/- 6.2. Spell et al., (2000) reported Ar/Ar ages of 109.0 +/- 1.0, 101.1
+/- 1.1, 97 Ma for hornblende, biotite, and k-feldspar respectively. These data suggest crystallization in the Devonian followed by a Cretaceous metamorphic event.
Variably deformed plutons of the Cretaceous age Rahu suite, mixed with metasedimentary rock of uncertain affinity crop out along the Tasman coast in the
Charleston region and inland in the southern Paparoa range. Common to these exposures is a ductile fabric of varying strength, which led Nathan (1975) to propose the term
Charleston Metamorphic Group (CMG) for the granites, orthogneisses and paragneisses.
Thus, these rocks were first grouped on the basis of texture rather than age. Early workers
30 considered the CMG to be as old as Precambrian although Aronson (1968) reported a Rb-
Sr mineral age of 104-212, suggesting metamorphism in the Cretaceous. Tulloch and
Kimbrough (1989) reported a conventional U/Pb zircon age of 114 +/- 18 for four orthogneiss samples collected near Charleston, and suggested an early Cretaceous crystallization age for at least some of the CMG. This conclusion was supported by
Ireland and Gibson (1998) who reported U/Pb age of 119 +/- 2 Ma on a zircon core from a sample near Charleston, and a 109 +/- 5 Ma rim using an ion microprobe.
Excellent age control indicates the portion of the CMG most central to this study is characterized by crystallization in the early Cretaceous followed by metamorphism within 5 Ma.
Unconformably overlying the Cambro-Ordovician rocks and the meta-igneous basement are mid to late Cretaceous sediments deposited in onshore and offshore fault bounded basins, which trend dominantly WNW-ESE and NNE-SSW. (Nathan, 1986;
Bradshaw, 1989; Bishop, 1992). Basin fill includes alluvial fan, fluviatile and lacustrine sediments and reaches thicknesses > 4 km (Bradshaw, 1989). In the study area these are dominated by the mid-Cretaceous Pororari Group which comprises breccia conglomerates or “fanglomerates” (Nathan, 1986). Laird (1990) showed that Pororari
Group deposits were derived from active normal fault scarps and deposited in half grabens trending WNW-ESE. Bishop (1992) on the basis of onshore and offshore basin geometry concluded that WNW-ESE trending basins were temporally associated with
Cretaceous NE-SW extension, and NNE-SSW trending basins may postdate continental rifting and the opening of the Tamsan Sea at 84 Ma. A major “break up” unconformity
31 and marine transgression mark this transition. Volcanic rocks intrude many of the
Cretaceous-Paleocene sedimentary basins suggesting volcanism was coeval with extension (Gage, 1952; Nathan, 1978; Newman, 1985; Sewell et al., 1988).
Late Cretaceous lamprophyre dikes of alkaline, basic and acidic compositions intrude older rocks in many localities in north Westland (Hamil, 1971; Nathan, 1978).
Bishop (1992) compiled dike orientation data presented by Suggate (1957), Warren
(1967), Hamil (1972), Nathan (1978), Grindley & Oliver (1979), White (1987), Laird
(1988), and Roder & Sugggate (1990) and showed the distribution to be bimodal with a main trend at 120° and a secondary trend at 020°. The dominant orientation is consistent with a NE-SW extension direction.
3.2.2 Paparoa metamorphic core complex
The unconformable, older over younger contacts of Greenland Group turbidites and CMG gneisses, complemented by new age data for CMG units led Tulloch and
Kimbrough (1989) to propose a metamorphic core complex model for crustal extensional deformation. Many geological attributes of the PMCC make it generally conformable to the core complex model synthesized from observations made in western North America
(Crittenden et al., 1980). The most significant departure from this model, is, that the
PMCC is characterized by two opposite-dipping crustal detachments, making it a
“symmetrical” core complex. Ductile deformation in lower plate rocks near detachment surfaces indicates upper plate transport down to the NE on the northern end of the core and down to the SW on the southern end of the core (Tulloch and Kimbrough, 1989).
Tulloch and Palmer (1990) have shown that motion on the Ohika (northern) detachment
32 may have ceased before motion on the Pike (southern) detachment. These workers
suggest, on the basis of an 85 Ma age for a hydrothermally altered mylonite below the
Pike detachment, that motion on the Pike detachment is linked to continental rifting and
the opening of the Tasman Sea at 84 Ma. If this assertion is true, deformation should
strongly favor the southern end of the core. We test this hypothesis by presenting
rigorous structural and kinematic data from both the northern and southern ends of the
core.
3.2.3 Cooling history of the PMCC
Isotopic studies performed on samples from plutons that intruded the lower plate
during deformation allowed previous workers to reconstruct cooling histories (Tulloch
and Palmer, 1990; Muir et al., 1994, 1995, 1997; Ireland and Gibson, 1998; Spell et al.,
2000). A 109.6 +/- 1.7 Ma crystallization age for the Buckland Granite by Muir et al.,
(1995) and a 111.0 +/- 0.8 monazite Th/Pb age was determined by Ireland and Gibson
(1998). Spell et al., (2000) show the Buckland Granite was unroofed immediately after
emplacement, cooling at a rate of ∼35°-60°C/Myr until ∼92 Ma, when the rates slowed
considerably to an average of 8°C/Myr. A similar cooling history was reconstructed by
Spell et al., (2000) for the Okari granitic gneiss in the southern Paparoa range. These
samples indicate rates of ∼50°C/Myr between 110 and 90, then slower cooling at a rate of
∼5°C/Myr through 80 Ma.
Tulloch and Palmer (1990) presented a variety of isotopic results obtained from clasts in the Hawk’s Crag Breccia (member of the Pororari Group). The petrology of the clasts led Tuloch and Palmer to propose the Buckland Granite, the Blackwater Pluton and
33 possibly the Steele Pluton as sources. Muscovite K-Ar age data suggest the source was
uplifted through the 350°C isotherm at ∼112 Ma. This age is slightly older than the ages
determined by Spell et al., (2000). Using a depositional age for the Hawk’s Crag Breccia
of ∼104 Ma (Raine, 1984; Nathan et al., 1986) Tulloch and Palmer (1990) suggest uplift
to the surface (a 10 km vertical distance assuming a geothermal gradient of 35°C/km)
was accomplished in less than ∼7 Ma, suggesting a minimum uplift rate of 1.4mm/yr (or
1.1 mm/yr at a gradient of 45°C/km). Sericite ages obtained from hydrothermally altered samples close to the Ohika and Pike detachment faults indicate ages of ∼98 Ma and ∼85
Ma respectively.
The reconstructed thermal histories determined by previous workers are consistent with an initial period of rapid cooling followed by protracted cooling rates preceding continental rifting. This trajectory is common to other metamorphic core complexes, however cooling rates tend to vary. For example, the South Mountains metamorphic core complex in Arizona cooled from ∼700°C to ∼100°C in less than 5 million years, equating to an average rate of 192 +/- 74°C/Ma (Livaccari, 1995). The
PMCC displays a cooling history that lasted 4 to 5 times longer. The Shuswap
Metamorphic Core Complex in southern British Columbia, has a thermal history akin to the PMCC. A rate of 50°C/Ma persisted for the first 8 Ma, followed by slower cooling at
16°C/Myr for the next 12 Ma (Vanderhaeghe et al., 2002). While initial rates for the
Shuswap and the PMCC are nearly identical, the second phase of cooling differs by a factor of two. In sum, the PMCC thermal history displays the charcteristic core complex
34 trajectory of initial rapid cooling followed by slower rates, however overall the PMCC is an example of a core complex that cooled relatively slowly.
3.3 DUCTILE DEFORMATION IN THE LOWER PLATE
The metamorphic core of the PMCC is exposed in the central region of the Paparoa range, and along the northwest coast of the South Island. Discontinuous exposure occurs for approximately 10 km between Cape Foulwind south of Charelston (Figure 3.1). In this section we present the ductile structure of the metamorphic core based on detailed structural transects, cross-cutting relationships and regional correlations.
3.3.1 Zone of mylonitic fabrics
Detailed structural mapping within a 4 km long coastal transect (WHC on Figure
3.1) south of Morrissey Creek (Figure 3.2) reveals variably deformed CMG gneisses and a polyphase folded sequence of protomylonitic-ultyramylonitic rocks. CMG gneisses south of White Horse Creek are characterized by granitic compositions that include plagioclase, quartz, biotite ± muscovite. CMG gneisses south of WHC are characterized by a gneissic foliation (S1) that dips to the SSE and mineral lineations (L1) that plunge moderately to the SE. Minor, asymmetric (s-type, z-type, and m-type) folds (F3) of this gneissic layering are visible at many locations along the coast (Figure 3.2). Fold hinges range from being curved to kinked. Wavelengths are on the order of tens of centimeters.
South of WHC, F3 fold axes plunge to the SE and axial planes dip moderately to the S.
North of WHC, F3 fold axes plunge to the SW-NE and axial planes dip to the N. At site
WHC 12 (Figure 3.2) two additional styles of folding affect the CMG. Earlier, tight to
35 Figure 3.2. White Horse Creek structural transect. a.) Map of structural data; b.) Cross- section showing the folded mylonitic front and WHC strain gradient ; c.)-e.) Outcrop
scale field sketches of F2,F3 folds; f.)-i.) Lower hemisphere, equal-area projections of structural data; j.) Block diagram illustrating the geometry of F2-F4 in the highest strain zone.
36 isoclinal folds (F2) of compositional layering in the gneiss are characterized by sub- horizontal axial planes, display sheared out limbs, and are refolded about a younger, upright open fold (F3). The upright, open fold is characterized by a moderately NNW dipping axial plane, and its upright limb is affected by minor kink folding. Thus, this single outcrop displays three generations of superposed, coaxial
(shallow NE-SW plunging) folds in non-mylonitized CMG gneiss.
Protomylonitic-ultramylonitic rocks, classified using clast:matrix proportion as suggested by Passchier and Trouw (1996), crop out discontinuously along the WHC transect. Mylonitic rocks are characterized by grain boundary reduction and dynamic recrystallization in quartz (Figure 3.10), planar alignment of micaceous minerals into well developed S and C surfaces, and feldspar porphyroclasts that commonly are rotated and fractured. Mylonites generally display well-developed foliations, defined by aligned quartz ribbons and biotite ± muscovite crystals, and lineations, defined by quartz rodding, quartz-mica aggregates, and feldspar rodding. Strain appears to be partitioned heterogeneously along the coast, based largely on the percentage of quartz recrystallization and fold tightness (Figure 3.2). The highest strain zone is located between sites WHC3-4, where the most spectacular mylonites and ultramylonites are visible at low tide.
Detailed structural mapping of mylonites exposed along the coast and Coast
Road, together with the aforementioned non-mylonitized CMG structures, allowed us to propose the geometry for the mylonitic shear zone shown in Figure 3.2. Map pattern variation in the dominant mylonitic foliation (S2) within the WHC transect suggests a
38 thickness of ∼50 m for the mylonitic front. A variety of structures affect rocks within the mylonitic front. Early, tight to isoclinal (interlimb angle < 10°) intrafolial folds (F2) with axes that plunge shallowly to the SW and NW are present within the highest strain zone
(Figure 3.2). Folds of S2 occur in four dominant styles: (1) Tight to isoclinal, intrafolial folds (F2) that plunge shallowly NE-SW. (2) Asymmetric, tight-is s and z type folds (F3) of foliation display both rounded and chevron type hinges and plunge SW-NE. (3)
Upright open folds of foliation (F3) with moderate NNW dipping axial planes and shallow WSW plunging axes (Figures 3.2, 3.8). (4) Regional scale (wavelength = tens to hundreds of meters) gentle warps in the foliation (F4) about shallow WSW-ENE punging axes (Figure 3.2). These relationships suggest a sequence of at least four events of deformation, beginning with pre-mylonitic structures south of WHC, followed by the development of mylonitic fabrics and penecontemporaneous F2 folds, outcrop scale F3 folding and regional scale F4 warping.
3.3.2 Central core zone
CMG orthgneisses and paragneisses crop out discontinuously on headlands, cliffs, and bays north and south of Charleston forming a ∼10 km long, measurable structural section.(Figure 3.3). Penetrative ductile fabrics dominate the northern portion of the transect while in the south only weak foliations, mineral and intersection lineations are present. On a regional scale, the foliation is folded into two broad arches with shallow
ESE and ENE plunging axes. Mineral lineations defined by quartz-mica aggregates plunge in accordance with the arch axial surface, generally NNE-SSW. Three styles of
39 folding are observed in the central core zone. Cross-cutting relationships allowed us to determine the following sequence: (1) Asymmetric, tight-intrafolial folds of foliation and compositional layering (F2). (2) Asymmetric, s and z type parasitic folds (wavelength <
10 cm) of foliation (F3 ). (3) Regional scale warping of the foliation and earlier formed axial planes. F2 folds tend to plunge S, SE, and NE while F3’s plunge shallowly to the W.
F4, regional arches plunge shallowly ENE-WSW.
3.3.3 Ductile deformation along the northern border of the core
In the northern Paparoa Range, the Greenland and Pororari Groups are separated from lower plate rocks by the NE dipping Ohika detachment fault (Tulloch and
Kimbrough, 1989; Tulloch and Palmer, 1990). Evidence of ductile deformation in lower plate rocks exposed along the road parallel to the Buller River is limited to a few exposed outcrops (Figure 3.4). Foliations dip to the WNW and mineral lineations plunge to the
NE and NW. These outcrops are <500 m from the mapped detachment fault, yet neither penetrative ductile fabrics nor mylonites are present in exposures along the road.
Approximately 3 km of nearly continuous coastal exposure south of the Cape
Foulwind lighthouse is dominated by the Foulwind Granite of Nathan (1975) (Figure
3.5). Two texturally distinct intrusive phases of the Foulwind Granite are found along the beach south of Cape Foulwind. A megacrystic variety rich in potassium feldspar crops out north of Siberia Bay and on Tauranga Bay’s bounding headlands. This unit, Kf1b of
Nathan (1976), is also rich in biotite which defines the local foliation planes, and quartz that has deformed plastically to form subgrains with undulose extinction. Potassium
41
feldspar megacrysts locally display asymmetric tails, useful for kinematic interpretation
(see section 3.5). South of Cape Foulwind, foliation is folded about two broad arches that plunge shallowly ENE. Quartz-mica mineral lineations plunge gently to the E in the south and SW-NE in the north. In contrast to areas farther south, no intrafolial or single outcrop scale folds are found anywhere along the transect.
A finer grained granitic phase crops out along the coast for less than 1000m beginning on the headland north of Siberia Bay. This phase is devoid of a penetrative ductile fabric, however thin (<5 m thick) proto-ultramylonite zones are developed within the granite (CF2,3). Ductile foliations in deformed rocks dip north and mineral lineations plunge to the NE. Texturally, these samples stand out in marked contrast from the deformed megacrystic phase. Quartz microtextures in samples collected from site CF3 indicate near complete recrystallization, similar to samples collected from the White
Horse Creek mylonite zone. Feldspar grains are deformed dominantly by microfracture and rotation. Quartz deformation textures in the megacrystic Foulwind Granites include subgrains, seriate grain shapes, and undulose extinction, however dynamic recrystallization is not observed. We discuss the localized development of mylonitic fabrics on the Foulwind transect in the next section.
3.4 BRITTLE AND SEMI-BRITTLE DEFORMATION
Brittle and semi-brittle faults affect lower plate rocks exposed along the coast in the north, south and center of the metamorphic core. In general, two styles of faulting are observed: (1) A strictly brittle phase dominated by SW, NE and SE dipping fault planes and slickenlines that plunge down-dip and obliquely down-dip. Offsets on these faults are
44 on the order of tens of centimeters. (2) A “semi-brittle” phase, so named because these faults are associated with localized development of ductile fabrics (discussed in the previous section) ranging from weakly deformed granitoid to ultramylonite. The magnitude of offset on the semi-brittle phase is difficult to constrain because of the absence of appropriate markers. In this section, we present fault-slip data from three domains within the core, southern central and northern. These data are used to generate kinematic solutions for brittle fault populations (Figures 3.6-3.8).
3.4.1 Brittle faults
Brittle faults cut all generations of ductile structure in lower plate rocks at numerous locations within the core (CF1, CH6-7, WHC1,4,10). We measured the orientations of these faults and found that most faults tend to occur in narrow zones, or groups. Here we describe fault geometries observed in the northern, central and southern domains within the core.
Northern domain
The first headland north of Siberia Bay contains an intensely faulted exposure of the Foulwind Granite. The exposure photographed in Figure 6a allowed us to measure a large population of minor (offset 5-10cm) faults in three dimensions. Most of the faults measured in this population lie within the hanging wall or footwall of a low-angle, NE dipping master fault. The exposed length of this fault is approximately 10 m; the fault zone is characterized by localized development of ductile strain and pseudotachlyte.
45
Brittle faults affecting this block generally dip moderately to the E and SE; slickenlines commonly plunge ENE and SSW. We grouped faults within 20 m of site CF1 into a data set and plotted them for comparison. Faults in this latter group (CF1 regional) dip preferentially to the SE and NE, slickenlines plunge to the NE and SSW.
Central domain
CMG gneisses within the Charleston transect are affected by two distinct styles of brittle faulting. A low-angle variety cuts coastal exposure between sites CH6-7. Most faults dip to the NE, and slickenlines plunge approximately down-dip. Ductile deformation in footwall rocks increases with proximity to the fault, and a thin (<5 cm thick) zone of protomylonite is common in the fault zone.
At site CH7 a block of CMG gneiss is cut by a series of SW dipping normal faults displaying a litsric “fault fan” geometry. Steeply dipping early faults splay into a slightly bowed-up, listric master fault. Slickenline orientation suggests motion was nearly down dip. A deformed pegmatite dike indicates meter-scale offset on the minor faults.
Southern domain
At site WHC1 along the Coast Road, brittle faults dip moderately to the SW and steeply to the NE dissecting the limbs of an upright open synform (F3) composed of protomylonites (Figure 8). Brittle faults also cut asymmetric s-folds (F3) and gently dipping crushed zones. Slickenlines on S-dipping faults generally plunge approximately down dip, while slickenlines on N-dipping faults tend to be shallow and obliquely down-
49 dip. A syn-tectonic pegmatite indicates offset is normal and on the scale of tens of centimeters.
3.4.2 Semi-brittle faults
Based on field observations (presented in section 3.2) and kinematic analysis
(presented in section 3.5) we suggest the presence of a “semi-brittle” (semi-ductile) structure capable of producing meter scale thicknesses of moderate to highly deformed rocks. The term semi-brittle, describes structures which accommodate stress by a combination of brittle and ductile deformation mechanisms, such as a brittle fault and a strain softened, ductiley deformed fault zone. We suggest at least two locations within the core (sites CF2,3) may have been affected by such structures. Texturally, rocks at these sites bear little resemblance to the massive, fine grained, undedformed phase of the
Foulwind Granite to which they belong (Figures 3.5, 3.9). Instead they are chacterized by localized, spatially distinct thicknesses of high strain rocks. Kinematic interpretations regarding semi-brittle structures are made in the next section.
3.5 KINEMATIC ANALYSIS
The structurally diverse, variably deformed metamorphic core of the PMCC provided an excellent opportunity to perform a scale independent kinematic analysis of ductile, semi-brittle, and brittle deformation. Here we present the techniques and results of this analysis.
3.5.1 Kinematic analysis of mylonitic rocks
50 Figure 3.9. Outcrop and thin section scale kinematic indicators. a,b.) Mylonitic fabric photographed in the field. S-C fabric and large beta-type feldspar clast directly below pencil indicate dextral motion, parallel to the pencil (lineation); c.) S-C fabric, imbricated, antithetically microfaulted feldspar in the center of field of view, sigma-type clast in the upper left-hand corner indicate dextral motion.Photograph taken under crossed polars with the gypsum plate inserted; d,f.) Plagioclase feldspar porphyroclasts sampled from site WHC3. Large beta-type clast in the upper third of "d."; sigma-type and delta-type clasts in the upper third of "f."; g,h.) S-C fabrics developed in the Foulwind
Granite at sites CF2 and CF3. These fabrics are interpreted as related to semi-brittle faulting. "g." indicates sinistral motion as C planes are oriented NE-SW and S planes are oriented NW-SE. Large beta-type clast in center of field of view. "h." displays well developed dextral S-C fabric.
51
Excellent coastal exposure and extensive sampling of mylonitic rocks allowed us to establish the kinematics of ductile flow associated with the main mylonite zone north of White Horse Creek (Figure 3.2). In the field, three-dimensional exposures facilitated the identification of S and C surfaces on faces perpendicular to foliation and parallel to mineral stretching lineation (XZ plane, where X is parallel to stretching lineation and Z is perpendicular to foliation). Easily visible (cm scale) asymmetric, sigma, beta, and delta type plagioclase feldspar porphyroclasts served as a check on sense of shear deduced from S/C relationships alone (Figure 3.9). Sense of shear information deduced in the field was cross-referenced with and complemented by results from a microscale analysis. At the scale of a thin section, kinematic indicators used include: S-C relationships, mica fish, asymmetric porphyroclast geometry, synthetic and antithetic microfaults affecting feldspars, and LPO in quartz. Our goal was to collect enough data to compare the kinematic signature of the main mylonitic shear zone within the PMCC to previously identified mylonitic zones, particularly those in the western North America (e.g.
Reynolds et al., 1990).
Our results indicate that the sense of shear in most samples within the mylonitic front was normal, top down to the S, SW or NE. Because all measurements were made on the XZ plane, sense of shear is also parallel to the gently plunging mineral lineation (SW-
NE). These results are consistent with 2D, NE-SW sub-horizontal flow. One significant exception to this general trend occurs at site WHC11 (Figure 3.2), where the mylonitic fabric dips N and the mineral lineation plunges NE (consistent with observations made by
Tulloch and Kimbrough,1989). Here, meso and microscale kinematic indicators suggest a
53 reverse, top to the SW sense of shear. This zone may be analogous to back-dipping mylonite zones in western North American core complexes, that commonly display sense of shear reversals. These reversals have been attributed to folding of mylonite zones post- crystallization of ductile kinematic information (e.g. Reynolds et. al., 1990).
Kinematic vorticity analysis
In addition to recording sense of shear distributions among the mylonites, we measured the axial ratios, long axis orientations and tail types on coarse grained plagioclase feldspar porphyroclasts, which are a ubiquitous feature of the highest strain zones (WHC3-4). Measurements were made on multiple outcrops within this zone, spanning a horizontal distance of approximately 40 m. We use this data set, complemented by an identical analysis on the microscale, to estimate the kinematic vorticity, or rotational component of flow within the main mylonitic shear zone (Simpson
& DePaor, 1993). Such an analysis requires that (1) clasts behave rigidly and rotate passively within the deforming matrix, and (2) results are interpreted two-dimensionally.
We address the first of these assumptions using deformation microtextures. All mylonites analyzed in this study are characterized by a strong ductility contrast between quartz and feldspar (this study; Tulloch and Kimbrough, 1989) where the former deformed through grain boundary migration and dynamic recrystallization and the latter deformed dominantly through microfracture and rotation. While localized deformation twinning in feldspar is present in some samples, Rf/φ (axial ratio/long axis orientation) data for 283
54 feldspar clasts measured on all three (XZ, YZ, XY) principle planes indicates Rf values are close to 1 on nearly all planes (Figure 3.10). This result supports rigid clast rotation.
A fundamental problem with using kinematic vorticity to completely describe deformation in shear zones is that vorticity results are two-dimensional measurements of flow fields that are commonly three-dimensional (Tikoff and Fossen, 1995; Jiang and
Williams, 1999; Jiang, 1999). Thin sections in this study examined on the XY and YZ planes display weakly developed asymmetries suggesting flow was three-dimensional, however, the plane of dominant asymmetry is consistently the XZ plane. As a result, we use the results to determine the relative contributions made by pure and simple shear, in the near vertical XZ plane.
We measured 100 clasts from planes parallel to lineation and perpendicular to foliation over a 40 meter section along the coast. The technique involves measuring the values for the long (X) and short (Z) axes of clasts, as well as the orientation (Φ) of the X axis relative to a fixed reference line. Along the WHC transect we found that foliation
(S2) served as a convenient reference line. We completed the same procedure on an oriented thin section where we measured 70 more clasts. We plotted the Rf/Φ data on three hyperbolic nets and after constructing eigenvectors following the technique outlined in Simpson & De Paor (1993) we determined Wk, the kinematic vorticity number, to be between 0.74-0.78 (Figure 3.10). A value of approximately 0.71 represents a shear zone, which has received equal contributions from pure and simple shear (Tikoff & Fossen,
1995; Law, 2002). The value we obtained is scale independent, and indicative of a shear
56 zone that has experienced thinning in the vertical plane where simple shear dominates over pure shear.
The domainal character of the core rocks exposed along the WHC transect allowed us to identify a strain gradient, defined by changes in , degree of rotation and transposition of D1, D2 structures, and the evolution of quartz and feldspar microstructures. In general, strain increases down-section (lowest strain - WHC6; highest strain - WHC3,4) as F2, F3 folds become tighter (Figure 3.2) and grain size reduction in quartz increases. Additionally, the direction of maximum elongation (tracked by the L1,
L2 stretching lineation) rotates from a position plunging shallowly SE in moderately deformed CMG gneisses to shallowly SW plunging in the main D2, D3 mylonitic zone.
The geometry of the WHC strain gradient, charcterized by vertical shortening, sub- horizontal extension and rotation, is consistent with Wk results that suggest thinning in the near vertical (XZ) plane.
3.5.2 Kinematic analysis of ductile fabrics outside the mylonite zone
Using the same methods as outlined above for outcop and thin section scale sense of shear determination, we were able to establish the kinematics of ductile flow outside the main mylonitic zone. North of Siberia Bay (Figure 3.5), field observation of S-C fabrics together with rotated, coarse potassium feldspar grains which commonly display tails composed of biotite ± muscovite and quartz indicate top down to the ENE and SW shear on opposite limbs of the warped foliation. On the southern coast of Siberia bay, at sites CF2 and CF3, local development of protomylonite, mylonite, and ultramylonite is characteristic of the NE dipping fabric. Protomylonitic rocks observed at site CF2 contain
57 well-developed S-C fabrics indicative of top down to the NE shear. Site CF3 indicates an increase in fabric intensity from protomylonite to ultramylonite over a vertical distance of
1 meter. The total thickness of high strain rocks at this site is approximately 3 meters.
Rotated feldspar porphyroclasts in the mylonites indicate sense of shear was top down to the NE. Less than 500m south of this outcrop, the granites become megacrystic again with penetrative ductile foliations and lineations. On the headland just north of Tauranga
Bay S-C fabrics, rotated feldspar grains, and E plunging lineations indicate sense of shear was top down to the E.
One sample collected from the lower Buller gorge (site BR2, Figure 3.4) displayed a visible S-C fabric in outcrop and thin section (Figure 3.9). These relationships, together with NE plunging lineations, indicate a top down to the NE sense of shear. Taken together, kinematic analysis of ductile fabrics in the north indicate sense of shear was dominantly top down to the NE. While ductile fabrics are common in the north, their intensity is greatly subdued when compared to fabrics observed in the southern end of the core. As a result, our understanding of the three-dimensional characteristics of ductile flow in these rocks is limited.
3.5.3 Kinematic analysis of fault-slip data
Using the program “Faultkin v.4.1” we calculated the principle axes of instantaneous shortening (Z) and extension (X) for the incremental strain tensor associated with each fault measured in this study (Figures 3.6, 3.7, 3.8, 3.11)
(www.geo.cornell.edu/geology/faculty/RWA/maintext.html)(Allmendinger,1993)
58
(Allmendinger et al., 1989). These are calculated using measurements of fault plane attitude, straie orientation, and sense of displacement. The program constructs an instantaneous strain ellipse for each fault within the plane perpendicular to the entered fault plane and parallel to straie orientation. In general, calculated Z axes were steep or sub-vertical, while calculated X axes plunge shallowly to the WSW-ENE. These data indicate the kinematics of brittle faulting are characterized by vertical shortening and sub-horizontal,
NE-SW extension.
3.6. CORRELATION OF STRUCTURES AND DEFORMATION SEQUENCE
Based on cross-cutting relationships, we break out a sequence of five events (D1-
D5) that may have occurred progressively or in a non-steady fashion during regional continental extension. D1 describes structures that formed prior to D2 mylonite generation. We interpret SE dipping foliations (S1) , SE plunging lineations (L1) and SE plunging fold axes (F3) within CMG gniesses south of WHC as older than the mylonites, based on their orientation and position within the strain gradient. Tight to isoclinal folds observed at site WHC12, previously termed F2’s based on fold tightness and refolded geometry, may represent flattened asymmetric folds observed south of this site, previously termed F3’s based on their geometry (Figure 3.2). Should this be true, the tight to isoclinal folds would also be F3s. In either case, structures south of WHC are interpreted to represent early stages of extension based on deformation textures and an oblique (∼90°) orientation with respect to the dominant extension direction.
60 Within the central core zone, early SE plunging folds (F2 ) may have also been part of D1 deformation. Ductile fabrics (S2, L2) exposed south of Cape Foulwind may have predated mylonite development, however stretching lineations plunge exclusively to the ENE-SW. This orientation is parallel to L2 myl in the south. Because of their geometrical similarity, we group ductile fabrics in the megacrystic Foulwind Granite with mylonitic structures (S2, L2, F2) and refer to them collectivecly as D2. Additionally, central core zone fabrics, S2 ,L2 are included because of their orientation.
D3 deformation involves folding of the mylonitic front and its host rocks. This event produced the upright, open F3 folds visible at sites WHC 12 (Figure 3.2) and WHC
1 (Figure 3.8). D4 describes the regional E-W arching about sub-horizontal axes displayed throughout the core, that folds axial planes of earlier formed structures (F4). D5 includes both semi-brittle and brittle faulting, as these cut all previously formed fabrics and structures.
3.7 GEOCHRONOLOGY
We determined U-Pb zircon ages for three samples within the core of the PMCC.
In this section we present the results and discuss how these data allow time constraints to be placed on D1- D5.
3.7.1 Sample locations and cross-cutting relationships
Sample 03-whc-1
61 Extracted from a granitic pegmatite on a roadcut along the Coast Road, < 1km south of
White Horse Creek. The pegmatite cuts the folded (F3) mylonitic fabric (S2) at site
WHC1, but is cut by a series of NE and SW dipping normal faults (Figure 3.8). This age places a minimum on the age of brittle faulting (D5) on the southern side of the core complex.
Sample 03-ch-2
Extracted from a granitic dike cross-cutting the locally folded (F3) gneissic fabric developed in the coastal exposure below, and west of the Charleston Cemetary. This age places a maximum on the age of fabric development and folding within the CMG in the central core zone.
Sample 03-whc-13
Extracted from a granitic pegmatite folded sub-concordantly with the local mylonitic fabric. This age places a minimum on folding (F3) within the mylonitic shear zone.
3.7.2 U-Pb Ages
3.8. DISCUSSION
In this section we discuss the results of our structural and kinematic analysis in the context of published models for regional extension.
3.8.1 Three-dimensional strain and kinematic evolution of the middle crust
62 Our structural analysis of the main mylonitic shear zone (WHC transect) suggests ductile deformation (D1-D4) included a three-dimensional flow pattern and may have been characterized by constriction (X - extension; Z - shortening; Y- shortening).
Evidence for constriction is based largely on the orientation of linear features (lineations, fold axes) as they are tracked through progressive deformation (section 5). Pre-mylonitic
(D1) folds regardless of tightness, and lineations (L1) progressively rotate into parallelism with the D2 strain field as the highest strain zone is approached from the south. These changes occur as strain is increasing within the WHC strain gradient. Three generations of coaxial, superposed folds (F2-F4) within CMG gneiss parallel to the dominant stretching direction at site WHC12 illustrate this geometry, and serve as an excellent qualitative indicator of constrictional strain. Additionally, within the mylonitic front north of site WHC12, F2, F3, and F4 myl folds have parallel, NE-SW plunging axes, parallel to the dominant stretching direction. (Figure 3.2). F4 folds found elsewhere in the core tend to have E-W sub-horizontal axes. This geometry implies a component of NNW-SSE shortening during extension, as F4 folds do not re-orient kinematic information crystallized during D2 or D3 deformation. These geometric relationships, together with independent determinations of the orientations of X and Z strain axes (kinematic vorticity) suggest three-dimensional strain in the ductile field was characterized by vertical shortening (Z axis), sub-horizontal, NE-SW extension (X axis), and NNW-SSE shortening (Y axis)(Figure 3.12).
Kinematic analysis of brittle fault-slip data indicates the orientations of maximum shortening and extension axes during D5 were parallel to those observed during ductile
63
deformation, (D1-D4). This result suggests the kinematics of ductile flow and brittle
faulting, and the orientation of X and Z strain axes remained constant during the
transition from ductile to brittle deformation.
3.8.2 Extensional mechanisms
Published models describing post-orogenic extensional mechanisms emphasize
the role of : (1) gravity-driven extension as a response to overthickened crust and reduced
intra plate stress (Coney & Harms, 1984; Burchfiel et al., 1982) as in the North American
Cordillera and the Himalaya; (2) thermal weakening of a broad zone of middle crust by
partial melting, and extension related, melt-enhanced deformation (Vanderhaghe &
Teyssier, 1997; Vanderhaghe & Teyssier, 2001) in regions such as the Shuswap
Metamorphic Core Complex in southern British Columbia. The Paparoa Metamorphic
Core Complex in western New Zealand is an excellent place to test aspects of these
models because the region was charterized by regional contraction and crustal thickening
during a ∼20 Ma period directly before the onset of regional extension at ∼112-108 Ma.
Post-orogenic extension in western New Zealand is somewhat unique, however, because extension eventually led to continental rifting and the opening of a new ocean basin.
The styles of crustal thickening and thinning during the period of Cretaceous orogenesis recorded by the rocks in western New Zealand sugggest that weakening of the middle crust caused by partial melting was not a dominant mechanism during the extensional process. Voluminous syn-tectonic magmatism during contraction and extension was likely important in reducing intra plate stress prior to and during extension, however migmatites recording melt-enhanced deformation are nearly absent at mid-upper
65 crustal levels. This observation is consistent with the mid-crustal architecture of a collapsed orogen composed domiantly of magmatic rocks as opposed to an orogen dominated by fertile, melt-producing meta-pelites. Tulloch and Palmer (1990) suggest the thickness of the crust in western New Zealand may have reached 45-55 km in the Early
Cretaceous. These workers suggest crustal thickening may have been important for the generation of syn-extensional granitic magmatism in the metamorphic core of the PMCC, represented by plutons of the Rahu Suite. Deformation in these rocks is consistently solid-state, suggesting extension was not driven by the collapse of the upper crust over a weak horizon in the middle crust. Data presented here and elsewhere suggest the dominant mechanism for extension in the PMCC was not related to a mechanical weakening of the middle crust caused by partial melting.
Late Cretaceous extension in western New Zealand coincided with a major change in plate boundary dynamics that affected the entire Pacific margin of Gondwana.
We suggest this change was the driving force behind and dominant mechanism for extension in the region. Two results in the recent literature help support this interpretation. First, shear zones affecting Cretaceous and older rocks shift from contractional style, vertically thickening pure shear dominated zones to extensional zones recording retrogression and vertical thinning at middle and lower crustal levels by 112-
108 Ma (Klepeis, 2002; Tulloch and Palmer, 19990; Gibson & Ireland, 1988; this study).
Second, thermal studies of lower crust exposed in Fiordland, indicate the lower crust was cooling rapidly and strong during the shift from contraction to extension (Daczko et al.,
2002; Klepeis et al., 2003). Additionally, partial melting in the middle and lower crust
66 produced low volumes of melt, particularly when compared to voluminous migmatites common where metapelites are subjected to heat and burial. We suggest the narrow, focused style of ductile thinning in the mid-crustal core of the PMCC is primarily the result of changing regional stress conditions in a column of lithosphere characterized by a strong lower crust and a plastic middle crust. We acknowledge that the middle crust was likely weakened to some degree by syn-extensional magmatism, but reject gravitational instability due to partial melting in the middle crust as a dominant mechanism driving extension.
An extensional model driven by plate boundary change is supported, additionally, by isotopic analyses of hydrothermally altered mylonites in the footwall of the Pike detachment fault (Tulloch and Palmer, 1990). A sericite age of ∼85 Ma has been suggested to represent the last stage of motion on the Pike detachment just prior to continental rifting and the opening of the Tasman Sea. It follows then, that the driving force behind late Cretaceous extension and the exhumation of mid-crustal rocks in the
PMCC may have been the same force that drove continental rifting and the generation of new ocean crust.
3.9. CONCLUSIONS
We recognize five events (D1-D5) of deformation within the core of the Paparoa
Metamorphic Core Complex produced during regional extension on the Pacific margin of
Gondwana in the Late Cretaceous. D1-D4 comprises a ductile history characterized by polyphase folding and fabric development in mylonitic (southern domain) gneissic
67 (central core zone) and granitic (northern domain) units. D5 describes semi-brittle and brittle faults that cut all previously formed ductile fabrics.
In the southern half of the core, D1-D4 produced a 4 km long section of interfolded mylonites and host rocks. Within the highest strain zone, three generations of folds (F2,F3,F4) have axes parallel to the orientation of the mineral stretching lineation and regional extension direction. Fold axes and mineral lineations oriented oblique (∼90°) to the extension direction are recorded by lower strained rocks within the WHC strain gradient. Sense of shear analysis of mylonitic fabrics indicates sub-horizontal stretching during top down to the NE and SW normal motion. Kinematic vorticity analysis of D2 mylonites performed on a suite of three sites in the field and a fourth on the microscale indicate 0.74 < Wk < 0.78. This result is consistent with a shear zone that has experienced thinning in the vertical (XZ) plane. Two-dimensional kinematic analyses, together with the geometry of D1-D4 structures, suggest a three-dimensional environment characterized by constrictional strain where the direction of maximum shortening (Z axis) is vertical and maximum extension (X axis) is sub-horizontal, NE-SW. Kinematic analysis of brittle fault-slip data indicates D5 was also characterized by vertical shortening and NE-SW sub-horizontal extension. We interpret these results as evidence that structural and kinematic parameters, namely sense of shear and the orientation of maximum extension and shortening axes, remained constant during the transition from ductile to brittle deformation in the middle crust during extension.
68 CHAPTER 4: Strain and kinematic analysis
4.1 INTRODUCTION
Any comprehensive study of rock deformation would be incomplete without an analysis of finite strain and the kinematics of flow. Strain analysis allows for the estimation and comparison of the magnitude and orientation of deformation parameters.
Kinematic analysis is important for reconstructing displacement history and interpreting the evolution of flow regimes. This chapter describes the background, methodology, results and analysis of a finite strain and kinematic analysis performed on ductiley deformed rocks during the Late Cretaceous extensional event in western New Zealand.
4.2 STRAIN
4.2.1 Background
Finite strain is the end result of one or more episodes of shape change, rotation, or translation experienced by a volume of rock (e.g. Davis & Reynolds, 1984; Twiss &
Moores, 1992). A common goal of structural geologists working with deformed rocks is the determination of the magnitude and orientation of the finite and incremental two- dimensional strain ellipse or three-dimensional strain ellipsoid for a sample, outcrop, or region. The concept of the strain ellipse and ellipsoid are fundamental to structural geology, and are based on the idea that an unstrained object may be represented as a circle or sphere, and any subsequent strain accumulated by that circle or sphere will result in the formation of an ellipse or ellipsoid. Implicit here is the concept of the instantaneous or infinitesimal strain ellipse, which records the shape change, rotation or translation of a
69 potentially infinite number of incremental strains. The end product of one or more episodes of incremental strain is the finite strain state of an object.
The challenge for field geologists working in deformed terranes became the identification of “strain markers” or objects that accurately record incremental and finite strain. A diverse and creative assortment of strain markers have been used by workers during the last three decades including shape change in ooids, boudins, and feldspar grains and rotation of veins and reaction zones (e.g. Fry, 1979; Sen and Mukherjee, 1972;
Borradaile, 1979; Klepeis et al., 1999). Choosing an appropriate strain marker in the field is difficult because natural objects often do not originate as circles or spheres, and shape change histories commonly deviate from the simple circle to ellipse morphology.
Based on these fundamental principles, a set of assumptions exists for any strain analysis performed on naturally deformed rocks. First, the nature of strain accumulation makes it difficult to determine, in the absence of external data, whether the strain being measured represents an infinitesimal or a finite strain, particularly in polyphase deformed terranes. For this reason, strain studies commonly are comparative analyses and the results are not necessarily viewed as true finite strains but rather as apparent strains. The aim of this strain analysis is to quantify a strain gradient defined by textural changes in deformed granitoid rocks, an increase in fold tightness, and rotation of structures toward parallelism with the dominant stretching lineation and extension direction. It is acknowledged, that while a strain gradient defined by external observations is used, the results likely do not represent true strains.
70 True strains are not calculated in this study because of the nature of the strain markers and the environment of deformation. This study fails to satisfy an important second assumption, that strain markers deform homogeneously. As indicated above, we measured the strain exhibited by granitoid rocks in a strain gradient associated with the
White Horse Creek high strain zone. Microtextural evidence suggests the conditions of deformation did not exceed 500°C, as feldspars deform dominantly by microfracture and quartz behaves by grain boundary migration during dynamic recrystallization. Studies have shown that at low strain rates, quartz deformation textures include undulose extinction and subgrain formation. Subgrains have been shown to form in a variety of orientations, from near circular to moderately elliptical. With increasing deformation, subgrains undergo grain boundary area reduction and finally, in mylonitic rocks, dynamic recrystallization. The shapes of quartz grains and subgrains were used as strain markers in this study, despite the admission that their deformation paths were likely heterogeneous.
A third and final important assumption to note is that factors other than deformation may influence the shapes and orientations of strain markers, resulting in apparently incorrect finite strain results. Volume loss can be a very important external factor to consider when using mineral grains as strain markers, particularly when using feldspars in low-temperature regimes. Dissolution of feldspar may cause grains to change shape before, during or after deformation, masking the true finite strain state. Knowledge of volume loss becomes critical when attempting to interpret three-dimensional strains.
Three dimensional strain ellipses have prolate (cigar-shaped) and oblate (pancake-
71 shaped) end member geometries. These ellipsoids are determined by combining two- dimensional strain results for three perpendicular faces. A Flinn diagram illustrates these concepts (Flinn, 1979), where each ellipsoid plots as a point somewhere in the field of constriction, flattening or plane strain. Volume loss in a sample shifts the K = 1 division
(a line with a slope of 1), where K > 1 indicates constriction and K < 1 indicates flattening. The more volume lost in a system, the farther to the right the line will shift.
Hence, without considering volume loss results may be interpreted as flattening when they are really constrictional.
4.2.2 Methods
This study employed the methods of Lisle (1985) and Ramsay and Huber (1983) in order to track the changes in magnitude and orientation of the finite strain ellipsoid through the White Horse Creek strain gradient. Two different analyses were performed; one suite of three samples using quartz as a strain marker and one suite of three samples using feldspar. The second analysis was conducted to produce a quantitative check on microtextural evidence suggesting feldspar behaved brittley during deformation. If quartz and feldspar truly deformed by different deformation mechanisms, a comparison of the results of these anlyses should reflect the behavioral variation.
In samples where quartz was used as a strain marker, ellipses were drawn around deformed, but not recrystallized, quartz grains and subgrains. Relic grains were used where they could be identified. In samples where feldspar was used, ellipses were drawn around whole feldspar grains. The methodology involved taking oriented, digital images of whole thin sections from three perpendicular planes (XY,XZ, and YZ, Figure 4.1),
72 importing these images into Adobe IllustratorTM, and tracing grains using the Illustrator ellipse tool. Axial ratios (ratios of long axis:short axis) and the φ angle (angle between the long axis and an arbitrary reference line) are calculated in the program (AI), then transferred to ExcelTM data sheets where harmonic and vector means are calculated.
Ramsay and Huber (1983) indicate that obtaining harmonic mean of Rf analyses is an acceptable technique for rapid determination of tectonic strain (Rs). This information allows the magnitude and orientation of sectional, two-dimensional strain ellipses to be calculated. The magnitude and orientation of the three-dimensional finite strain ellipsoid is calculated by entering the data for three sectional ellipses into the program Strain3D written by Basil Tikoff (University of Wisconsin) that uses the least squares of Means
(1984) method to compute six possible ellipsoidal geometries. Thus, the result of each sample analysis is a lower hemisphere, equal-area projection showing the trend and plunge of six possible X (direction of maximum elongation), Z (direction of maximum shortening) and Y (direction of intermediate shortening/elongation) orientations. A robust analysis is one where X,Y and Z orientations form three tight clusters.
4.2.3 Reults
Quartz
Despite the behavioral heterogeneity quartz displays during deformation, Rf/φ analysis of grains from deformed granitoid rocks in the White Horse Creek strain gradient indicates a modest, but detectable quantitative increase in strain (Figure 4.2).
Sample WHC6, the low strain sample collected in the field, dislpays the lowest Rf
74 Figure 4.2. Three-dimensional strain results from the quartz analysis. Lower hemisphere equal-area plots show the orientation of the principle ellipsoid axes (X,Y,and Z)
φ determined using the program Strain3D and Rf/ data from three perpendicular planes.
75 harmonic mean values (least elliptical). The XZ plane from this sample yielded a mean Rf of < 2, where 1 is a perfect circle. Sample WHC5 yielded higher mean Rf values; the XZ plane for this sample = 2.14. Sample WHC2, taken from a low strain pod within the mylonitic front near Morrissey Creek (Figure 3.2), yielded the highest mean Rf value for
XZ planes at 2.56. While these results clearly do not represent true strains, they do confirm what was observed in the field, the presence of a measurable strain gradient.
Three-dimensional strain analyses yielded the ellipsoids shown in Figures 4.2 and
4.3. An example of how to read one of these diagrams is given here for Sample WHC6.
The X-axis (direction of maximum stretch), the longest axis of the ellipsoid, is steep and plunges to the SW. The Z axis (direction of maximum shortening), the shortest axis of the ellipsoid, plunges shallowly to the NE. The Y axis, or intermediate axis, plunges shallowly to the SE. The relative lengths of the X, Y, and Z axes are directly proportional to the Rf values determined for the three sectional XZ , YZ and XY, strain ellipses.
Sample WHC5 shows a change in geometry, where the Z-axis is nearly vertical and maximum extension is shallow to the NW. WHC2 shows a third stage in the geometrical evolution, with a steep Z-axis and maximum extension shallow to the SW. The results of the three-dimensional strain analysis suggest that as strain was increasing from site
WHC6 to WHC2, the shape and orientation of the strain ellipsoid was evolving. The final orientation of the ellipsoid, recorded by sample WHC2, contains a direction of maximum extension parallel to the orientation of the mineral stretching lineation in the main mylonitic zone and the dominant regional extension direction (NE-SW). Three
77 Figure 4.3. Three-dimensional strain results from the feldspar analysis. Lower hemisphere equal-area plots show the orientation of the principle ellipsoid axes (X,Y,and
φ Z) determined using the program Strain3D and Rf/ data from three perpendicular planes.
78
Figure 4.4. a.) Flinn plot illustrating the three-dimensional strain results. After volume loss is accounted for, samples would likely plot in the field of constriction; b.) Shapes of feldspar grains measured in three-dimensional feldspar strain analysis.
80 dimensional strain results may also be plotted on a Flynn diagram, which places ratios of harmonic mean of Rf values determined in sectional analysis on the x and y axes such that y = Rf xy and x = Rf yz (Figure 4.4). Samples that plot in the upper half of the diagram display a constrictional strain geometry, samples which plot in the bottom half indicate flattening strains. As discussed in the background, volume loss in the system will shift the plane strain line separating the two fields to the right. Microstructural evidence from this study suggests volume loss was present in the system, thus making the shift a reality in this system. Depending on exactly what percentage of volume was lost, samples from this study would either plot well into the field of constriction or near the boundary indicating plane strain.
Feldspar
An identical analysis was performed on three samples containing abundant plagioclase feldspar porphyroclasts. These data indicate that Rf for feldspar grains is generally less than 1.5 (Figure 4.3). Additionally, ellipticity decreases towards the high strain zone; the inverse of what the quartz analysis indicated. These results suggest feldspar was not deforming by ductile processes during deformation. Comparison of all
XY and YZ sectional Rf data suggests that the shapes of feldspar grains are both prolate and oblate (Figure 4.4).
Despite the conclusion that feldspar deformed by brittle deformation mechanisms during deformation, three-dimensional analysis of feldspar Rf/φ data yielded ellipsoids that indicate a preferred alignment of long axes on principle planes. While the orientation
82 of X and Y axes are variable, Z axes are generally steep to vertical. This is the same orientation as the Z axis in the finite strain ellipsoid determined by the quartz analysis.
4.3 KINEMATIC ANALYSES
4.3.1 Background and methods
Kinematic analyses, mainly sense of shear determination and kinematic vorticity
(rate of rotation in a flow field) analysis , play a significant role in this study. These are powerful tools which may be very useful in describing the environment of deformation for a given shear zone (Lister and Williams, 1983; Simpson and De Paor, 1993; Passchier
& Trouw, 1996). However, due to the complex behavioral patterns in heterogeneous naturally deforming systems, a certain amount of caution is required when interpreting the results of kinematic analyses. This section first describes the fundamental assumptions inherent in general kinematic analysis, then discusses these assumptions with particular reference to the kinematic vorticity analysis used in this study.
Kinematic analyses of ductile fabrics are the measurement of instantaneous and finite flow in rocks. Rocks deformed through tectonic processes are generally termed
“tectonites” and may be dominated by planar (S-tectonite), linear (L-tectonite) or a combination of planar and linear elements (L-S tectonite). L-S tectonites allow for the identification of principle, XY, YZ, and XZ planes, based on the orientation of the stretching lineation and the foliation. The XZ plane (Figure 4.1), the plane parallel to lineation and perpendicular to foliation, has been long thought to be the plane where the most asymmetry occurs. Asymmetrical relationships formed on the XZ plane, have been used as kinematic indicators under the assumption that the stretching lineation is parallel
83 to the transport, or shear direction (eg. Hanmer and Passchier, 1991). This assumption may not always be valid, as will be discussed later in this section. Asymmetrical relationships which have been used as sense of shear indicators include S/C fabrics, mica fish, oblique foliations, asymmetric tails developed on porphyroclasts, synthetic and antithetic microfaulted grains, and preferred orientation of mineral c-axes (Hanmer and
Passchier, 1991). This study employs macroscopic and microscopic S/C fabrics and porphyroclast tails, as well as synthetic and antithetic microfaulted feldspar grains. As a result of this kind of analysis, two-dimensional sense of shear is determined and is always parallel to the stretching lineation for a given sample.
These techniques have been used by numerous workers in deformed terranes and are occasionally interpreted as relating to the orientation of an applied tectonic stress, such as the emplacement of an allochthon. However, two important assumptions must be discussed before such interpretations may be made. The first of these assumptions concerns the deformation path followed by naturally deforming rocks. For some kinematic interpretations to be valid, a progressive (as opposed to a time-separated) deformation path must be assumed. For example, in the case of the emplaced allochthon, if fabric asymmetry indicates reverse motion on a 30° plunging lineation, an interpretation assuming progressive deformation might suggest this fabric was related to the thrusting. However, if a time-separated deformation path is assumed, there are an infinite variety of paths that may lead to a certain finite deformation (Jiang and White,
1995; Jiang and Williams, 1999).
84 The progressive vs. time-separated deformation path complication to kinematic analysis may be addressed by invoking corroborating evidence, often in the form of field data, which allows for a sequence of events and/or a strain gradient to be established. In this study, the White Horse Creek strain gradient allows this assumption to be satisfactorally addressed. Fold axes and lineations plunge moderately to the SE south of
White Horse Creek in deformed, though not mylonitized granitoid rocks. North of WHC still within the same unit, multiple generations of folds are present with axes plunging gently to the NE-SW. Continuing north, granitoids are mylonitized with stretching lineations and F2,F3 folds plunging gently SW-NE. These relationships suggest that the
ISA, or instantaneous stretching axis, evolved from an orientation plunging shallowly SE to one plunging shallowly SW and NE. The occurrence of earlier or later stages of ductile deformation, however, may not be ruled out.
The second assumption that is owed discussion is based on the theory that every deforming system has the potential to be characterized by three-dimensional flow. This is not to say that all deformation is three-dimensional, nor does it mean that two- dimensional analyses are not useful. Studies have shown that assuming a two- dimensional strain field may increase the likelihood of incorrect interpretation (Lin et al.,
1998; Williams, 2002). Lin (1998) showed that changes in the orientation of the main stretching lineation in a shear zone indicate the strain field was likely three-dimensional and deformation likely triclinic. Williams (2002) showed that in some cases, the ISA
(instantaneous stretching axis) may not be parallel to lineations measured in the field.
85 These factors effectively shift the burden toward disproving a three-dimensional
environment of deformation.
The results of this study are consistent with a three-dimensional, evolving strain
field during Late Cretaceous extensional deformation. Microstructural investigation of all
three principle planes for many samples confirms the assumption that the XZ plane is the
dominant plane for asymmetry. Additionally, the XZ planes measured in this study are
characterized by the highest Rf values. However, auxiliary planes did record changes in
strain, suggesting a three-dimensional flow field. Three-dimensional strain analysis
indicates that strains were likely constrictional, and that ellipsoids had prolate
geometeries with maximum extension oriented SW-NE in the final state. Additionally,
the orientation of F2, F3, and F4 fold axes within the mylonitic front suggests constriction
as a first order approximation. In sum, this study (1) acknowledges the assumption that
strain fields are likely three-dimensional, and (2) describes the 3D kinematic evolution of
the strain field during extension.
4.3.2 Kinematic vorticity analysis
The aim of any kinematic vorticity analysis is to quantify the relative
contributions of pure shear (coaxial deformation) and simple shear (non-coaxial
deformation) in a system. Passchier and Trouw (1996) summarize a number of sensitive
vorticity gauges including deformed sets of veins, lattice-preferred orientations, mantled
porphyroclasts, and oblique foliations. This study employs the rigid rotation of
porphyroclasts and the geometry of asymmetric σ-type, β-type, and δ-type tails. It is generally accepted that rigid objects embedded in a viscous medium will rotate during
86 non-coaxial flow in accordance with the bulk kinematics of the system. Passchier (1987) showed that during two-dimensional general shear, rigid porphyroclasts whose axial ratio is above a critical value may become stationary at high strain as they rotate into parallelism with the asymptotic flow apophyses. Passchier used this critical value, below which no stable clast orientations exist, as a measure of kinematic vorticity. Simpson and
De Paor (1993) describe the technique for measuring kinematic vorticity using systems of rigid porphyroclasts containing asymmetric tails. Rf and positive or negative φ are plotted on a hyperbolic net (Figure 3.10). The cosine of the acute angle between the first eigenvector (parallel to the reference line, S2 in this study) and the second eigenvector
(forms the limit of the field of backrotation) is Wk, the kinematic vorticity number. The equation is written:
Wk = cos(v)
where v = angle between eigenvectors
An increasing v value indicates an increase in the contribution of pure shear to the system. A system completely dominated by pure shear has a Wk of 0, and a system dominated completely by simple shear has a Wk of 1. General shear describes all values between 0 and 1. When Wk = .71 the system has received roughly equal contributions from pure and simple shear (Tikoff & Fossen, 1995).
Kinematic vorticity analysis has been proven to be useful for describing flow regimes in two-dimensional strain fields characterized by monoclinic symmetry and steady-state deformation (e.g. Passchier, 1998, Holcombe and Little, 2001). However, a problem with all determinations of Wk is that it may have been variable over a volume of
87 rock and may have changed with time. As a result, some workers have suggested the term mean vorticity, or Wm, as a more appropriate description of the result. For simplicity, this study consistently describes kinematic vorticity as Wk.
Passchier and Trouw (1996) suggest these difficulties may be addressed by measuring vorticity using a variety of gauges that re-equilibrate at different rates. For example, LPO in quartz is a gauge which re-equilibrates much faster than a deformed vein set, particularly if the veins pre-date deformation. This approach seeks to define changes in Wk during progressive deformation. A variety of gauges applied to a scale independent sampling and measuring strategy is the most thorough method to date for determining Wk.
An additional limitation of kinematic vorticity anlaysis is that it is a two- dimensional measurement of a three-dimensional flow field. While in many cases it can be argued that the XZ plane is the dominant asymmetric plane, a two-dimensional analysis is, by definition, not completely accurate. Successful application of vorticity measurement techniques to the third dimension has yet to be achieved (Tikoff and
Fossen, 1995). Tikoff and Fossen (1995) have shown that assumptions made during two- dimensional analysis are not valid in a three dimensional system. These workers indicate knowledge of the orientation of the flow apophyses and instantaneous strain axes with respect to the shear zone boundary is not sufficient for determining three-dimensional
Wk. Despite these assumptions, this study includes the results of scale independent, two- dimensional kinematic vorticity analysis. This analysis is useful on account of the following: (1) This study has produced a detailed account of the environment of
88 deformation, i.e. good understanding has been reached regarding the deformation mechanisms favored by the participating mineral constituents; (2) This study has tracked the orientation of the ISA (instantaneous stretching axis) during deformation, using mineral stretching lineation as a proxy and by performing an Rf/φ analysis on a strain gradient defined in the field. The final observed orientation of the ISA is parallel to the mineral lineation, transport direction, and inferred shear zone boundary; (3) This study is interested in using the results of the analysis for a two-dimensional interpretation of whether the shear zone was thinning or thickening in the vertical, or Z direction.
4.3.3 Shear sense determinations
Shear sense analysis results Analysis of macroscopic and microscopic fabrics in the field and in the lab indicates the kinematics of flow were characterized by sub-horizontal,normal motion parallel to the plunge of the mineral stretching lineation. On the northern half of the core,
Cape Foulwind and the Buller Gorge, this generally meant top down to the E-NE motion.
In the southern half of the core, WHC and Pike Stream, fabrics suggest both top down to the SW and top down to the NE shear. In general, these results are consistent with a NE-
SW extension direction
Discussion
Excellent exposure along the coast north and south of White Horse Creek allowed us to track the orientation of shear sense indicators through the main mylonitic zone associated with the Pike detachment fault. Models describing the structural evolution of
89 the mylonitic front associated with metamorphic core complexes make predictions regarding the orientation and distribution of kinematic indicators as well as the gross structure of the mylonitic zone (see Chapter 2 for discussion of models). Detailed observation of kinematic indicators in the field and in samples examined microscopically in this study allow for comparison with these models, particularly the model proposed by
Reynolds and Lister (1990) (Figure 2.2). In this model, the mylontic front is folded asymmetrically such that a shallow dipping mylonitic zone has synthetic kinematics with the main detachment fault, and a steeper back dipping limb displays opposite (or apparent reverse) kinematics. Reynolds et al. (1990) suggest the apparent reverse kinematic indicators are evidence the mylonitic front was folded after the crystallization of kinematic information. Our results suggest some agreement with this model (Figure 3.2).
Between site WHC1, the first mylonitic exposure driving north along the Coast Road, and site WHC11 structural and kinematic data suggests a geometry consistent with the folded geometry proposed by Reynolds and Lister (1990). Site WHC1 is folded at the mesoscale about a W-plunging axis and contains top down to the SSW kinematics. Site
WHC11 dips to the NNW and contains NE-plunging mineral lineations. Kinematic indicators from this site are consistent with reverse, top to the SW motion. The mylonitic front disappears below the surface north of this site, re-emerging at site WHC4 where it dips to the SSW. The reverse-sense conclusion at site WHC11 is drawn from kinematic indicators observed in the field and checked under the microscope.
North of site WHC11, the mylonitic front is shallowly warped about NE-SW plunging axes, and contains both SW and NE shallowly plunging mineral lineations.
90 Kinematic indicators in this region are consistent with top down to the SW and top down to the NE normal motion. This geometry may suggest the influence of syn-extensional magmatism on the gross geometry and kinematics of the core complex. Buoyant, granitic magmatism is known to have accompanied extension (e.g. Buckland Granite). Syn- extensional magmatism during exhumation is known to “bow” the lower plate during extension, often resulting in a dome-and-basin pattern (e.g. Lister and Davis, 1988). This process may help explain NE plunging lineations and top down to the NE kinematic indications in a zone dominated by top to the SW shear. Thus, the proposed geometry and kinematics of the mylonitic front associated with the Pike detachment fault is in agreement with some features of published models as well as generally accepted styles of extensional deformation.
Exposure in the northern half of the core allowed us to analyze fabrics for kinematic interpretation at Cape Foulwind and in the Buller Gorge. The level of deformation in the north does not equal the level observed in the south on a penetrative scale, however isolated, thin mylonitic zones are observed. These mylonites contain S/C fabrics and rotated porphyroclasts consistent with top down to the NE motion. The
Foulwind Granite (megacrystic variety) is affected by N-dipping foliations and mineral lineations that plunge from E to N. Kinematics are consistently top down to the E-NE, save for one site near the Foulwind lighthouse where a fold reverses the kinematic trend.
The exposure on the roadside on the southern bank of the Buller River reveals only one site with a fabric suitable for kinematic analysis. Here, a well developed S/C fabric
91 indicates top down to the NE normal motion. These results are consistent with top down the NE motion on the Ohika detachment fault.
4.3.4 Results of kinematic vorticity analysis
Thin section, outcrop, and multiple outcrop scale measurements of rigid particles in a once viscous matrix enabled this study to report on Wk within the main mylonitic zone associated with the Paparoa metamorphic core complex (Figure 3.10). The distribution of the 175 plagioclase feldspar clasts measured allowed for determination of the first and second eigenvectors of flow (Simpson & De Paor, 1993). The limit of the field of backrotation, bound by the second eigenvector, displays a range between 38° and
42°. The greater the area displayed by the field of backrotation, the greater the contribution from pure shear to the system. A second eigenvector at 90° to the first suggests a Wk of 0. Thus, our results indicate 0.74 < Wk < 0.78, where Wk = 0.71 (Tikoff
& Fossen, 1995) describes a shear zone which has received equal input from pure and simple shear.
Discussion
These results suggest that the main mylonitic zone within the core of the PMCC was characterized, during its final phase of movement in the ductile field, by vertical thinning and sub-horizontal stretching. A value for Wk between 0.74 and 0.78 suggests conditions approximated general shear, however simple shear seems to have been the dominant process. This result proves ductile thinning of the middle crust was an important mechanism of extension during early stages of deformation.
92 4.4 SUMMARY AND CONCLUSIONS
Efforts made by this study to quantify finite strain state, infer strain paths and establish the kinematics of flow complement a rigorous structural analysis and together these data contribute to a near complete understanding of structural, kinematic and deformational processes at multiple length scales. This study shows that the dominant deformation mechanisms at the grain scale are grain boundary migration and dynamic recrystallization in quartz and rigid rotation accompanied by microfaulting in feldspars.
Strain analysis shows that Rf values for all feldspars are generally close to 1 with no significant spatial variation. Rf/φ analyses using quartz show a quantitative increase in strain with proximity to the zone dominated by mylonitic fabrics. Additionally, quartz analyses indicate a finite strain ellipsoid characterized by NE-SW horizontal stretching and vertical shortening. Quartz results plotted on a Flinn diagram indicate that three dimensional strains were likely constrictional, consistent with qualitative results
(structural geometry of F2,F3,F4) interpreted from field data. Kinematic vorticity analysis indicates that the main mylonitic zone (WHC) was characterized by general shear (0.74 <
Wk < 0.78) though simple shear was dominant. The range of Wk indicates the main mylonitic zone was characterized by thinning in the vertical plane and NE-SW, sub- horizontal extension. These results are interpreted here as an indication that ductile thinning of the middle crust was an important mechanism during Late Cretaceous extension.
The data and interpretations presented in this section are consistent with field studies in other extensional metamorphic core complexes (Fletcher and Bartley, 1994). In
93 the central Mojave metamorphic core complex (CA, USA) Fletcher and Bartley (1994) show, using a combination of finite strain and fold orientation, constrictional strains produced by a non-coaxial shear zone. Syn-mylonitic folds have orientations parallel to the stretching lineation in the main shear zone, suggesting a component of compression in the Y direction. These workers also show that the geometry of folds in a vertical strain gradient evolves from upright and open and tight and isoclinal (also consistent with this study and others e.g. Harris et al., 2002). Hence, the results presented here and elsewhere make predictions about structural geometries and strain patterns in continental extensional terranes, or at least in metamorphic core complexes. These may include, but are not restricted to:
•two-dimensional, non-coaxial strain •three-dimensional, constrictional strain •superposed, extension parallel folding •ductile shear zones characterized by vertical thinning
94 CHAPTER 5: Thesis Summary and Conclusions
5.1 SUMMARY AND CONCLUSIONS
This study adds structural and kinematic data from the metamorphic core within the Paparoa Metamorphic Core Complex to a growing body of knowledge regarding late
Cretaceous extension in western New Zealand. This thesis describes the geometry, kinematics and sequence of deformation in the middle crust, as it was exhumed during regional extension between ∼112-84 Ma. The methods used to characterize the style of extensional deformation incorporate field data and lab analyses including: (1) a structural analysis of structures recorded by mid-crustal core rocks; (2) a rigorous kinematic analysis of ductile and brittle phases of deformation including kinematic vorticity; (3) a detailed analysis of ductile and brittle deformation mechanisms as well as three- dimensional strain analysis.
Results from the structural analysis indicate five events (D1-D5) of deformation within the core of the Paparoa Metamorphic Core Complex were produced during regional extension on the Pacific margin of Gondwana. D1-D4 comprises a ductile history characterized by polyphase folding and fabric development in mylonitic (southern domain), gneissic (central core zone) and granitic (northern domain) units. D5 describes semi-brittle and brittle faults that cut all previously formed ductile fabrics.
In the southern half of the core, D1-D4 produced a 4 km long section of interfolded mylonites and host rocks. Within the highest strain zone, three generations of folds (F2,F3,F4) have axes parallel to the orientation of the mineral stretching lineation and regional extension direction. Fold axes and mineral lineations oriented oblique (∼90°)
95 to the extension direction are recorded by lower strained rocks within the WHC strain gradient. Sense of shear analysis of mylonitic fabrics indicates sub-horizontal stretching during top down to the NE and SW normal motion. Kinematic vorticity analysis of D2 mylonites performed on a suite of three sites in the field and a fourth on the microscale indicate 0.74 < Wk < 0.78. This result is consistent with a shear zone that has experienced thinning in the vertical (XZ) plane. Two-dimensional kinematic analyses, together with the geometry of D1-D4 structures, suggest a three-dimensional environment characterized by constrictional strain where the direction of maximum shortening (Z axis) is vertical and maximum extension (X axis) is sub-horizontal, NE-SW. Kinematic analysis of brittle fault-slip data indicates D5 was also characterized by vertical shortening and NE-SW sub-horizontal extension. We interpret these results as evidence that structural and kinematic parameters, namely sense of shear and the orientation of maximum extension and shortening axes, remained constant during the transition from ductile to brittle deformation in the middle crust during extension.
Results from this study are consistent with ductile thinning of the middle crust within narrow focused zones as a dominant mechanism for extension under ductile P-T and strain rate conditions. This style of deformation contrasts sharply with the style of post-orogenic extension characterized by weakening of the middle crust by partial melting during crustal thickening (Shuswap Metamorphic Core Complex, BC). While both core complexes developed after a period of crustal thickening, deformation in the
Shuswap MCC is distributed over a broad, diffuse zone and directly related to the collapse of the upper crust over a weak layer in the middle crust. As a result of these
96 observations, weakening of the middle crust is not considered to be the driving force behind late Cretaceous extension in the PMCC.
Regional extension coincided with a major change in plate boundary dynamics that affected the entire Pacific margin of Gondwana in the mid-late Cretaceous. We suggest the narrow, focused style of ductile thinning in the mid-crustal core of the PMCC is primarily the result of changing regional stresses in an extending column of lithosphere characterized by a strong lower crust and a plastic middle crust. Additionally, we suggest extension was not controlled by buoyancy forces in a weak, gravitationally unstable middle-crust. An extensional model driven by plate boundary change is supported, additionally, by isotopic analyses of hydrothermally altered mylonites in the footwall of the Pike detachment fault (Tulloch and Palmer, 1990). A sericite age of ∼85 Ma obtained from these rocks has been suggested to represent the last stage of motion on the Pike detachment just prior to continental rifting and the opening of the Tasman Sea. Thus, the driving force behind late Cretaceous extension and the exhumation of mid-crustal rocks in the PMCC may have been the same force that drove continental rifting and the generation of new ocean crust.
5.2 SUGGESTIONS FOR FUTURE WORK
Future work on extensional deformation in western New Zealand should focus on establishing the link between the PMCC (extension in the middle crust) and the Doubtful
Sound Shear Zone, an extensional shear zone that cuts lower crustal rocks. Extensional processes on a full crustal scale are not completely understood. Questions to pursue include: How do we reconcile a SW directed, dominant extension direction in the middle
97 crust and a NE directed, dominant extension in the lower crust? If no other detachments are found, a full-crustal “conjugate” system may be a possible model to test. Efforts should be focused on attaining a better understanding of extensional processes between the lower and middle crust.
Lastly, conclusions drawn in this study regarding indicate a major plate boundary change as the dominant mechanism driving Late Cretaceous extension in Western New
Zealand. If this is true, then one must ask why a spreading center was born on the Pacific margin of Gondwana at that time? One hypothesis to test might focus on the end of subduction related magmatism coeval with the accretion of an island arc terrane in the
Early Cretaceous. Slab break-off at this time may have been an important process that perturbed convection currents in the upper mantle, and caused the plate boundary to reorganize.
98 COMPREHENSIVE BIBLIOGRAPHY
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114 APPENDICES
115 APPENDIX A: KINEMATIC INDICATORS
116
Figure A-2-3. This sample from the Cape Foulwind transect shows the characteristic, megacrystic, protomylonitic fabric within the Foulwind Granite. An S-C fabric is defined by aligned biotite grains indicating dextral motion
119
APPENDIX B: IGNS PETLAB DATABASE SPREADSHEET
128 COLL_TYPE COLL_NUM COLLECTORS COLL_DATE FIELDNUMBEIN_SITU LOC_TYPE SHEET EAST aaa 2-Jan-2004 a Yes M A44 1 P 03-cf-1b W.C. Simonson 1/20/07 03-cf-1b Yes K29 2382000E P 03-cf-3a W.C. Simonson 1/20/07 03-cf-3a Yes K29 2382000E P 03-whc-4b W.C. Simonson 1/30/07 03-whc-4b Yes K30 2378000E P 03-br-8 W.C. Simonson 1/21/07 03-br-8 Yes K29 2402000E P 03-br2a-xy W.C. Simonson 1/21/07 03-br2a-xy Yes K29 2403000E P 03-cf-2a W.C. Simonson 1/20/07 03-cf-2a Yes K29 2382000E P 03-cf-2 W.C. Simonson 1/20/07 03-cf-2 Yes K29 2382000E P 03-cf-4b W.C. Simonson 1/20/07 03-cf-4b Yes K29 2382000E P 03-cf-1a W.C. Simonson 1/20/07 03-cf-1a Yes K29 2382000E P 03-br2a-yz W.C. Simonson 1/21/07 03-br2a-yz Yes K29 2403000E P 03-cf-3b W.C. Simonson 1/20/07 03-cf-3b Yes K29 2382000E P 03-ps-3 W.C. Simonson 2/2/07 03-ps-3 Yes K31 2385000E P 03-ps-2 W.C. Simonson 2/2/07 03-ps-2 Yes K31 2385000E P 03-br2a-xz W.C. Simonson 1/21/07 03-br2a-xz Yes K29 2403000E P 03-whc-17 W.C. Simonson 30-Jan-2007 03-whc-17 Yes K30 2377000E P 03-ps-4 W.C. Simonson 2-Feb-2007 03-ps-4 Yes K31 2385000E P 03-whc-8 W.C. Simonson 30-Jan-2007 03-whc-8 Yes K30 2378000E P 03-br-2 W.C. Simonson 21-Jan-2007 03-br-2 Yes K29 2403000E P 03-whc-4a W.C. Simonson 30-Jan-2007 03-whc-4a Yes K30 2378000E P 03-ch-7 W.C. Simonson 28-Jan-2007 03-ch-7 Yes K29 2379000E NORTH COUNTRY LOC_METHOD PRECISION SITE_DESCRIPTION ROCK_DESCRIPTIO 1 Afghanistan Surveyed 0.1 a a 5938000N New Zealand Map - 1:50,000 scale 50 Headland north of Siberia B megacrystic granite 5937000N New Zealand Map - 1:50,000 scale 50 Southern end of Siberia Baymegacrystic granite 5913000N New Zealand Map - 1:50,000 scale 50 WHC beach porphyroclastic mylo 5929000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite 5929000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite 5938000N New Zealand Map - 1:50,000 scale 50 Siberia Bay deformed granite 5938000N New Zealand Map - 1:50,000 scale 50 Siberia Bay megacrystic granite 5938000N New Zealand Map - 1:50,000 scale 50 South of Foulwind lighthous megacrystic granite 5938000N New Zealand Map - 1:50,000 scale 50 South of Foulwind lighthous megacrystic granite 5929000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite 5937000N New Zealand Map - 1:50,000 scale 50 Southern end of Siberia Baymylonite 5888000N New Zealand Map - 1:50,000 scale 50 within Pike Stream mylonite 5888000N New Zealand Map - 1:50,000 scale 50 within Pike Stream deformed granite 5929000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite 5911000N New Zealand Map - 1:50,000 scale 50 WHC beach mylonite 5888000N New Zealand Map - 1:50,000 scale 50 within Pike Stream mylonite 5912000N New Zealand Map - 1:50,000 scale 50 WHC beach granitic gneiss 5929000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite 5913000N New Zealand Map - 1:50,000 scale 50 WHC beach deformed granite 5921000N New Zealand Map - 1:50,000 scale 50 Charleston Coast deformed granite STRATIGRAPHIC_UNIT STRATAGE_MIN STRATAGE_MAX ROCK_CODESAMPLE_TYPANALYSIS_T ARCHIVE_SIT a Quaternary Quaternary aaaGracefield Foulwind Granite Carboniferous Carboniferous grnt TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Berlins Porphyry, Railway Quartz Diorite Cretaceous Cretaceous grnt TS other Other Buckland Granite Cretaceous Cretaceous grnt TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Buckland Granite Carboniferous Carboniferous grnt TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Buckland Granite Cretaceous Cretaceous grnt TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous gneiss TS other Other Buckland Granite Cretaceous Cretaceous grnt TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous gneiss TS other Other JOBNUM COMMENTS_REPORTS_PAPERS AZIMUTH DIP DIP_DIR MARKED PETROGRAPHY_BY aaa 11NTopside a Simonson & Klepeis, in prep. 96 49 N W.C. Simonson Simonson & Klepeis, in prep. 220 85 NW W.C. Simonson Simonson & Klepeis, in prep. 239 75 NW W.C. Simonson Simonson & Klepeis, in prep. 39 56 SE W.C. Simonson Simonson & Klepeis, in prep. 151 5 SW W.C. Simonson Simonson & Klepeis, in prep. 44 77 NW W.C. Simonson Simonson & Klepeis, in prep. 56 84 NW W.C. Simonson Simonson & Klepeis, in prep. 82 38 SE W.C. Simonson Simonson & Klepeis, in prep. 54 73 NW W.C. Simonson Simonson & Klepeis, in prep. 88 80 N W.C. Simonson Simonson & Klepeis, in prep. 47 83 NW W.C. Simonson Simonson & Klepeis, in prep. 65 90 SE W.C. Simonson Simonson & Klepeis, in prep. 175 28 W W.C. Simonson Simonson & Klepeis, in prep. 22 80 SE W.C. Simonson Simonson & Klepeis, in prep. 45 79 NW W.C. Simonson Simonson & Klepeis, in prep. 206 76 SE W.C. Simonson Simonson & Klepeis, in prep. 102 58 NE W.C. Simonson Simonson & Klepeis, in prep. W.C. Simonson Simonson & Klepeis, in prep. 64 90 W.C. Simonson Simonson & Klepeis, in prep. 1 81 E W.C. Simonson PET_DATE HAND_SPECIMEN_DESC MINERALOGY_DESCRIPTION TEXTURAL_DESCRIPTION 2-Jan-2004 a a a megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite porphyroclastic mylonite Plagioclase feldspar + quartz + biotite +/- mus porphyroclastic mylonite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite granitic gneiss Plagioclase feldspar + quartz + biotite +/- mus granitic gneiss deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite COLL_TYPE COLL_NUM COLLECTORS COLL_DATE FIELDNUMBEIN_SITU LOC_TYPE SHEET EAST aaa 2-Jan-2004 a Yes M A44 1
P 03-cf-6 W.C. Simonson 20-Jan-2007 03-cf-6 Yes K29 2379000E P 03-cf-4 W.C. Simonson 20-Jan-2007 03-cf-4 Yes K29 2382000E P 03-whc-15b W.C. Simonson 30-Jan-2007 03-whc-15b Yes K30 2378000E P 03-whc-11 W.C. Simonson 30-Jan-2007 03-whc-11 Yes K30 2378000E P 03-whc-15a W.C. Simonson 30-Jan-2007 03-whc-15a Yes K30 2378000E P 03-ch-6a W.C. Simonson 28-Jan-2007 03-ch-6a Yes K29 2379000E P 03-whc-19b W.C. Simonson 30-Jan-2007 03-whc-19b Yes K30 2377000E P 03-whc-19 W.C. Simonson 30-Jan-2007 03-whc-19 Yes K30 2377000E P 03-ps-1 W.C. Simonson 2-Feb-2006 03-ps-1 Yes K31 2385000E P 03-whc-16 W.C. Simonson 30-Jan-2007 03-whc-16 Yes K30 2378000E P 03-br-3 W.C. Simonson 21-Jan-2007 03-br-3 Yes K29 2405000E P 03-br-4 W.C. Simonson 21-Jan-2007 03-br-4 Yes K29 2403000E NORTH COUNTRY LOC_METHOD PRECISION SITE_DESCRIPTION ROCK_DESCRIPTIO 1 Afghanistan Surveyed 0.1 a a
5921000N New Zealand Map - 1:50,000 scale 50 Tauranga Bay megacrystic granite 5938000N New Zealand Map - 1:50,000 scale 50 South of Foulwind lighthous megacrystic granite 5913000N New Zealand Map - 1:50,000 scale 50 WHC beach mylonite 5912000N New Zealand Map - 1:50,000 scale 50 WHC beach mylonite 5913000N New Zealand Map - 1:50,000 scale 50 WHC beach mylonite 5921000N New Zealand Map - 1:50,000 scale 50 Charleston Coast deformed granite 5911000N New Zealand Map - 1:50,000 scale 50 WHC beach mylonite 5911000N New Zealand Map - 1:50,000 scale 50 WHC beach mylonite 5888000N New Zealand Map - 1:50,000 scale 50 within Pike Stream protomylonite 5914000N New Zealand Map - 1:50,000 scale 50 WHC beach protomylonite 5928000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite 5928000N New Zealand Map - 1:50,000 scale 50 Buller Gorge rd. deformed granite STRATIGRAPHIC_UNIT STRATAGE_MIN STRATAGE_MAX ROCK_CODESAMPLE_TYPANALYSIS_T ARCHIVE_SIT a Quaternary Quaternary aaaGracefield
Foulwind Granite Carboniferous Carboniferous gneiss TS other Other Foulwind Granite Carboniferous Carboniferous grnt TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous grnt TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Charleston Metamorphic Group Cretaceous Cretaceous mylon TS other Other Berlins Porphyry, Railway Quartz Diorite Cretaceous Cretaceous grnt TS other Other Berlins Porphyry, Railway Quartz Diorite Cretaceous Cretaceous grnt TS other Other JOBNUM COMMENTS_REPORTS_PAPERS AZIMUTH DIP DIP_DIR MARKED PETROGRAPHY_BY aaa 11NTopside a
Simonson & Klepeis, in prep. 288 36 NE W.C. Simonson Simonson & Klepeis, in prep. 42 64 NW W.C. Simonson Simonson & Klepeis, in prep. 230 74 SE W.C. Simonson Simonson & Klepeis, in prep. 36 74 SE W.C. Simonson Simonson & Klepeis, in prep. 240 84 NW W.C. Simonson Simonson & Klepeis, in prep. 290 50 SW W.C. Simonson Simonson & Klepeis, in prep. 220 86 SE W.C. Simonson Simonson & Klepeis, in prep. 45 90 W.C. Simonson Simonson & Klepeis, in prep. 40 78 SE W.C. Simonson Simonson & Klepeis, in prep. 250 75 NW W.C. Simonson Simonson & Klepeis, in prep. 241 72 NW W.C. Simonson Simonson & Klepeis, in prep. 52 81 NW W.C. Simonson PET_DATE HAND_SPECIMEN_DESC MINERALOGY_DESCRIPTION TEXTURAL_DESCRIPTION 2-Jan-2004 a a a
megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite megacrystic granite Plagioclase feldspar + quartz + biotite +/- mus megacrystic granite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite mylonite Plagioclase feldspar + quartz + biotite +/- mus mylonite protomylonite Plagioclase feldspar + quartz + biotite +/- mus protomylonite protomylonite Plagioclase feldspar + quartz + biotite +/- mus protomylonite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite deformed granite Plagioclase feldspar + quartz + biotite +/- mus deformed granite APPENDIX C: STRUCTURAL DATA
137 02WHC1 foliation lineation 02WHC2-4 foliation strike dip dip direction trend plunge strike dip dip direction 115 40 sw 158 14 204 21 n 90 87 s 169 10 196 19 n 100 70 sw 175 5 207 20 n 142 34 sw 188 15 187 15 n 147 32 sw 187 15 203 18 n 157 25 sw 177 26 196 10 n 142 15 sw 182 11 206 24 n 152 20 sw 179 12 208 20 n 140 15 sw 172 2 200 15 n 125 30 sw 190 43 199 17 n 115 70 sw 180 22 225 8 n 130 49 sw 190 20 198 11 n 105 62 sw 167 1 190 12 n 140 30 sw 180 5 205 20 n 162 35 sw 165 6 195 16 n 156 47 sw 182 7 205 15 n 165 15 sw 196 18 n 150 20 sw 186 17 n 158 24 sw 52 43 s 162 22 sw 60 21 s 95 8 sw 4830s 118 15 sw 85 21 s 152 28 sw 79 22 s 70 43 s 212 26 n 94 20 s 212 32 n 185 15 w 185 16 w 02WHC2-4 lineation 02WHC5,6 foliation trend plunge trend plunge strike dip 210 10 245 8 75 25 se 218 9 225 16 65 30 se 192 19 231 13 13 40 se 225 5 229 11 80 32 se 239 20 232 18 5 24 se 224 14 237 26 0 35 e 226 7 228 11 240 64 se 212 13 235 23 285 15 s 215 16 225 15 13 40 se 205 6 7544s 209 14 24 40 se 208 16 202 24 lineation 221 10 trend plunge 225 15 208 15 229 9 170 38 240 11 187 40 235 13 210 15 225 8 220 16 230 15 135 30 252 9 115 20 223 8 234 12 232 3 230 6 222 9 228 9 224 4 220 5 218 8 03WHC7-12 foliation 03WHC14,15 foliation lineation strike dip strike dip plunge trend 85 35 85 51 146 20 14 228 72 46 85 50 46 14 4 46 122 40 207 17 17 260 101 24 lineation 217 32 8 219 41 18 plunge trend 219 20 9 223 49 9 42 162 85 11 10 212 44 20 34 138 66 15 9 210 310 6 10 131 201 27 7 211 301 11 9 149 170 37 2 45 90 36 10 138 97 11 3 206 59 20 54 174 49 10 9 219 72 26 59 160 65 30 8 224 79 27 2 75 120 15 19 228 57 27 11 45 85 22 7 60 77 43 11 154 77 15 17 235 47 47 10 325 205 20 7 236 76 48 3 125 205 33 6 233 90 19 30 138 192 30 7 225 110 33 24 146 80 22 14 221 56 35 36 154 110 20 13 222 32 30 26 143 160 10 12 226 36 7 20 162 203 7 17 232 282 15 36 200 184 16 7 230 15 55 14 143 210 17 3 220 72544135 5 217 65 26 36 125 2 40 78 35 66 32 936 68 37 03N of WHC15 foliation lineation 03WHC11 foliation lineation strike dip plunge trend strike dipplunge trend 320 5 9 34 100 35 2 205 349 4 4 66 0 12 7 60 345 15 16 25 85 16 6 48 115 5 2 218 320 15 16 46 89 14 10 230 322 14 14 57 121 21 21 215 293 11 16 43 152 24 20 211 324 11 7 28 152 20 17 205 310 6 9 44 90 17 17 207 340 10 8 30 120 19 3 220 243 18 65 15 0 207 205 5 2 30 300 6 5 39 03WHC16 foliation lineation 24 2152 348 6 8 41 strike dipplunge trend 313 1 2 211 156 26 12 228 338 11 11 48 60 20 14 228 30 3 10 32 8720446 23 20 6 38 146 20 35 185 355 19 8 219 46 14 17 260 291 9 9 223 207 17 63 17 10 212 132 51 160 9 9 210 292 7 7 211 181 19 182 9 CF1,4 foliation lineation strike dip strike dip trend plunge trend plunge 42 32 320 15 10 56 17 40 45 30 0 19 15701758 15 6 358 11 10 54 14 62 267 21 316 17 21 90 5 42 621315 11 19 16 15 42 80 12 80 11 6 65 7 47 55 15 45 20 15 65 7 213 70 19 310 24 21 110 2 225 95 21 321 25 4 61 25 40 295 11 311 19 30 65 31 17 70 25 317 29 35 40 15 47 90 11 288 22 5 340 27 43 315 24 61 43 34 29 17 37 265 5 252 16 21 240 10 233 330 15 276 19 6 244 270 34 316 15 24 215 75 32 5 250 58 26 16 230 46 20 15 256 11 19 15 237 16 22 19 247 24 18 22 14 48 32 16 37 312 27 11 37 322 15 14 34 291 12 11 75 320 14 9 62 185 17 19 34 216 22 13 15 274 20 18 34 CF2 foliation lineation CF3 foliation strike dip trend plunge strike dip 300 52 38 55 310 23 331 36 36 36 315 35 10 25 21 45 320 16 355 27 23 37 292 15 335 22 21 85 346 17 316 19 19 56 315 34 308 19 22 47 355 7 295 27 30 45 336 11 315 26 27 42 338 16 340 24 27 37 335 17 280 32 32 35 322 27 355 27 27 45 332 32 31 45 lineation 298 31 32 41 trend plunge 303 27 36 52 23 56 307 30 32 52 26 57 312 31 31 44 16 55 311 29 34 46 11 62 320 33 37 52 17 62 32 63 946 12 46 16 52 16 46 27 35 CF5 foliation lineation CF6 foliation strike dip trend plunge strike dip 48 26 15 75 60 40 22 85 65 40 51 20 19 85 51 26 60 36 15 100 62 25 55 20 14 77 55 24 54 32 29 75 64 23 62 28 15 85 72 36 72 34 10 65 62 27 56 37 15 74 60 37 4 82 lineation 70 27 17 79 80 40 13 83 trend plunge 66 37 3 72 29 100 67 42 5 84 36 89 61 46 11 74 9 73 76 46 34 77 11 84 73 43 11 92 82 30 11 84 84 32 570 64 53 64 55 BR 2 foliation lineation BR6 foliation strike dip trend plunge strike dip 35 59 25 24 210 42 273 30 25 23 199 31 264 24 21 22 194 32 252 23 26 15 196 31 271 17 23 22 265 21 18 17 274 19 14 13 CH3-8 foliation 273 16 12 22 262 17 12 20 262 16 25 20 stike dip 266 27 17 11 325 7 276 16 26 20 265 24 266 26 26 20 270 20 271 27 5 244 125 21 68 12 40 5 66 22 04 80 18 76 5 348 11 280 13 328 16 325 6 260 30 195 10 252 21 272 25 262 24 262 20 267 22 270 19 4151496 CH1 foliation 326 25 25 53 78 30 strike dip 281 34 89 9 253 45 245 30 32 10 126 11 CH9-10 foliation 95 27 102 12 CH3-8 lineation strike dip 192 16 135 20 120 15 plunge trend 148 16 89 24 120 19 84 17 11 59 90 16 116 20 22 85 101 14 111 16 23 25 112 24 2456825 lineation 8 221 105 35 4 225 80 22 plunge trend 12 90 55 16 11 169 79626161532 14 87 85 16 6 28 13 93 42 20 27 185 374301612193 554429 13 216 070101414198 15 57 14 186 20 52 16 198 26 14 19 202 24 358 15 200 21 352 23 348 16 336 15 112 CH2 foliation lineation Pike Stream foliation strike dip plunge trend strike dip 260 46 30 22 225 10 265 19 21 16 180 30 246 16 7 356 255 37 275 11 21 9 185 53 315 34 21 350 190 35 266 24 12 330 320 37 252 9 24 356 322 40 310 21 27 27 338 17 350 35 4 29 180 42 278 31 20 55 202 30 306 27 32 46 175 22 284 26 20 54 188 36 295 26 21 32 325 26 307 27 22 60 186 19 282 13 12 52 315 24 341 25 8 285 192 34 338 28 20 55 176 40 348 26 25 52 192 36 325 24 22 45 310 32 20 56 270 26 25 8 Pike Stream lineation CH1-2 F2,3 axes N of WHC3 F3 axes plunge trend plunge trend plunge trend 31 215 10 30 7 303 10 223 15 18 17 252 16 196 31 126 22 263 14 210 11 280 27 223 16 279 30 200 12 281 WHC11-4 F3 axes 37 214 4 2 12 203 36 36 plunge trend 27 205 14 21 11 245 15 210 579 16 210 All F3 kinks axes 950 10 206 11 226 24 215 plunge trend 11 240 10 210 11 245 1 59 27 230 5 79 10 224 10 203 9 50 9 220 11 190 11 226 11 191 11 240 159 WHC3-4 F2 axes 10 224 9 220 plunge trend 35 238 39 270 11 231 37 225 30 225 20 235 22 250 13 200 24 260 18 202 9 268 72312245 17 225 19 255 17 235 16 230 14 248 WHC7-10 F3 axes 18 265 18 275 plunge trend 14 305 36 114 7 195 35 110 7 252 29 120 14 250 21 125 16 283 33 136 16 264 10 55 5 310 20 30 15 285 12 165 17 305 24 172 21 292 26 124 7 303 17 252 22 263 17 228 12 21 20 235 24 255 WHC 1
Sense Fault Fault Striae Striae T-axis T-axis P-axis P-axis of Slip Strike Dip (rhr) Trend Plunge Trend Plunge Trend Plunge NR 145 51 240 51 237 6 35 84 NL 120 48 45 45 138 11 37 44 NR 128 46 240 44 49 0 319 79 NR 142 68 265 64 242 22 29 64 NR 130 67 335 49 263 23 14 41 NL 335 80 350 56 39 28 278 45 NL 355 75 42 70 74 29 283 58 NL 155 87 115 87 243 44 67 45 NR 162 21 255 21 74 24 257 66 NR 110 59 245 50 219 10 332 65 NL 168 51 105 37 358 4 92 39 NR 304 74 50 73 38 29 207 61 NL 316 66 45 66 46 21 227 69 NR 120 46 222 45 216 1 312 84 NL 138 46 222 46 225 1 118 87 NR 141 40 235 41 53 5 257 85 NR 116 75 225 75 210 30 19 59 NL 122 56 119 51 174 12 68 51 NL 132 57 222 57 222 12 42 78 NR 314 80 44 80 44 35 224 55 WHC 10
Sense Fault Fault Striae Striae T-axis T-axis P-axis P-axis of Slip Strike Dip (rhr) Trend Plunge Trend Plunge Trend Plunge NL 290 67 20 67 20 22 200 68 NL 7 2 60 123 53 146 12 26 67 NL 278 76 290 41 335 17 230 39 NL 287 78 298 41 343 18 238 38 NL 230 60 320 60 320 15 140 75 NR 205 60 325 56 307 14 79 70 NL 206 63 267 60 286 17 144 69 NL 330 50 320 48 15 6 277 52 NL 9 4 68 104 24 144 1 53 33 NL 9 7 76 119 56 163 24 41 49
CH7 fan
Sense Fault Fault Striae Striae T-axis T-axis P-axis P-axis of Slip Strike Dip (rhr) Trend Plunge Trend Plunge Trend Plunge NR 123 58 233 57 221 13 4 74 NL 149 28 232 28 54 17 224 73 NL 152 29 218 27 46 17 194 70 NR 110 45 262 25 55 12 309 53 NR 130 40 263 32 65 10 312 66 NR 115 67 226 66 211 22 10 67 NL 134 40 213 40 38 5 171 83 NL 124 55 200 55 208 10 62 78 NR 147 63 260 61 245 17 34 70 CF1 block
Sense Fault Fault Striae Striae T-axis T-axis P-axis P-axis of Slip Strike Dip (rhr) Trend Plunge Trend Plunge Trend Plunge TR 123 30 140 10 110 47 343 29 NL 218 49 239 22 90 11 192 47 NL 279 38 290 8 139 27 255 41 NR 1 5 68 200 29 151 5 244 33 NL 2 0 50 60 37 265 1 357 62 NL 2 8 62 85 58 106 16 331 68 NL 3 1 51 80 43 101 2 6 68 NR 185 80 346 61 297 30 66 48 NR 291 60 82 40 49 6 147 54 NL 219 50 223 5 79 23 184 31 NL 2 2 53 70 45 92 4 353 67 NR 192 70 22 61 311 29 77 47 NL 172 17 175 1 11 42 158 43 NL 2 4 45 63 32 266 7 9 61 NR 282 86 103 32 53 20 152 24 NL 3 2 43 42 10 253 23 4 40 NR 7 8 40 238 16 31 20 277 47 NR 7 9 32 242 10 38 28 273 47 NR 8 4 30 232 17 33 24 264 55 NR 8 0 42 235 21 29 16 278 51 NL 2 4 45 63 32 266 7 9 61 NL 3 2 43 42 10 253 23 4 40 NL 3 0 53 35 6 252 21 354 30 NL 6 2 22 55 2 256 41 35 41 NR 6 5 36 211 22 10 18 250 57 CF1 reg
Sense Fault Fault Striae Striae T-axis T-axis P-axis P-axis of Slip Strike Dip (rhr) Trend Plunge Trend Plunge Trend Plunge NR 9 7 30 231 23 36 20 263 62 NL 310 19 12 17 199 28 359 61 NL 295 35 12 34 197 10 346 78 NR 190 50 328 39 124 1 33 64 NL 2 0 34 45 16 248 23 10 51 NL 355 35 60 32 250 12 23 73 NR 327 80 204 76 67 45 231 44 NR 322 82 222 80 54 46 231 44 NR 6 5 35 167 34 342 10 191 78 NR 5 6 34 159 33 334 11 182 77 NL 7 5 37 117 27 316 14 75 62 NR 8 7 35 273 6 62 33 301 38 NR 284 74 16 74 14 29 193 61 NL 325 67 34 66 49 22 251 67 NL 334 54 10 39 38 2 305 59 NR 180 14 342 4 150 39 356 48 NR 244 24 352 23 167 22 5 67 NR 103 44 248 29 44 10 298 58 NR 8 6 39 251 12 43 24 288 44 CH6-7
Sense Fault Fault Striae Striae T-axis T-axis P-axis P-axis of Slip Strike Dip (rhr) Trend Plunge Trend Plunge Trend Plunge NL 351 18 49 15 237 29 36 59 NR 338 29 70 29 249 16 72 74 NL 332 19 50 19 233 26 44 63 NR 285 12 57 9 230 36 66 53 NR 310 19 52 19 229 26 58 63