A Thesis Proposal Presented By. Paul Michael Betka To. The Faculty of the Geology Department of The University of Vermont 4/24/07

Transcription

1 EVOLUTION AND TECTONIC SIGNIFICANCE OF DUCTILE DEFORMATION IN THE LOWER CRUST DURING CONTINENTAL EXTENSION FROM THE BREAKSEA SOUND SHEAR ZONE, FIORDLAND, NEW ZEALAND A Thesis Proposal Presented By Paul Michael Betka To The Faculty of the Geology Department of The University of Vermont 4/24/07 Accepted by the Faculty of the Geology Department, the University of Vermont, in partial fulfillment of the requirements for the degree of Master of Science specializing in Geology. The following members of the Thesis Committee have read and approved this document before it was circulated to the faulty: Advisor Keith A. Klepeis

2 Evolution and tectonic significance of ductile deformation in the lower crust during continental extension from the Breaksea Sound shear zone, Fiordland, New Zealand Paul M. Betka, Geology Department, University of Vermont, 2007 Abstract The recent discovery of deep (~18kb, ~60km) granulite and eclogite exposures within a lower crustal extensional shear zone in Fiordland, New Zealand provides evidence of interactions between the lower crust and lithospheric mantle during continental extension. Field studies of oceanic core complexes along the mid-atlantic ridge document the evolution of the crust-mantle boundary during oceanic extension. New data from a lower continental crust extensional shear zone in Fiordland, suggest that some processes controlling extension in the lower continental crust are similar to those occurring in oceanic core complexes. During oceanic extension, hydration of mantle rocks brought near the surface creates weak lithologies that localize large amounts of strain in a core-complex style detachment.. This study examines field evidence from a lower continental crust extensional shear zone that suggests strain localization in the lower crust and upper mantle are controlled by retrograde metamorphism during continental extension. ii

3 Table of Contents ABSTRACT... II TABLE OF CONTENTS...III INTRODUCTION... 1 OBJECTIVES... 1 BACKGROUND INFORMATION... 2 CONTINENTAL EXTENSION... 2 OCEANIC EXTENSION... 3 PROPOSED RESEARCH... 4 FIELD SITE... 5 ANALYTICAL TECHNIQUES... 5 INITIAL RESULTS AND SIGNIFICANCE... 8 TIMELINE REFERENCES iii

4 Introduction Continental lithosphere consists of the continental crust and lithospheric mantle, separated by the Mohorovicic discontinuity. The continental crust can be itself subdivided into three seismically and compositionally different layers: the upper crust (0 - ~15km), the middle crust (~15 - ~30km), and the lower crust (30km-~50km) (Mooney et al., 1998, Christensen and Mooney, 1995, Rudnick and Gao 2003). Along active plate boundaries, strength contrasts between different layers of the crust and the lithospheric mantle have proven to be an important driving force behind the development (or collapse) of orogenic belts (eg. Wijns 2005, Beaumont 2004, Koons 2003, Jackson 2002). More specifically, during continental extension the strength of the lower crust proves to be the dominant factor for determining how continents will extend. For example, when the lower curst is hot and weak, models predict that localized detachment faults zones will develop that accommodate large amounts of extensional strain. Conversely, if the lower crust is cool and strong, extensional strain will be spread throughout several normal faults distributed evenly within the crust (Buck 1991, Hopper and Buck 1996, Wijns 2005). Objectives This study investigates how rocks in the lower crust behave during a phase of continental extension. Rocks exposed in Fiordland, New Zealand provide a well-constrained tectonic setting to test the behavior of lower crust during extension. I will test models of continental extension that rely on the strength of the lower crust proposed by Buck (1991), Hopper and Buck (1996), Wijns (2005), and Karson et al. (2006) using field data from lower crust rocks exposed in Doubtful Sound shear zone that are currently understood as part of a lower crustal metamorphic core complex (Gibson and McDougall, 1988). I plan to focus on two factors influencing deformation in the lower crust: the geometry of ductile flow, and relationships between metamorphism and strain localization. On the basis of previous work, and preliminary field study results I hypothesize that: 1) hydration of mantle eclogites and granulites near the crust/mantle boundary cause metamorphic and rheologic heterogeneities similar to those of oceanic core complexes; and 2) strain localization within a weak amphibolite facies lithology defines a unique boudinage, or dome and basin geometry extension. 1

5 Background Information Continental Extension Several numerical models exist explaining the behavior of the lower crust during continental extension. Three models outlined by Buck (1991) describe the way continents extend depending on the strength of the lower crust: core-complex mode, wide rift mode, and narrow rift mode. Hopper and Buck (1996) modify these models of extension by assuming a three-layer rheology of continental lithosphere. They describe middle-upper crust and lithospheric mantle as brittle/plastic layers sandwiching a thin and viscous lower crust. Hopper and Buck (1996) conclude that the mode of extension depends highly on the strength of the lower crust. Very weak and low viscosity lower crust above an equally weak mantle lithosphere will undergo core-complex mode extension, where one localized detachment zone accommodates strain from the uplifting metamorphic core footwall block (Figure 1). Wide rift mode extension occurs when a weak lower crust is sandwiched by more viscous mantle lithosphere and upper crust. In this case, crustal buoyancy forces between the weak lower crust and rigid upper and middle crust are the dominant driving force for extension, and strain is spread over several normal faults across a large area (eg. basin and range province of N. America). A strong lower crust favors narrow rift mode extension. This occurs when crustal heat flow is low, and the viscosity of the lower crust is too high to flow easily, so strain is accommodated by normal faults localized into a narrow rift zone (eg. East African rift) (Hopper and Buck, 1996, Buck, 1991) (Figure 1). Numerical modeling by Wijns et al. (2005) also suggests that mechanical stratification of the crust controls the mode of continental extension. Wijns et al. (2005) separates continental crust into two layers, the upper and lower crust. In this case, the strength ratio between the upper and lower crust are again the primary control on type of extension. When there is a low strength ratio they predict distributed faulting mode extension, where strain is evenly accommodated by faults or plastic deformation in the crust. When the strength ratio between upper and lower crust is large, Wijns et al. (2005) predicts core complex mode extension, and large amounts of strain are localized in detachment fault zones. 2

6 Figure 1. Modes of continental extension and lithospheric strength profiles (after Buck 1995). Oceanic Extension Recent work by Schroeder and John (2004) and Karson et al. (2006) documents the presence of core complexes in oceanic crust along the Mid-Atlantic Ridge. Along these oceanic core complexes, extension occurs by the hydrous alteration of upwelling mantle dunite, peridotite, and harzburgite into amphibolite and greenschist facies shear zones. As the footwall of the shear zone becomes un-roofed it is hydrated and experiences retrograde metamorphism. A localized detachment shear zone develops in the weaker lithology and begins to accommodate extension as the core complex is uplifted. Extension along these core-complex shear zones serves as the plate boundary in the absence of active magmatism (Karson et al., 2006) (Figure 2). 3

7 Figure 2. Oceanic core complex showing strain localization into hydrated facies (after Karson et al., 2006). Proposed Research Numerical models of continental extension commonly assume a homogeneous rheology of the lower crust, suggesting that it is behaving uniformly. Field-based models of oceanic extension provide examples of crust-mantle interactions that occur much closer to the surface and are more easily observed. These models suggest that during oceanic extension hydration metamorphism controls the rheology of the crust by weakening mantle rocks, localizing strain into detachment fault zones (Schroeder and John, 2004, Karson et al., 2006). Similarly, during continental extension in Fiordland, preliminary results indicate that metasomatism of granulite and eclogite material representative of the mantle and lower crust created zones of weakening that accommodate strain during extension. These observations pose two fundamental questions about the behavior of the lower continental crust and lithospheric mantle during continental extension: does retrograde metamorphism of lower crust and upper mantle rocks cause weaker lithologies in the lower crust and upper mantle that accommodate strain during deformation? Do the lower continental crust and upper mantle behave similarly to processes documented in other settings, including oceanic core complexes? 4

8 Field Site Shoreline outcrops in Fiordland, New Zealand expose moderately to highly deformed eclogite, granulite, and amphibolite facies gneiss. Emplacement of these gneisses was synchronous with regional deformation and collisional orogenesis during the early Cretaceous (c. 120 Ma) that caused burial and granulite facies metamorphism (c. 16kb) in the lower crust (Hollis et al., 2004). Geochronologic data by Flowers et al. (2005) indicates that shortly after burial these gneisses experienced a rapid phase of exhumation beginning around 113 Ma. Several workers (Gibson et al., 1998, Spell et al., 2000, Scott and Cooper, 2006) have attributed exhumation of this magmatic arc to the development of middle and lower crust extensional shear zones. Early Cretaceous continental extension in western Fiordland marks the beginning of continued continental extension and exhumation until about 90 Ma. Onset of seafloor spreading (c. 85 Ma) attributed to the breakup of Gondwana marks the cessation of continental extension in Fiordland and the rifting of the Tasman Sea (Mortimer, in press). Outcrops along the Doubtful Sound shear zone discovered by King (2006) and Oliver (1980) exposed on the northern shore of Resolution Island provide field evidence of lower crustal shearing during extension. Geobarometery conducted on eclogite samples collected within this shear zone by Geoff Clarke, and Matthew De Paoli gives pressures of 18kb +/-1 indicating depths ~60km (personal communication, 2007). During a field season in 2007 an abundant amount of structural data was collected from this shear zone that describe the geometry of ductile flow. Field relations also suggest a critical relationship between metamorphic reactions in the lower crustal rocks and strain localization. Analytical Techniques Microstructural and petrologic analysis of deformed rocks in thin section provides valuable evidence describing the sequence and magnitude of metamorphic and deformational events. Describing the relationship of metamorphic reactions (eg. garnet altering to hornblende) relative to deformational events indicates the importance of metamorphism and rheology in controlling deformation, or accommodating strain. I plan to: 1) complete micro-structural and petrologic thin section analyses on samples collected from this site to, a) establish the timing and type metamorphic reactions relative to deformation, b) conduct kinematic, and strain studies to determine the vorticity of flow and style of deformation; and 2) use structural data including 5

9 attitudes of foliations, lineations, and kinematic indicators to describe the geometry of ductile flow during extension. I will use microstructure analytical techniques outlined by Passchier and Trouw (2005) and Vernon (2004) including describing quartz and feldspar textures, geometry of metamorphic tails on rotated porphyroblasts, c-s fabrics, and any other applicable methods. I will also attempt to employ the Rf/Φ technique on rotated plagioclase porphyroblasts in hand sample (Lisle, 1985). Macroscopic structural data collected from field sites can be plotted on a stereonet and interpreted to describe strain symmetry and geometry of ductile flow (eg. Gapias et al., 1987). Deformation textures for quartz and feldspar: Textures displayed by quartz and feldspar phases, visible in thin section, approximate a temperature range during deformation. For example, the lowest temperature deformation textures begin to occur in quartz when temperatures during deformation exceed 200 C. At this temperature quartz will display textures including deformation bands, and undulose extinction (Hirth and Tullis, 2002). Describing deformation textures displayed by quartz and feldspar constrains a temperature range on the rock during deformation between 200 C and 550 C, or suggests that these temperatures were exceeded and the deformation mechanisms in quartz and feldspar were reset (Passchier and Trouw, 2005). Deformation textures in feldspar will be particularly important for describing the temperatures of deformation within the amphibolite facies shear zones. Timing of metamorphic and deformational events: Deformed rocks also preserve textures from multiple deformation and associated metamorphic events. Comparing deformation textures, and metamorphic mineral assemblages across a suite of samples will describe the relative timing of a sequence of deformation and metamorphism. This will explicate the mineral phases that accommodated strain (eg. ductile quartz), occurred prior to deformation (eg. rotated garnet porphyroblasts), and the relative timing of metamorphic reactions during a given deformational event. Discerning the sequence and relationship between metamorphic and deformational events will help reconstruct the processes that control the development of a shear zone. 6

10 Kinematics: Kinematic indicators in a shear zone record the flow regime occurring in a rock during deformation. Textures in strained mineral phases that grew before or during deformation are referred to as kinematic indicators. Kinematic indicators include textures such as: metamorphic tails on rotated porphyroblasts, c-s fabrics in foliations, preferred orientations of elongate mineral phases, symmetry or asymmetry of porphyroblasts, lattice preferred orientations of c-axis of quartz and feldspar crystals, and several other textures. Vorticity of flow describes the rotational strain from an instantaneous stretching axis and can be used to quantify the ratio of pure-shear verses simple shear in a shear zone (Passchier and Trouw, 2005, Vernon, 2004, references therein). This kinematic information can be extrapolated and used to describe processes occurring in a shear zone on the map scale. Bulk kinematics uses mesoscale (outcrop-scale) structural data to describe the geometry of deformation in a shear zones. Attitudes of lineations, and foliations from a shear zone plotted on a stereonet can show the pervasive shear zone stretching direction. The median pole to foliation, and the vector on the foliation plane normal to the pervasive stretching direction are also plotted and describe the three-dimensional orientation of a strain ellipse. Symmetry and orientations of outcrop scale structural data is also useful for determining sense of shear, and bulk deformation history (Gapais et al. 1987). Rf/φ Technique: The Rf/φ technique describes the finite strain in a rock by measuring the aspect ratios and orientations of strain ellipsoids compared to assumed strain markers. Strain ellipsoids in ductile shear zones could be feldspar clasts, rotated porphyroblasts, quartz aggregates, boudins, and several other markers. Pervasive foliation in the shear zone, and the size or shape of crystals before deformation provides a reference frame for measuring strain. Plots of φ vs. Rf can be compared to several mathematically derived curves that will help quantify the strain ratio of a shear zone (Lisle, 1985). Determining phase equilibrium by thin section analysis: Metamorphic reactions that do not run to completion often preserve replacement textures or reactions rims on the mineral phases within a rock. Examining these textures including 7

11 pseudomorphs, symplectites, reactions rims, or coronas describes the types and progress of metamorphic reactions that occurred in the rock. Pseudomorphs and symplectites occur when one mineral phase is completely replaced by a new mineral phase during changing pressure and temperature conditions. In this case, the new mineral phase will keep the shape of the mineral that it is replacing. Reaction rims and coronas occur when metamorphic reactions along grain boundaries do not fully run to completion, and partial replacement rims around the mineral core record the progress, and direction of the reaction. Initial Results and Significance Mapping of outcrop-scale (~10m 2 ) exposures of this shear zone indicate that shear strain is localized in an anastomosing array of high strain amphibolite facies shear fabrics that envelope meter-scale, low-strain pods of eclogite and granulite. Eclogite and granulite display crosscutting clinozoisite- and hornblende-bearing veins, indicating they experienced brittle deformation during extension under high-pressure amphibolite-facies conditions. Amphibolite facies fabrics contain very strongly aligned hornblende and plagioclase stretching lineations that plunge strongly toward both ~25 and ~205. Foliations in amphibolite shear fabrics wrap the pod-shaped lenses of eclogite and granulite, forming dome-and-basin structures (Figure 3). Initial results indicate that metasomatism of granulite and eclogite weakened the lower crust sufficiently to allow it to accommodate strain during continental extension, resembling strain localization mechanisms recently documented in oceanic core complexes. This suggests that strain localization within the lower crust, determined by retrograde metamorphism, is the driving mechanism for accommodating strain in the lower crust. Notably, the bulk strength of the lower crust, and its ability to control the methods by which continents extend, may rely on rheologic heterogeneities developed through metamorphism and hydration in the lower crust during continental extension. For example, even in extensional settings where the lower crust is strong (eg. eclogite) strain localization processes may still accommodate large enough amounts of strain to develop core complex style extension. 8

12 Figure 3. Form map, profile sketch, and stereonet from a field site illustrating anastomosing high strain fabrics that envelope low strain mafic pods. 9

13 Timeline Spring 2007: Conduct field season in Fiordland New Zealand. Begin processing field data send off samples to be made into thin sections. Defend proposal. Summer 2007: Analyze thin sections from New Zealand for microstructural and petrologic textures. Continue to analyze field data from New Zealand, including making stereonet plots, field maps and cross sections. Submit GSA abstract. Fall 2007: Write up results and prepare a presentation for GSA. Defend progress report. Take a Geochemistry class, learn SEM/Microprobe techniques? November January 2007/2008: Compile results of thesis, work on publication(s?) and writing up thesis. Spring 2007: Write and defend thesis. 10

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15 King, D. S. (2006), Shear zone processes in the mid to lower crust and the structural evolution of central Fiordland, New Zealand, M.S. thesis, 150 pp., Univ. of Vermont, Burlington, February. Kloppenburg, A., S. H. White, et al. (2001). "Structural evolution of the Warrawoona Greenstone Belt and adjoining granitoid complexes, Pilbara Craton, Australia: implications for Archean tectonic processes." Precambrian Research 112: Koons, P. O., R. J. Norris, et al. (2003). "Influence of exhumation on the structural evolution of transpressional plate boundaries: An example from the Southern Alps, New Zealand." Geology 31(1): 3-6. Law, R. D., M. P. Searle, et al. (2004). "Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet." Journal of the Geological Society, London 161: Lisle, R. J. (1985). Geological Strain Analysis A Manual for the Rf/phi Technique. Oxford, Pergamon Press. Marcotte, S. B., K. A. Klepeis, et al. (2005). "Intra-arc transpression in the lower crust and its relationship to magmatism in a Mesozoic magmatic arc." Tectonophysics 407: Mooney, W. D., G. Laske, et al. (1998). "CRUST 5.1: A global model at 5 X 5 deg." Journal of Geophysical Research 103(B1): Mortimer, N. New Zealand's geological foundations. 11th International Gondwana Symposium, Institute of Geological & Nuclear Sciences. Passchier, C. W. and R. A. J. Trouw (2005). Micro-tectonics 2nd edition. New York, Springer Berlin Heidelberg. Rudnick, R.L., and S. Gao (2003), The Composition of the Continental Crust, In The Crust (ed. R.L. Rudnick), 3, Treatise on Geochemistry (eds. H.D. Holland and K.K. Turekian), Elsevier-Pergamon, Oxford, pp Schroeder, T. and B. E. John (2004). "Strain localization on an oceanic detachment fault system, Atlantis Massif, 30*N, Mid-Atlantic Ridge." Geochemistry Geophysics Geosystems 5(11). Scott, J. M. and A. F. Cooper (2006). "Early Cretaceous extensional exhumation of the lower crust of a magmatic arc: Evidence from the Mount Irene Shear Zone, Fiordland, New Zealand." Tectonics 25(TC3018). Spell, T. L., I. McDougall, et al. (2000). "Thermochronologic constraints on the breakup of the Pacific Gondwana margin: The Paparoa metamorphic core complex, South Island, New Zealand." Tectonics 19:

16 Vernon, R. H. (2004). A practical guide to Rock Microstructure. Cambridge, Cambridge University Press. Wijns, C., R. Weinberg, et al. (2005). "Mode of crustal extension determined by rheological layering." Earth and Planetary Science Letters 236: