Geometric analysis and scaling relations of deformation bands in porous sandstone

Displacement, length and linkage of deformation bands have been studied in Jurassic sandstones in southeastern Utah. Isolated deformation bands with lengths (L) that span more than three orders of magnitude show similar displacement (D) profiles with more or less centrally located maxima and gently increasing gradient toward the tips. Soft- and hard-linked examples exhibit steeper displacement gradients near overlap zones and immature hard links, similar to previously described fault populations. The deformation band population shows power-law length and displacement distributions, but with lower exponents than commonly observed for populations of larger faults or small faults with distinct slip surfaces. Similarly, the D_max-L relationship of the deformation bands shows a well-defined exponent of ca 0.5, whereas the general disagreement for other fault populations is whether the exponent is 1 or 1.5. We suggest that this important difference in scaling law between deformation bands and other faults has to do with the lack of well-developed slip surfaces in deformation bands. During growth, deformation bands link to form zones of densely spaced bands, and a slip surface is eventually formed (when 100 m < L < 1 km). The growth and scaling relationship for the resulting populations of faults (slip surfaces) is expected to be similar to ‘ordinary’ fault populations. A change in the D_max-L scaling relationship at the point when zones of deformation bands develop slip surfaces is expected to be a general feature in porous sandstones where faults with slip surfaces develop from deformation bands. Down-scaling of ordinary fault populations into the size domain of deformation bands in porous sandstones is therefore potentially dangerous.     

Polygonal deformation bands

We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1–5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.     

Permeability upscaling of fault zones in the Aztec Sandstone,Valley of Fire State Park,Nevada, with a focus on slip surfaces and slip bands

The effect of open and filled slip surfaces on the upscaled permeability of two fault zones with 6 and 14 m strike-slip in an eolian Aztec Sandstone, Nevada, USA is evaluated. Each fault zone is composed of several fault components: a fault core, bounded by filled through-going slip surfaces referred to as slip bands, and a surrounding damage zone that contains joints and deformation bands. Slip band geometry, composition, and petrophysical properties are characterized. Measurements and modeling show that slip band permeabilities can vary over 12 orders of magnitude depending on the degree of fill within the slip bands. The slip bands along with other fault zone components are represented in finite volume numerical calculations and the impact of various slip-band representations on upscaled fault zone permeability is tested. The results show 2 orders of magnitude variation in upscaled fault zone permeability in the fault-normal direction and a factor of 2 variation in the fault-parallel direction. The numerical results presented here are compared to the earlier numerical results in which structured Cartesian grids were used for the numerical simulations, and are in qualitative agreement with earlier calculations but use about a factor of 250–400 fewer numerical cells.     

Damage zone and slip-surface evolution over μm to km scales in high-porosity Navajo sandstone, Utah

Detailed mapping of throw variations and deformation along two km-scale normal faults in the high-porosity Navajo sandstone, Utah, has been used to investigate fault growth in this lithology. The faults consist of one or more through-going, striated, slip-surfaces, accommodating the greater part of the offset surrounded by a damage zone consisting of deformation band clusters and short, unconnected slip-surfaces. In contrast to previous models for deformation in this lithology, we find that the nucleation of slip-surfaces begins where measurable throw is negligible and deformation bands are forming and increasing in number. The microstructure and porosity of deformation bands and slip surfaces are distinct and independent of the amount of offset that they accommodate, i.e. they represent different and yet contemporaneous deformation mechanisms. The point where measurable throw begins to accumulate (the fault tip) is marked by the first through-going connected slip-surface. Increase in throw towards the centre of the fault results in a three-dimensional strain field, producing orthorhombic structural geometries within the damage zone. We find that the total width of the damage zone increases as offset is accumulated. For these faults, the damage zone width is approximately 2.5 times the total fault throw.     

Deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte-bearing brittle faults: A matter of a few seconds

Tectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites. In this study, we document the occurrence of recrystallization of quartz to ultrafine-grained (grain size 1–2 μm) aggregates along microshear zones (50–150 μm thick) in the host rock adjacent to pseudotachylytes from two different faults within quartzite (Schneeberg Normal Fault Zone, Eastern Alps), and tonalite (Adamello fault, Southern Alps) in the brittle crust. The transition from the host quartz to microshear zone interior includes: (i) formation of high dislocation densities; (ii) fine (0.3–0.5 μm) polygonization to subgrains defined by disordered to well-ordered dislocation walls; (iii) development of a mosaic aggregate of dislocation-free new grains. The crystallographic preferred orientation (CPO) of quartz towards the microshear zone shows a progressive misorientation from the host grain, by subgrain rotation recrystallization, to a nearly random CPO possibly related to grain boundary sliding. These ultrafine aggregates appear to be typically associated with pseudotachylytes in nature. We refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating. Microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip. Thermal models indicate that the process of recrystallization, including recovery processes, occurred in a time lapse of a few tens of seconds.     

Experimental investigation of the influence of slip velocity and temperature on permeability during and after high-velocity fault slip

We used a rotary shear apparatus to investigate changes of fluid transport properties in a fault zone by real-time measurement of gas flow rates during and after shearing of hollow sandstone cylinders at various slip rates. Our apparatus measures permeability parallel to the slip plane in both the slip zone and wall rocks. In all cases, permeability decreased rapidly with increasing friction, but recovered soon after slip, reaching a steady state within several tens of minutes. The rate of reduction of permeability increased with increasing slip velocity. Permeability did not recover to pre-slip levels after low-velocity (ca. 0.0019 m/s) tests but recovered to exceed them after high-velocity (ca. 0.29 m/s) tests. Frictional heating of gases at the slip surface increased gas viscosity, which increased gas flow rate to produce an apparent permeability increase. The irreversible permeability changes of the low-velocity tests were caused by gouge formation due to wearing and smoothing of the slip surface. The increase of permeability after high-velocity tests was caused by mesoscale fracturing in response to rapid temperature rise. Changes of pore fluid viscosity contributed more to changes of flow rate than did permeability changes caused by shear deformation, although test results from different rocks and pore fluids might be different.     

Seismic slip recorded in tourmaline fault mirrors from Elba Island (Italy)

This paper reports the first example of fault mirrors developed in an unusual protolith, consisting of tourmaline crystals with interstitial goethite. The deformation mechanisms active in the fault zone have been investigated from the outcrop to the nanoscale, aiming to identify possible traces of frictional heating at seismic slip rate, as observed for other fault mirrors in different protoliths. The investigation revealed the superposition of two main deformational stages. The first was dominated by brittle processes and produced a cataclastic/ultracataclastic principal slip zone, a few mm thick; the second was associated with seismic slip and produced a sharp discontinuity (the principal slip surface) within the cataclastic/ultracataclastic zone. The mirror-like coating, a few microns thick, occurs on the principal slip surface, and is characterized by 1) absence of interstitial goethite; 2) occurrence of truncated tourmaline crystals; 3) highly variable grain size, from 200 μm to 200 nm; 4) tourmaline close packing with interlobate grain boundaries, and 5) tourmaline random crystallographic orientation.Micro and nanostructural investigations indicate the occurrence of thermally-activated processes, involving both interstitial goethite and tourmaline. In particular, close to the principal slip surface, goethite is completely decomposed, and produced an amorphous porous material, with local topotactic recrystallization of hematite. Tourmaline clasts are typically characterized by strongly lobate boundaries, indicative of reaction and partial decomposition at grain boundaries. TEM observations revealed the occurrence of tourmaline nanograins, a few tens of nm in size, characterized by rounded shape and fading amorphous boundaries, that cannot be obtained by brittle processes. Lastly, the peculiar interlobate microstructure of the mirror surface is interpreted as the result of grain boundary recrystallization processes taking place by deformation at high-T conditions. Our results show that tourmaline fault mirrors recorded localized high-T processes triggered by frictional heating and can be therefore considered as reliable traces of ancient earthquakes.     

Growth, linkage, and termination processes of a 10-km-long strike-slip fault in jointed granite: the Gemini fault zone, Sierra Nevada, California

Field-based structural analysis of an exhumed, 10-km-long strike-slip fault zone elucidates processes of growth, linkage, and termination along moderately sized strike-slip fault zones in granitic rocks. The Gemini fault zone is a 9.3-km-long, left-lateral fault system that was active at depths of 8–11 km within the transpressive Late-Cretaceous Sierran magmatic arc. The fault zone cuts four granitic plutons and is composed of three steeply dipping northeast- and southwest-striking noncoplanar segments that nucleated and grew along preexisting cooling joints. The fault core is bounded by subparallel fault planes that separate highly fractured epidote-, chlorite-, and quartz-breccias from undeformed protolith. The slip profile along the Gemini fault zone shows that the fault zone consists of three 2–3-km-long segments separated by two ‘zones’ of local slip minima. Slip is highest (131 m) on the western third of the fault zone and tapers to zero at the eastern termination. Slip vectors plunge shallowly west-southwest and show significant variability along strike and across segment boundaries. Four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at, or below, greenschist facies conditions. Taken together, we interpret the fault zone to be a segmented, linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations, complex fault-segment interactions, and asymmetric slip-distance profiles.     

Growth processes,dimensional parameters and scaling relationships of two conjugate sets of compactive shear bands in porous carbonate grainstones,Favignana Island,Italy

Three main sets of deformation bands are identified in the Lower Pleistocene carbonate grainstones of Favignana Island (Italy). A bedding-parallel set is interpreted to contain compaction bands, based on the lack of evidence for shear. The other two sets are oriented at a high-angle to bedding, forming a conjugate pair comprised of compactive strike-slip shear bands. In this study, we focus on the compactive shear bands documenting their development, as well as analyzing their dimensional parameters and scaling relationships.Single compactive shear bands are thin, tabular zones with porosity less than the surrounding host rocks, and have thicknesses and displacements on the order of a few mm. The growth process for these structures involves localizing further deformation within zones of closely-spaced compactive shear bands and, possibly, along continuous slip surfaces within fault rocks overprinting older zones of bands. During growth, single bands, zones of bands and faults can interact and link, producing larger structures. The transitions from one growth step to another, which are controlled by changes in the deformation behavior (i.e. banding vs. faulting), are recorded by different values of the dimensional parameters for the structures (i.e. length, thickness and displacement). These transitions are also reflected by the ratios and distributions of the dimensional parameters. Considering the lesser porosity values of the structures with respect to the host rock, the results of this contribution could be helpful for mapping, assessing, and simulating carbonate grainstone reservoirs with similar structures.     

Spatial distribution of deformation bands in damage zones of extensional faults in porous sandstones: Statistical analysis of field data

The distribution of deformation bands in damage zones of extensional faults in porous sandstones has been analyzed using 106 outcrop scanlines along which the position and frequency of deformation bands have been recorded. The analysis reveals a non-linear relationship between damage zone width and fault throw, a logarithmic decrease in deformation band frequency away from the fault core, as well as a fractal spatial distribution associated with clustering of the deformation bands. Furthermore, damage zones appear wider in the hanging wall than in the footwall, although the deformation band density is similar on both sides. Statistical trends derived from the database imply that fault growth in porous sandstones can be considered as a scale invariant process. From an initial process zone, the damage zone grows by a constant balance between the development of new deformation bands in the existing damage zone and the creation of new bands outside. Moreover, as the width of the damage zone increases throughout the active lifetime of a fault, the distribution of the deformation bands in the damage zone remains self-similar. Hence band distribution and damage zone width for seismically mapped faults can be predicted from the relationships found in this paper.     

The application of lithogeochemistry in the search for unconformity-type uranium deposits, Northern Saskatchewan, Canada

Lithogeochemical-mineralogical haloes around unconformity-type uranium deposits in northern Saskatchewan can expand the size of drill targets up to fifteen times. The deposits are located at or near the unconformity between Aphebian metamorphosed basement rocks in close proximity to Archean granitoids and overlying unmetamorphosed sandstones of the Helikian Athabasca Group. Deposits studied include Key Lake (Deilmann), Midwest Lake and Eagle Point.Unconformity-type deposits are associated with broad alteration haloes in the overlying sandstones and more restricted haloes in the basement rocks. The haloes in the sandstone are localized around steeply dipping fault structures and are characterized by zones of intense leaching of specular hematite and detrital heavy mineral layers and by changes in chemistry related to clay mineral alteration and tourmalinization.Clay mineral alteration haloes are common in the sandstone and basement host rocks. Interstitial clays consist mainly of kaolinite and illite with lesser amounts of chlorite. The ratio K₂O/AI₂O₃ proved useful in delineating illite-kaolinite patterns in the sandstone and in the uppermost portions of the paleoweathering profile in basement rocks. The Midwest deposit is characterized by a broad bell-shaped zone (500 m across strike) of high K₂O/AI₂O₃ ratios (>0.18) in which illite is the dominant clay mineral. The sandstone above the Deilmann deposit, on the other hand, is characterized by silicification and kaolinitization with low K₂O/AI₂O₃ ratios (<0.04). This kaolinite cap overprints a preexisting illite zone.Anomalously high boron values are characteristic of the three deposits considered in this study. Boron anomalies are similar in extent to the anomalous clay mineral alteration haloes. Altered sandstones commonly contain aggregates of radiating magnesium-rich dravite needles within the clay matrix. The interpretation of boron patterns is problema tical however, mainly because of the detrital tourmaline component in sandstones and metamorphic tourmalines present in the Aphebian metasediments.Trace elements such as U, Ni, As and Co are generally of limited use in expanding targets in sandstone because their haloes are restricted to a few tens of metres horizon-anomalies along the steeply dipping zones of mineralization related to fault structures. Uranium (>3 ppm) does form an anomaly more than 200 metres laterally across the Midwest deposit. At the Deilmano deposit anomalous uranium dispersion is restricted to within a few metres of high grade ore.In the basement rocks, the various layers of the paleoweathering profile are geochemically overprinted up to 250 m from mineralization. Bleaching related to illitization and chloritization is associated with enrichment in K₂O, MgO, B, S, U, As, Ni and P₂O₃. The ratio Fe₂O₃/MgO is useful in delineating chloritization in the upper portion of the paleo weathering profile.Deposits with large root extensions in the Aphebian metasediments such as Eagle Point, have intense dravite-chlorite-illite alteration zones which are restricted to within a few metres of mineralization across strike. ‘Quartzite” units are alteration related. The complexity of the basement lithology inhibits the use of individual elements as alteration guides other than in the intensely altered zone. The application of multivariant techniques, element ratios and clay mineralogical work prove useful in identifying the mineralogical changes at Eagle Point.     

Strain compatibility and fault linkage in relay zones on normal faults

Relay zones on normal faults are unlikely to have tabular geometries as depicted in idealised models. Rotation of a relay ramp between non-parallel and non-planar relay-bounding faults will inevitably lead to strain compatibility problems causing open gaps or overlaps within the relay zone. Linkage of relay-bounding faults does not evolve from a single branch point. Rather, linkage occurs at multiple points along the fault tip lines giving rise to initially discontinuous branch lines. Where linkage occurs along a discontinuous slip-aligned branch line, displacement at different levels within the relay zone is partitioned between variable amounts of ramp rotation and slip across the branch line. The linking fault propagates when strain compatibility can no longer be maintained by continuous deformation processes, such as thickening or thinning of incompetent layers within the relay ramp. Step-like changes in vertical displacement vs. distance (d − x) profiles on horizons containing apparently intact relay ramps are probably indicative of incipient breaching and can be used predict the presence of a slip-aligned branch line in the sub-surface. Despite the complexity of the strain distribution within relay zones, the total vertical displacement across the relay remains geometrically coherent at all levels.     

A microstructural study of fault rocks from the SAFOD: Implications for the deformation mechanisms and strength of the creeping segment of the San Andreas Fault

The San Andreas Fault zone in central California accommodates tectonic strain by stable slip and microseismic activity. We study microstructural controls of strength and deformation in the fault using core samples provided by the San Andreas Fault Observatory at Depth (SAFOD) including gouge corresponding to presently active shearing intervals in the main borehole. The methods of study include high-resolution optical and electron microscopy, X-ray fluorescence mapping, X-ray powder diffraction, energy dispersive X-ray spectroscopy, white light interferometry, and image processing.The fault zone at the SAFOD site consists of a strongly deformed and foliated core zone that includes 2–3 m thick active shear zones, surrounded by less deformed rocks. Results suggest deformation and foliation of the core zone outside the active shear zones by alternating cataclasis and pressure solution mechanisms. The active shear zones, considered zones of large-scale shear localization, appear to be associated with an abundance of weak phases including smectite clays, serpentinite alteration products, and amorphous material. We suggest that deformation along the active shear zones is by a granular-type flow mechanism that involves frictional sliding of microlithons along phyllosilicate-rich Riedel shear surfaces as well as stress-driven diffusive mass transfer. The microstructural data may be interpreted to suggest that deformation in the active shear zones is strongly displacement-weakening. The fault creeps because the velocity strengthening weak gouge in the active shear zones is being sheared without strong restrengthening mechanisms such as cementation or fracture sealing. Possible mechanisms for the observed microseismicity in the creeping segment of the SAF include local high fluid pressure build-ups, hard asperity development by fracture-and-seal cycles, and stress build-up due to slip zone undulations.     

Architectural characteristics and petrophysical properties evolution of a strike-slip fault zone in a fractured porous carbonate reservoir

This paper describes the structural, petrophysical and hydromechanical properties relationships between a small fault zone and the porous layered carbonate series which host it. In a gallery located at 250-m depth, the deformation of a 22-m thick section of layered carbonates-, affected by a strike slip-fault have been characterized by means of structural (Q-value), acoustic velocities (V_p), porosity and uniaxial compressive strength (σ_c) measurements conducted in situ at the meter scale, and on laboratory samples at the infra-centimeter scale. A clear influence of the layers initial properties on fault architecture and properties evolution is underlined. In the porous layers with a low σ_c, there is an important accommodation of the deformation by micro-mechanisms resulting in a progressive decrease in the porosity toward the fault core. In the low-porosity layers with a high σ_c, deformations are accommodated toward the fault core by: an increase in the fracture porosity, in the micro-cracks porosity, and by displacements along pre-existing fractures resulting from a joint roughness decrease. The fault zone appears as relatively stiff and low permeable zones intercalated with low stiffness and high fracture permeability zones that extend one to tens of meters from the fault following the initial properties contrasts and geometry of the sedimentary layers.     

填埋场衬垫系统界面本构模型研究

典型垃圾填埋场衬垫系统由土/土工膜、土工膜/土工织物等多个界面组成,这些界面抗剪强度低,成为潜在的滑裂面,容易使填埋场发生沿界面的失稳破坏。本文根据大量土/土工膜界面直剪试验的研究,针对该界面的变形机理和应变软化特性,在前人研究的基础上确定了接触面的厚度,定义了接触面的切向应变和法向应变,将界面的变形分为3个阶段:弹性阶段,塑性阶段和残余阶段。同时提出了适合于土与土工膜界面的弹塑性本构模型。该本构模型参数较少,物理意义明确,运用简单方便。最后,结合Filz接触面直剪试验和钱学德单剪试验的结果,将通过该模型得到的拟合结果与之进行了对比验证分析。研究结果表明:本文提出的土与土工膜界面弹塑性本构模型是合理可行的,且建模方便,适用于工程实践。     

Smoothing and re-roughening processes: The geometric evolution of a single fault zone

The geometry of a fault zone exerts a major control on earthquake rupture processes and source parameters. Observations previously compiled from multiple faults suggest that fault surface shape evolves with displacement, but the specific processes driving the evolution of fault geometry within a single fault zone are not well understood. Here, we characterize the deformation history and geometry of an extraordinarily well-exposed fault using maps of cross-sectional exposures constructed with the Structure from Motion photogrammetric method. The La Quinta Fault, located in southern California, experienced at least three phases of deformation. Multiple layers of ultracataclasite formed during the most recent phase. Crosscutting relations between the layers define the evolution of the structures and demonstrate that new layers formed successively during the deformation history. Wear processes such as grain plucking from one layer into a younger layer and truncation of asperities at layer edges indicate that the layers were slip zones and the contacts between them slip surfaces. Slip surfaces that were not reactivated or modified after they were abandoned exhibit self-affine geometry, preserving the fault roughness from different stages of faulting. Roughness varies little between surfaces, except the last slip zone to form in the fault, which is the smoothest. This layer contains a distinct mineral assemblage, indicating that the composition of the fault rock exerts a control on roughness. In contrast, the similar roughness of the older slip zones, which have comparable mineralogy but clearly crosscut one another, suggests that as the fault matured the roughness of the active slip surface stayed approximately constant. Wear processes affected these layers, so for roughness to stay constant the roughening and smoothing effects of fault slip must have been approximately balanced. These observations suggest fault surface evolution occurs by nucleation of new surfaces and wear by competing smoothing and re-roughening processes.     

Dynamic analysis and two types of kink bands in quartz veins deformed under subgreenschist conditions

In order to clarify deformation mechanisms and behaviours of quartz in a low-temperature regime in the earth's crust, microstructural analyses, particularly on kink bands have been carried out for quartz veins moderately deformed under subgreenschist conditions. Both the dominance of subbasal deformation lamellae and geometry of kink bands suggest that basal (0001) slip was the sole active slip system in the deformed quartz. On a morphological basis, kink bands in the quartz were classified into two types: type I is characterized by conjugate and narrow bands with angular hinge zones, and type II by a wide monoclinal band. Dynamic analyses using deformation lamellae and kink bands have revealed that type I kink bands were formed in grains with basal plane (sub-)parallel to the compression axis, whereas type II kink bands were formed in grains with basal planes inclined to it. Using a numerical model of kinking of elastic multilayers modified after Honea and Johnson (Tectonophysics 30, 197–239, 1976), changes of the level of yielding stress for kinking and the width of kink bands as a function of the angle θ between the slip plane and the compression axis have been examined. The theory predicts that type I kink bands were formed at a higher stress level than type II kink bands, and hence occurrence of type I kink bands suggests that a significant strain hardening occurred in the deformed quartz veins. The theory also well explains the fact that the width of type I kink bands (θ=0 to 10°) is narrower by an order of magnitude than type II kink bands (θ=10 to 80°).     

The Salado River fault: reactivation of an Early Jurassic fault in a transfer zone during Laramide deformation in southern Mexico

The Salado River fault (SRF) is a prominent structure in southern Mexico that shows evidence of reactivation at two times under different tectonic conditions. It coincides with the geological contact between a structural high characterized by Palaeozoic basement rocks to the north, and an ～2000 m thick sequence of marine and continental rocks that accumulated in a Middle Jurassic–Cretaceous basin to the south. Rocks along the fault within a zone up to 150 m across record crystal-plastic deformation affecting the metamorphic basement of the Palaeozoic Acatlán Complex. Later brittle deformation is recorded by both the basement and the overlying Mesozoic sedimentary rocks. Regional features and structural textures at both outcrop and microscopic scale indicate two episodes of left-lateral displacement. The first took place under low-to medium-grade P-T conditions in the late Early Jurassic (180 Ma) based on the interpretation of ⁴⁰Ar/³⁹Ar ratios from muscovite within the fault zone; the second occurred under shallow conditions, when the fault served as a transfer zone between areas with differing magnitudes of shortening north and south of the fault. In the southern block, fold hinges were dragged westward during Laramide tectonic transport to the east, culminating in brittle deformation characterized by strike–slip faulting in the Mesozoic sedimentary rocks. North of the fault, folds are not well defined, and it is clear that the fold hinges observed in the southern block do not continue north of the fault. Although the orientation and kinematics of the SRF are similar to major Cainozoic shear zones in southern Mexico, our new data indicate that the fault had become inactive by the time of Oligocene volcanism.     

Microstructure,c-axis pattern,microstrain and kinematics of some S-C mylonites in grenville gneiss

In a major tectonic zone late extension related SC mylonites locally overprint the predominant coarser quartz microstructures, which are related to earlier thrusting. Some of the SC mylonites display a microstructural evolution which began with the formation of deformation bands in the coarser pre-existing microstructure and continued with the formation of asymmetric quartz microfoliations, either by continued formation of deformation bands or fine new grains oblique to the deformation band boundaries. The orientation of boudinaged and passively reoriented rutile needles show that (i) the formation of deformation bands was preceded and accompanied by the accumulation of strain; (ii) that the deformation bands and oblique microfoliation which formed directly from them lie close to the finite stretching direction; whereas (iii) other microfoliations form oblique to deformation bands and extended rutile needles near the probable instantaneous stretching direction. The latter are therefore interpreted to be strain insensitive, steady-state foliations. The crystallographic preferred orientation of the original deformation bands appears to determine that of the microfoliations, the two types of microfoliations showing distinct but related patterns. The element common to both types is the presence of two maxima near Y in a YZ girdle—a feature inherited from the deformation bands, which were formed in the initial stages of shortening of the aggregate, favourably disposing it for rhomb slip and providing nucleation sites for subsequent recrystallization.The data confirm that, despite the fact that a variety of microstructures and crystallographic microfabrics result from recrystallization processes, kinematic information is usually recoverable from the crystallographic microfabrics owing to the primacy of intracrystalline slip processes.