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Empirical relationships between thickness and displacement for different types of deformation zones in quartzofeldspathic rocks are presented. Faults exhibit a linear correlation of thickness with displacement and an average displacement/thickness ratio of 63; mylonite zones may show a similar correlation (with an average ratio of 2), though the data are problematic. The thickness-displacement relationships provide information on the growth of deformation zones with time and the role of strain softening and hardening during progressive displacement.

Microstructural observations of naturally faulted granitic gneisses show that feldspar grains are weaker than quartz grains at temperatures below 325γC and at depths of less than 8–10 km. Feldspar grains sustained most of the deformation by grain-scale faulting, slip along cleavage-controlled fractures and cataclasis. Fracture of feldspar grains within fault zones promoted their alteration to kaolinite. Quartz grains also deformed by fracturing, and often healed to form quartz porphyroclasts and mosaics in a comminuted matrix of feldspar and kaolinite. Syntectonic alteration of the feldspar grains may have weakened the fault zones over time and resulted in foliated textures within the fault zones. This study of naturally deformed rocks confirms published experimental results on the behavior of granitic rocks at low temperatures and pressures and, taken together, these data show that the theology of the upper 10 km of the crust is greatly influenced by cataclastic processes in feldspar.

The narrow zone of mid-Cenozoic detachment terrane in the southwestern United States can be widened and traced across the southern strands of the San Andreas fault zone into the eastern margin of the Peninsular Ranges Batholith. The Yaqui Ridge core complex and detachment fault, in southern Borrego Valley, California, exemplifies the nature of detached terranes in southcentral California. The detachment fault dips 10°–40° to the northeast, and separates a lower core of gneissic Late Cretaceous granodiorite from an unconsolidated, unmetamorphosed megabreccia of Eocene to Miocene age. Foliations in the lower plate generally conform to the strike and dip of the overlying detachment fault, and become less distinct away from the fault. The megabreccia which forms the upper plate is composed of unsorted, fairly well rounded clasts characteristic of batholithic and metasedimentary rocks of the region. Plio-Pleistocene lacustrine sediments unconformably overlie the megabreccia in some areas. The detachment fault and upper plate are expressed as a series of klippen which parallel Yaqui Ridge along the northeast and southeast flanks. The fault is marked by a typically narrow (∼6–10 cm) band of intensely sheared cataclasite. A chlorite-breccia zone occurs below the cataclasite, and gradually grades into the less deformed, regionally foliated gneissic granodiorite. On the southeast flank, what appears to be an extension of the cataclasite and detached upper plate is exposed only locally, near the southeast nose of the antiform. Here, the upper plate consists of blocks of pale gray to tan, aphanitic fault gouge. Superimposed upon and cutting through the detachment-related features is a broad zone of left-oblique-slip faulting termed the Yaqui Ridge shear zone. This fault zone trends WNW-ESE, parallel to Yaqui Ridge, and separates the detachment fault from the ridge, merging with the detachment fault near the eastern margin. Further east, the shear zone widens into a massive zone of gouge >400 m wide and disappears under the sediments of Borrego Valley and the Salton Trough. To the west, the shear zone slices into the eastern margin of the Peninsular Ranges Batholith and extends the marginal plutons in an E–W direction. Four dominant tectonic episodes can be recognized from exposures at and adjacent to Yaqui Ridge: (1) A Late Cretaceous synkinematic cataclasis and metamorphism accompanying and following emplacement of the Yaqui Ridge and related plutons, and forming a pervasive regional NW-trending foliation and NE-trending lineation. (2) A subsequent mid-Cenozoic, shallow, low temperature event of detachment faulting and folding which produces localized brittle cataclasis which overprints the earlier NW-trending fabrics. (3) The Late Miocene-Early Pliocene development of Yaqui Ridge shear zone along the northeast margin of Yaqui Ridge, shearing, rotating and overprinting the fabrics formed during the Late Cretaceous plutonism and regional deformation. (4) A Pleistocene episode of high-angle faulting and folding which produce a series of E-W-trending anticlines and synclines in Upper Pliocene to Pleistocene sediments. Linear elements which have formed throughout the several Late Cretaceous to Cenezoic deformations are concordant.

Fault plane structures and microstructures have been examined from a fault in Cambro-Ordovician quartzites exposed on the north coast of the Cantabrian zone, N.W. Spain. Along the fault there is a 2–3 cm wide cataclasite zone bounded by plastically deformed intact quartzite, which is restricted to a few metres from the fault plane. In thin section, the cataclasite consists of angular fragments of quartz in a fine-grained matrix of quartz cement. Cathodoluminescence reveals extensive intra- and transgranular microcracking. Deformation lamellae (D.L.) are widespread in the intact quartzite; electron microscopy shows that they are defined by a banded dislocation substructure. The microstructures indicate a fault history consisting of plastic deformation restricted to a metre-scale zone, followed by cataclasis and dilational fracture. The early plasticity is interpreted as evidence for the propagation of a process zone. The D.L. are used as an indicator of power-law-breakdown creep, which occurs above a critical normalized stress, estimated as at least 170 MPa from available data. Dislocation densities also indicate a stress higher than 116 MPa. The process zone as defined by formation of D.L. is modelled using fracture mechanics theory. For the likely size of the zone, the remote applied stress for fault propagation is shown to be less than 100 MPa.

The cataclastic rocks in the Alpine Fault Zone fall into three distinct groups: (a) fault pug, fault breccia, and shattered rock, (b) cataclasite, mortared and brecciated rock, and (c) mylonite, augen mylonite, ultramylonite, and blastomylonite. The first group is related to Quaternary movements at the junction of schist and granite; the zone of shattered schist is several chains or more in width, and the fault pug and fault breccia a few feet to more than 100 ft wide. The second group is conspicuous in the granitic rocks, the structureless cataclasite being 100–200 ft thick, and the zone of brecciated and mortared granite a mile or more in width, The mylonite group is characterised by fluxion banding and forms a belt 1,000–2,500 ft wide, largely within the granite. The age of formation of the mylonite and cataclasite is uncertain but a key tectonic horizon is the blastomylonite, which shows features of both dislocation and regional metamorphism. The evidence of the blastomylonite, and the compar...

Many pseudotachylytes and their related fault rocks are found in the Hidaka metamorphic belt representing an ancient crustal section. On the basis of field observations of the pseudotachylytes and related fault rocks, nature of seismogenic faulting in the Hidaka crust is examined. The field observations suggest the following conclusions. (1) Two structural types of pseudotachylytes are distinguished: layer-parallel and layer-oblique. The latter are scattered in the metamorphic belt, but the former occur only in the southern part of the metamorphic belt. (2) An abundance of the layer-parallel pseudotachylytes suggests that earthquakes occurred repeatedly and frequently in the southern part, where complicated and duplicated crustal structures occur with many low-temperature thin mylonite zones. The southern part with such crustal structures was an ancient seismogenic area containing asperities and having a radius of a few tens of kilometers in the Hidaka crust. In the seismogenic area, the layer-parallel pseudotachylytes resulted from seismic slip on the mylonitic foliation within the low-temperature mylonite zones with strong preferred orientation of micas. (3) The layer-parallel pseudotachylytes and the subsequent layer-oblique pseudotachylytes post-date the latest and very-low-temperature mylonitization in the metamorphic belt. The former pseudotachylytes formed just above the upper side of the brittle-plastic transition zone.

The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220°C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m.y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.

A new approach for revealing the brittle origin of C-surfaces as localized high shear strain zones in S-C mylonites (mylonites with C-surfaces cutting through a mylonitic S-foliation) is presented. A compiled worldwide catalog of width (W) and displacement (D) data for shear zones indicates that ductile mylonites show a constant W/D ratio of 10−0.3 and ratios of brittle ‘cataclasites’ vary in magnitude from 10−1 to 10−3, implying that the ratio is a diagnosis for discriminating ductile and brittle shear zones. A newly measured W-D data of shear displaced minerals along C-surfaces in granitic S-C mylonites from the Hatagawa shear zone in northeast Japan is added on the worldwide W-D catalog, being plotted on a brittle origin with the high W/D ratio of 10−1.5. Using this result and a tribochemical wear theory which accounts for wear formation under hydrothermal conditions, C-surfaces in the S-C mylonite might have been formed by cataclastic deformation under the lithosphere stress level of ca. 300 MPa at temperature of 400°C with water for granite. This result suggests a high lithosphere stress level at the depth of the S-C mylonite formation where deformation is predominantly plastic.

We determined the internal and permeability structures of the nappe boundary between the Tamba and the Ultra-Tamba belt in the Inner Zone of Southwest Japan. Protolith of hanging wall is Permian sandstone and that of footwall is Jurassic pelitic breccia. These are stuck tightly with cataclasite zone of 3 m in width. Permeability data and structural observations of the fault zone suggest the following; (1) cataclasite is distributed in the fault zone, but most deformation is localized in 0.5 to 3 cm width foliated cataclasite zones in the hanging wall, (2) experiments at effective pressures, Pe, of 40 to 100 MPa show that permeability of the hanging wall is low (10-20 m2 at Pe > 80 MPa), while it is higher by 1-3 orders of magnitude in the footwall and the fault zone. The permeability structure implies that high pore fluid pressure could be sustained within the fault zone, which could have promoted the thrust movement.

The Alpine Fault in New Zealand is a major plate-bounding structure that typically slips in ∼M8 earthquakes every c. 330 years. To investigate the near-surface, high-velocity frictional behavior of surface- and borehole-derived Alpine Fault gouges and cataclasites, twenty-one rotary shear experiments were conducted at 1 MPa normal stress and 1 m/s equivalent slip velocity under both room-dry and water-saturated (wet) conditions. In the room-dry experiments, the peak friction coefficient ( μ p = τ p / σ n ) of Alpine Fault cataclasites and fault gouges was consistently high (mean μ p = 0.67 ± 0.07). In the wet experiments, the fault gouge peak friction coefficients were lower (mean μ p = 0.20 ± 0.12) than the cataclasite peak friction coefficients (mean μ p = 0.64 ± 0.04). All fault rocks exhibited very low steady-state friction coefficients ( μ ss ) (room-dry experiments mean μ ss = 0.16 ± 0.05; wet experiments mean μ ss = 0.09 ± 0.04). Of all the experiments performed, six experiments conducted on wet smectite-bearing principal slip zone (PSZ) fault gouges yielded the lowest peak friction coefficients ( μ p = 0.10–0.20), the lowest steady-state friction coefficients ( μ ss = 0.03–0.09), and, commonly, the lowest specific fracture energy values ( E G = 0.01–0.69 MJ/m 2 ). Microstructures produced during room-dry and wet experiments on a smectite-bearing PSZ fault gouge were compared with microstructures in the same material recovered from the Deep Fault Drilling Project (DFDP-1) drill cores. The near-absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments. Mechanical data and microstructural observations suggest that Alpine Fault ruptures propagate preferentially through water-saturated smectite-bearing fault gouges that exhibit low peak and steady-state friction coefficients.

The Alpine Schist immediately west of the Main Divide of the Southern Alps is a west‐dipping duplex system consisting of an imbricated stack of rock slabs, each c. 250–1000 m thick. The imbricated stack has low grade, little deformed pumpellyite‐actinolite facies semischists at the base, overlain by progressively higher grade and more deformed schists. The structurally highest slab mapped consists of multiply‐deformed biotite zone schist. The duplex lies on the hanging wall of the northwest‐dipping Main Divide Fault Zone, which separates semischist from structurally underlying greywacke. Rock slabs are internally disrupted by faults subparallel to layering, and consist of lozenge‐shaped blocks of 10–100 m. Fault zones separating rock slabs consist of

We have investigated the structures and stable isotope geochemistry of fault rocks within thrust faults and normal faults of the central Apennines in order to understand the fluid circulation during late Miocene-early Pliocene contraction and late Pliocene-Pleistocene extension of the thrust belt. Stable isotope data were obtained for 70 carbonate host rocks and 80 fault rocks. Cataclasites, veins, and slickenfibers of the thrust faults have δ13C values ranging from 0 to 3‰, which are similar to the host rocks' values, and δ18O values from 25 to 34‰, which are slightly lower than the 28 to 34‰ δ18O values of the host rocks. Cemented cataclasites, veins, and slickenfibers of the normal faults have δ13C values ranging from −5 to 3‰ and δ18O values from 19 to 30‰, which are significantly lower than the host rocks' values. These data are consistent with the contraction-related structures having formed in a semiclosed hydrologic system. In contrast, the extension-related structures formed in semi-open to open system conditions. During shortening, there was little to no advection of large quantities of isotopically distinct fluids from the siliciclastic wedge on top of the subducting Adriatic plate and/or from the overlying groundwater aquifers. Only during exhumation and extension of the orogen were fluids able to penetrate downward into large normal faults that at present are seismically active.

Summary The lack of a predictive tool for calculating fault permeability in carbonate reservoirs has led to an increasing amount of research towards the permeability structure of carbonate-hosted fault zones. However, a better understanding of fault rock distributions and their potential petrophysical properties is required to predict the impact of faults in carbonate reservoirs. This research combines structural, microstructural and petrophysical data from a series of carbonate-hosted fault zones in Malta, enabling an understanding of the fault zone permeability structures in various lithofacies, whilst highlighting the heterogeneity on all scales of carbonate-hosted fault zones. From the studied exposures, fault displacements of 30 m are required for a continuous fault core, but 100–200 m displacement is required for a continuous cataclasite veneer. Fault rocks have reduced macro-scale heterogeneity compared to host rocks, whilst the outcrop scale heterogeneity is increased. Fault rock permeability measurements show that permeability is reduced relative to the host rock for all faulted lithofacies. Cataclasite exhibits the lowest permeability (geometric average = 10−3 mD). These reductions are large enough to have a noticeable impact on fluid flow over production scales for the c.60% of high porosity fault rocks, however less than 10% of fault rocks derived from low porosity host rocks exhibit these permeability reductions. 30% of fault rocks derived from high porosity host rocks exhibit permeability reductions sufficient to behave as a significant barrier to fluid flow.

We describe different fault rocks within a normal fault bordering the Neogene Valles-Penedes basin in NE Spain. Through this fault, the Lower Cretaceous limestones of the footwall are in contact with the Miocene siliciclastic sediments of the hangingwall. In the Lower Cretaceous protolith, cataclasites and breccias are a mixture of tectonic and karstic products resulting from multi-stages movement and development of the fault. The localisation of frictional processes generated a cohesive fault breccia and the cataclasite during the progressive opening of the hydrological system. Later, when the fault reached the surface, meteoric diagenetic processes leaded to the formation of calcretes and karstic features and generating the breccia with the pisolithic matrix.

Compressional wave-velocity anisotropy has been measured to 1 kb confining pressure in several mylonites, in one cataclasite, and in their protoliths. No significant difference in the velocity or density was found between the protolith and the tectonically altered rock. At 1 kb, strong seismic anisotropy (greater than 10%) was present in only two of nine mylonites measured. The anisotropy was due to their aphanitic texture, high content of layered silicates, and pervasive foliation. Most mylonitic rocks studied were not strongly anisotropic despite a well-developed macroscopic foliation. Where anisotropy was present, velocities usually were slowest perpendicular to the foliation and fastest parallel to both foliation and lineation. These preliminary results suggest that ductile fault zones in the crust are not strong seismic reflectors unless mylonites in the fault zone are aphanitic and contain large amounts of foliated phyllosilicates. It is also possible that anomalous environmental conditions such as high pore pressure in the fault zone in situ may explain observed seismic reflectivity.

Fluid inclusions and structural fabric in fault rock in the exhumed footwall of the Wasatch fault allow measurement of fluid pressure, temperature, and composition and estimation of rheological characteristics of fault behavior. Hydrothermally altered and deformed Oligocene quartz monzonite of the Little Cottonwood stock forms a partially preserved footwall carapace at the southern end of the Salt Lake segment of the Wasatch normal fault zone. Cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall. An early epidote-chlorite-sericite-magnetite alteration assemblage developed in phyllonite and cataclasite, and a later laumontite-prehnite-hematite alteration assemblage developed during cataclasis. Fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains. Fluid inclusions that record the transition temperature from quasi-plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite, epidote, and sericite and were trapped at minimum temperatures of 223°C–353°C and minimum fluid pressures of 1150–2800 bars. The fluids average 13 mol% CO2 and 8.2 wt % NaCl. A radiometric age of hydrothermal sericite suggests that the age of the alteration is 17.6±0.7 m.y. If maximum fluid pressure is near lithostatic, the minimum depth of formation of the alteration was 11 km at 337°C consistent with a thermal gradient of 30°C/km. The ratio of pore fluid pressure to lithostatic pressure varied from 0.52 to nearly 1.0 and indicates that effective normal stress on the fault at these depths may have approached zero. Structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault. Cleavage lineation and S-C bands in the phyllonite indicate that deformation at temperatures above 350°C and depths greater than 11 km was dominated by aseismic creep. Unstable fracture propagation in cataclasite developed during seismic slip at shallower levels. The downdip, seismogenic length of the Salt Lake segment is estimated as 16 km using a fault zone dip of 45° and a minimum depth of 11 km for the quasi-plastic/frictional transition. The segment strike length is 35 km, and the seismogenic area is about 600 km2, compatible with generation of M = 7.0 earthquakes.

A glass-coated half-meter-size boulder was sampled by the Apollo 17 crew at station 8 near the foot of the Sculptured Hills. The rock proved to be a coarse-grained (0.5-cm) plagioclase-orthopyroxene cumulate, and the samples are the only true norites returned from the lunar surface. Photographs of the boulder showed it to contain at least nine structural surfaces and four glass veins. Orientation and inspection of three of the returned samples resulted in the identification of six surfaces and one vein. One of the structural surfaces visible in the boulder was identified as primary cumulus planar lamination, which was folded through an angle of at least 35° between two oriented samples, whereas fracture sets representing the other surfaces were coincident. The boulder is believed to be a sample of the deeper highlands or submare lunar crust, derived from a depth of 8 to 30 km and somewhat shock-metamorphosed during at least two excavation events. The chemical composition of the norites, when determined, should be of special interest in view of the large amount of literature concerning glass, cataclasite, hornfels, and “basalt” of noritic composition returned by other Apollo missions. However, the cumulus texture of the boulder precludes its being representative of any magmatic liquid composition, suggests that the lunar crust is heterogeneously layered, and that plagioclase sank, not floated, in magmatic liquids that formed the lunar crust.

The Bantimala Complex and overlying Cretaceous sediments of South Sulawesi, Indonesia, were intensely faulted in the Plio-Pleistocene as a result of the northward movement of the Banada Sea microplate with respect to Western Indonesia. The area is dominated by NNW-striking sinistral wrench faults. The structures in these faults formed at less than 250°C. Zones of intense rotation and faulting, 200 m wide, parallel the major faults. They consist of lenses of a wide range of sedimentary and metamorphic rocks separated by narrow zones of foliated cataclasite. Within the lenses, folding is non-cylindrical and there is a weak stretching lineation parallel to the shear direction. Extension fractures and veins occur at 70° to the stretching lineation. Sinistral and dextral faults within the lenses are symmetrical to the major faults. The fabric within these lenses suggests moderate horizontal extension rather than strong simple shear. Within the cataclasite, the foliation is composed of anastomosing microfaults. The sense of shear is demonstrated by offset on these microfaults and on smaller fractures within the microlithons. Elongate clasts are horizontally aligned within the cleavage and have symmetrical drag patterns. Many of the equidimensional clasts have asymmetrical drag patterns consistent with sinistral shear.

The Aguilon nappe, Sierra Alhamilla, southeast Spain, is a north-closing recumbent isoclinal fold with a greatly thinned lower limb. This large-scale geometry can be demonstrated using small-scale structures. The fold nappe is detached along the lower limb and lies on a carpet of mylonitic schist, mylonite, and cataclasite. Formation of the nappe must have involved northward relative transport—a point of current controversy in the Betic Zone. During emplacement, the nappe underwent extensive ductile deformation, accomplished partly by pressure solution of silica. In the highly deformed footwall rocks, however, quartz was deformed by crystal-plastic processes. The change in deformational mechanism suggests a downward increase in deviatoric stress, which is compatible with a gravitational mechanism of nappe emplacement. The core of the low-grade nappe is occupied by medium-grade schist. The abrupt change in grade suggests that the schist contact is an extensional fault that excised part of the original metamorphic zonation and was subsequently refolded in the nappe. Such relations are common in the Betic Zone and suggest extension by listric normal faulting, a characteristic feature of nappes emplaced by gravity spreading.

The Mineral Mountains intrusive complex records the interaction since Oligocene time between magmatic, structural, and hydrothermal processes. These processes continue to the present, as exemplified by the Roosevelt Hot Springs geothermal system, which is structurally controlled and driven by a pluton emplaced during the Pleistocene. The intrusive rocks of the complex show changes in composition from an early calc-alkaline suite (Oligocene) to a slightly alkaline main intrusive sequence (Miocene) that makes up most of the range. At about 9 Ma, the intrusive activity was associated with low-angle faulting. The oldest volcanic rocks are dated at ∼7 Ma. Volcanic activity continued in the region until ∼0.5 Ma and may be active still. Examples of the youngest volcanism include a sequence of rhyolite flows, domes, and pyroclastic deposits that were erupted along the crest of the range between 0.8 and 0.5 Ma. Minor basaltic flow and pyroclastic eruptions also were localized in the northern part of the range. A major low-angle detachment fault, containing as much as 200 m of cataclasite, is preserved in the southern Mineral Mountains. This fault separates lithologies of the Mineral Mountains intrusive complex from an overlying Paleozoic sedimentary sequence. We propose that this zone developed at the brittle-ductile transition and has been exposed because of the extreme uplift of the range. The range also preserves a series of listric faults which may sole into a major low-angle fault at depth. High-angle normal faults strike both north and east. Both sets have important controls on the flow of fluid in the Roosevelt Hot Springs geothermal field. The east-west structures are presently seismically active. Hydrothermal activity has been associated with all intrusive events in the Mineral Mountains. Base-metal mineralization accompanied the older events, whereas precious-metal mineralization accompanied the younger. We propose an evolution of the intrusive complex, involving the interaction of magmetic, structural, and hydrothermal components. From oldest to youngest, each phase of activity reflects emplacement of magma into progressively higher levels of the crust. Uplift reflected by this progression has been quite rapid at times, as shown by fission track dating. As a consequence, the Mineral Mountains constitute a profound structural high in the region. This extreme uplift probably was caused by diapiric rise of this area under the influence of repeated magmatic intrusion.

Near Parkfield in the southern Diablo Range, the smooth summit of Table Mountain, a narrow ridge 12 miles long, is capped by a subhorizontal sheet of weakly foliate serpentinite breccia composed of massive serpentinized-peridotite blocks set in a slickensided matrix of crushed serpentine. The breccia was extruded by plastic flowage from longitudinal fault fissures now occupied by steeply dipping serpentinite breccia dikes that lie near the crest of a complex anticline whose core is Jurassic-Cretaceous(?) Franciscan tectonic breccia made of exotic blocks set in a cataclasite matrix. The fissure feeders probably tapped a long-dormant mass of Franciscan serpentinite that was mobilized by fault disruption and high lateral stresses during the Pliocene-Pleistocene orogenesis of the Diablo Range. Upon reaching the surface, the breccia buried an uneven ridge crest of hills and saddles beneath a gently arched summit carapace. The breccia then moved downslope in earthflow fashion to form narrow, sloping ramps that filled draws between resistant spurs on the flanks of the ridge. On the lower slopes, the extrusion again coalesced to form an evenly sloping piedmont plate through which rejuvenated streams have cut steep gorges. Extensive landslides with prominent headwall scarps have scarred eroded edges of the extrusive sheet, and a degradational sequence of Quaternary pediment gravel surfaces at the base of Table Mountain also postdates the extrusion. The piedmont plate rests discordantly across a steeply dipping anticline limb of marine Cretaceous Panoche Group and a largely marine Tertiary sequence that includes Pliocene-Pleistocene(?) continental gravels. The Tertiary beds rest unconformably on the Panoche Group, but the Panoche Group lies upon the underlying Franciscan tectonic breccia along a folded thrust. The thrust emplacement followed widespread tectonic brecciation of the Franciscan, but preceded deposition of Eocene sandstones containing Franciscan detritus. The folded thrust is cut locally by younger, steep reverse and strike-slip faults that offset Tertiary strata.