Showing papers on "Fault (geology) published in 1982"

Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine

Abstract: Plane indentation experiments on unilaterally confined blocks of plasticine help us to understand finite intracontinental deformation and the evolution of strike-slip faulting in eastern Asia. Several large left-lateral strike-slip faults may have been activated successively, essentially one at a time. The experiments suggest that the penetration of India into Asia has rotated (≈25°) and extruded (≈800 km) Indochina to the southeast along the then left-lateral Red River fault in the first 20 to 30 m.y. of the collision. This process can account for the opening of the South China Sea before late Miocene time. Extrusion tectonics then migrated north, activating the Altyn Tagh fault as a second major left-lateral fault and moving southern China hundreds of kilometres to the east. As this occurred, Indochina kept rotating clockwise (as much as 40°), but the sense of motion reversed on the Red River and other strike-slip faults in the south. Opening of the Mergui basin and Andaman Sea (up to the present) also appears to be a simple kinematic consequence of the extrusion. Recent rifts in northeastern China and Yunnan may be considered incipient analogs of the South China and Andaman Seas. Other Tertiary tectonic features such as the sedimentary basins of the Gulf of Thailand may be explained as collisional effects, if one uses our experiments as a guide. The experiments also suggest that a major left-lateral strike-slip fault and rift system will propagate across the Tien Shan, Mongolia, and Baikal to the Sea of Okhotsk.

Evolution of pull‐apart basins and their scale independence

Abstract: Pull-apart basins or rhomb grabens and horsts along major strike-slip fault systems in the world are generally associated with horizontal slip along faults. A simple model suggests that the width of the rhombs is controlled by the initial fault geometry, whereas the length increases with increasing fault displacement. We have tested this model by analyzing the shapes of 70 well-defined rhomb-like pull-apart basins and pressure ridges, ranging from tens of meters to tens of kilometers in length, associated with several major strike-slip faults in the western United States, Israel, Turkey, Iran, Guatemala, Venezuela, and New Zealand. In conflict with the model, we find that the length to width ratio of these basins is a constant value of approximately 3; these basins become wider as they grow longer with increasing fault offset. Two possible mechanisms responsible for the increase in width are suggested: (1) coalescence of neighboring rhomb grabens as each graben increases its length and (2) formation of fault strands parallel to the existing ones when large displacements need to be accommodated. The processes of formation and growth of new fault strands promote interaction among the new faults and between the new and preexisting faults on a larger scale. Increased displacement causes the width of the fault zone to increase resulting in wider pull-apart basins.

The Basin and Range Province: Origin and Tectonic Significance

Abstract: The Basin and Range province is a vast arid tract of regionally corrugated, angular topography of high relief in the western Cordillera. It is characterized by evenly spaced parallel mountain ranges and intervening desert basins (Fig­ ure la). The range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward, interrupted here and there by low fault scarps that paral­ lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges. Thermal springs located at, or near, range-bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting. In the southern part of the province, especially in southeastern California and southwestern Arizona, range fronts have been worn back by erosion, leaving a thin veneer of gravel on an erosion-cut, bedrock surface that slopes gently outward. The range-bounding faults of these mountain blocks are buried at the outer edge of such pediments, often at considerble distances from the erosional remnants of the ranges themselves. The American physiographer N. M. Fenneman (1928, 1931) named the Basin and Range province and defined its general boundaries. As thus circum­ scribed, the province includes some 800,000 km2 of area in eight western states. Later students (Pardee 1950, Lawrence 1976, Reynolds 1979, Eaton 1979b) have observed that many of the fundamental geological and geo­ physical characteristics of the province are found well beyond the boundaries drawn by Fenneman, which were based on physiography alone. As a tec­ tonophysical entity, its areal extent is greater than 1 million km2, more than 10% of the area of the United States (Figure 1) . Fenneman (1931) subdivided the province into five physiographic sections, the largest of which is the Great Basin (see Figure 1b) . It is not, as its name implies, a single regional depression with a common topographic center, but is characterized instead by isolated networks of interior drainage, divisible into

Three-dimensional finite difference simulation of fault dynamics: Rectangular faults with fixed rupture velocity

Abstract: We analyze three-dimensional finite difference solutions for a simple shear-crack model of faulting to determine the effects of fault length and width on the earthquake slip function. The fault model is dynamic, with only rupture velocity, fault dimensions, and dynamic stress-drop prescribed. The numerical solutions are accurate for frequencies up to 5 Hz, and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies. Near the hypocenter, the slip velocity exhibits a square-root singularity whose intensity increases with hypocentral distance. At distances greater than the fault width, w , growth of the velocity intensity ceases, and the slip function becomes nearly invariant with distance along the fault length. Closed-form expressions are developed for the dependence of static slip ( s ∞), slip rise time ( TR ), and slip velocity intensity ( V ) on fault geometry. Along the center line of a long, narrow fault, at hypocentral distances exceeding w , these expressions reduce to s ∞ ≈ w Δτ/μ, TR ≈ 0.5 w/vR , and V ≈ √ w /2 vR Δτ/μ, where Δτ is the dynamic stress drop, μ the shear modulus, and vR the rupture velocity. The numerical results imply that uniform-dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high-frequency ground motion relative to low-frequency ground motion. A further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width, but will be independent of rupture length.

Inversion of field data in fault tectonics to obtain the regional stress — I. Single phase fault populations: a new method of computing the stress tensor

Abstract: Summary. We attempt a general definition of the inverse problem of computing the components of the regional stress tensor from a set of field data including the measurements of the strike and dip of several faults, and the directions and senses of relative motion along these faults (as indicated by slickenslides). In previous treatments of this problem, no experimental errors could be taken into account except those in measuring the pitch of slickenslides; thus, errors in the orientation of the fault (strike and dip), which have considerable practical importance, were neglected. In our work, all experimental errors in the field measurements are taken into account, so that the agreement between the computed stress tensor and the set of field measurements can be rigorously checked.

The relationship between Quaternary volcanism in central Mexico and the seismicity and structure of subducted ocean lithosphere

Abstract: Late Quaternary volcanism in central Mexico is related to the subduction of young ocean lithosphere at the Middle America. Trench. Along-arc variations in seismicity, volcano structure, and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab. Morphological and petrographic characteristics of major volcanoes within the Trans-Mexican Volcanic Belt (TMVB) serve to distinguish two calc-alkaline subprovinces: 1. A western arc, averaging 60 km in width, associated with aseismic subduction of the Rivera plate. The main cones of this region are dominated by two-pyroxene andesites, comprise volumes ⩽70 km 3 , and stand less than 3,000 m above sea level. 2. A broad central and eastern arc related to subduction of a gently inclined segment of the Cocos plate bounded by the Rivera transform and the Tehuantepec Ridge. Major volcanic edifices possess summit elevations in the range 4,000 to 6,000 m, have appropriately larger volumes (typically > 200 km 3 ) and are constructed with a high proportion of amphibole-bearing lavas. The boundary between these subprovinces is marked by a north-south–oriented structural depression, the Colima Graben, and it coincides with a 100-km offset in the “volcanic front.” Extensional tectonism in the Colima Graben, accompanied by mixed calc-alkaline and alkaline volcanism of potassic affinity, is likely related to a hinge-type transform fault which marks the Cocos-Rivera plate juncture in the downgoing slab. A third segment of ocean floor is presently interacting with continental lithosphere south of the Gulf of Tehuantepec, where Quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse Cocos-NOAM-Caribbean triple junction. The northern limit of this triple junction is defined by the seismically active Isthmus fault, which may be related to alkaline volcanic activity at San Andres Tuxtla. A tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate-depth earthquakes along the arc is directly dependent upon the age of the subducted slab. Ocean lithosphere younger than approximately 20 m.y. is subducted aseismically at convergence rates approaching 9 cm/yr. The length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m.y. The TMVB overlies the aseismic extension of this young ocean lithosphere. Several aspects of this study have a bearing on the segmented nature of converging margins in general: 1. The tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction. 2. The complete record of volcanism in the TMVB over the past million years can be related to the present plate configuration. 3. Alkaline and calc-alkaline volcanism have developed contemporaneously at a converging plate margin. 4. Lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab.

Subsidence in Late Paleozoic basins in the northern Appalachians

Abstract: During the interval between continental collision in the Devonian and continental breakup in the Triassic the northern Appalachians became the site of a wide plate boundary zone of dominantly right-lateral strike slip. As is typical of intracontinental transforms, tectonism was both diachronous and rapidly variable along strike through regimes of ‘pure’ strike slip, transpressional deformation, and rapid subsidence of extensional basins. Up to 9 km of mainly nonmarine, clastic sediments accumulated in these local depocenters, which subsided episodically in two stages: (1) an initial phase of stretching and thinning of the lithosphere, when subsidence was rapid, fault controlled, and often accompanied by volcanism and (2) a subsequent phase of gradual thermal subsidence, during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement. The largest depocenter, the Magdalen Basin, opened as a pull-apart between strike slip faults in Newfoundland and New Brunswick from late Devonian to early Carboniferous. Subsequent thermal subsidence affected a large area during medial and late Carboniferous, a phenomenon that is well recorded to the north and west, where no later tectonism occurred. In areas to the south and east of the basin, strike slip on other faults continued into the time of thermal subsidence, introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull-aparts.

The 1971 San Fernando earthquake: A double event?

Abstract: Evidence is presented which suggest that the 1971 San Fernando earthquake may have been a double event that occurred on two separate, subparallel thrust faults. It is postulated that the initial event took place at depth on the Sierra Madre fault zone which runs along the base of the San Gabriel Mountains. Rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km. A second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the Sierra Madre fault zone. The surface trace of this fault coincides with the San Fernando fault zone which was the principal fault associated with surface rupture. It is postulated that rupture propagated from a depth of 8 km to the free surface. The moments of the first and second events are approximately 0.7 × 10^(26) dyne-cm and 1.0 × 10^(26) dyne-cm, respectively. This model is found to explain the combined data sets of strong ground motions, teleseismic P and S waveforms, and static offsets better than previous models, which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth. Nevertheless, many features of the observed motions remain unexplained, and considerable uncertainty still exists regarding the faulting history of the San Fernando earthquake.

Strain, tilt, and rotation associated with strong ground motion in the vicinity of earthquake faults

Abstract: In the absence of near-field records of differential ground motion induced by earthquakes, we simulate the time histories of strain, tilt, and rotation in the vicinity of earthquake faults embedded in layered media. We consider the case of both strike-slip and dip-slip fault models and study the effect of different crustal structures. The maximum rotational motion produced by a buried 30-km-long strike-slip fault with slip of 1 m is of the order of 3 × 10 −4 rad while the corresponding rotational velocity is about 1.5 × 10 −3 rad/sec. A simulation of the San Fernando earthquake yields maximum longitudinal strain and tilt a few kilometers from the fault of the order of 8 × 10 −4 and 7 × 10 −4 rad. These values being small compared to the amplitude of ground displacement, the results suggest that most of the damage occurring in earthquakes is caused by translation motions. We also show that strain and tilt are closely related to ground velocity and that the phase velocities associated with strong ground motions are controlled by the rupture velocity and the basement rock shearwave velocity.

Number and orientation of fault sets in the field and in experiments

Abstract: Arrays of faults that can be grouped into multiple sets occur in the Entrada and Navajo Sandstones in southeastern Utah. The faults form a network that usually has a rhombohedral pattern both in map and cross-sectional views. Similar fault patterns were formed experimentally in cubic samples of sandstone, limestone, and granite deformed to failure with a polyaxial apparatus. The faulting theory of Anderson fails to explain both the number and the orientation of the faults observed in this study. However, the number and orientation of faults can be understood in terms of a theory of deformation of rock solely by slip along planes.

Fault Patterns Associated with Domes--An Experimental and Analytical Study

Abstract: Experimental (clay) and analytical models suggest that regional strain, either extension or compression, significantly affects the fault patterns produced by doming. Our models specifically simulate the shallow deformation produced by gentle doming of a homogeneous material with and without a simultaneously applied, regional horizontal strain. The models of circular domes show that without regional strain, normal faults develop on the crests and flanks. On the flanks, the normal faults have radial trends. With regional extension, normal faults on the crests of circular domes trend perpendicularly to the regional extension direction, whereas many normal faults on the flanks trend obliquely to it. Strike-slip faults trending 60° from the regional extension direction fo m near the peripheries. With regional compression, many normal faults on the crests and flanks of circular domes strike parallel with the regional compression direction. Strike-slip faults trending 30° from the regional compression direction also form on the flanks, and reverse faults striking perpendicularly to the regional compression direction develop on the peripheries. Our models show that regional strain affects the fault patterns produced by elliptical doming similarly. The fault patterns of our models resemble fault patterns of domed strata above salt diapirs and above uplifted blocks of basement.

Subduction seismicity and tectonics in the lesser Antilles arc

Abstract: We have studied the mechanisms of 17 earthquakes along the Lesser Antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate. No large thrust earthquakes occurred during the 1950-1978 study period; the three large (magnitude seven) events are all normal faults. One is a normal faulting event seaward of the trench. Its aftershock sequence includes strike slip events on differently oriented faults, probably due to lateral block motion in response to the main shock. A second indicates extension within the slab at depth. These observations suggest that subduction in this region is primarily decoupled and aseismic unless the time interval studied is unrepresentative. The third normal fault earthquake occurred within the upper plate with fault planes perpendicular to the arc and trench. This unusual geometry may represent a flexural response to the subduction of the Barracuda Ridge, a major bathymetric high with uncompensated excess mass at depth which seems analogous to flanking ridges found along some Mid-Atlantic Ridge fracture zones. Thus, the Barracuda Ridge is not buoyant and does not affect Benioff zone dip. Strike slip faulting occurs at depth in the subduction zone along a concentration normal to the arc and maymore » indicate a fossil fracture zone. There is no direct evidence in the shallow seismicity for the hypothetical North America-South America-Caribbean triple junction through some of the oceanic 'intraplate' seismicity is consistent with such a boundary.« less

Aftershocks of the Coyote Lake, California, earthquake of August 6, 1979: A detailed study

Abstract: Aftershock hypocenters and focal mechanism solutions for the Coyote Lake, California, earthquake reveal a geometrically complex fault structure, consisting of multiple slip surfaces. The faulting surface principally consists of two right stepping en echelon, northwest trending, partially overlapping, nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 Parkfield, California, earthquake. The overlap occurs near a prominent bend in the surface trace of the Calaveras fault at San Felipe Lake. Slip during the main rupture, as inferred from the distribution of early aftershocks, appears to have been confined to a 14-km portion of the northeastern sheet between 4- and 10-km depth. Focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex, with left stepping imbricate structure. Seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock. Aftershocks in this zone define a single vertical plane 8 km long between 3- and 7-km depth. Within the overlap zone between the two main slip surfaces, the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone, in close agreement with the stress perturbations predicted by crack interaction models. Aftershock activity in the overlap zone is not associated with a simple dislocation surface. Space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone. This alternation is consistent with local stress perturbations predicted by crack interaction models. We conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which, in turn, control the displacements. Table 5 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82-006; $1.00. Payment must accompany order.

Dating of a fault by electron spin resonance on intrafault materials.

Abstract: The total dose of natural radiation and the age were determined from paramagnetic defects in quartz grains at a fractured fault zone. Young age at the fault indicates that the accumulated defects in rocks were destroyed by high stress or high temperature at the time of the last fault movement, setting the clock time to zero. The technique was applied to quartz grains crushed by uniaxial compression in the laboratory to verify this interpretation.

Magnitude of crustal extension in the southern Great Basin

Abstract: Strike-slip faults in the southern Great Basin separate areas of Cenozoic upper crustal extension from relatively stable tectonic blocks. Linear geologic features, offset along the Garlock fault, Las Vegas Valley shear zone, and Lake Mead fault system, allow reconstruction of the southern Great Basin to a pre-extension configuration. The Sierra Nevada, Mojave Desert, Spring Mountains, and Colorado Plateau are treated as stable, unextended blocks that have moved relative to each other in response to crustal extension, with the Spring Mountains held fixed to the Mojave block. Our reconstruction indicates a minimum of 65% extension (140 km) between the southern Sierra Nevada and Colorado Plateau.

Tectonic geomorphology of the San Andreas fault zone in the southern Indio Hills, Coachella Valley, California

Abstract: Geomorphic investigation of the San Andreas fault zone in the Indio Hills indicates many tectonically produced landforms, including beheaded streams, right-lateral deflected and offset streams, sags, shutter ridges, pressure ridges, and fault scarps. Near Biskra Palms, an alluvial fan-pediment complex has an apparent cumulative offset of about 0.7 km along the Mission Creek fault zone (north branch, San Andreas fault). Many of the tectonic landforms, as well as the fracture pattern that has developed during the Pleistocene, are explainable by simple shear or uplift associated with a small left bend in the main trace of the Mission Creek fault. The ratio of vertical to horizontal displacement in the vicinity of the bend is about 0.04. Exposed in Pushawalla Canyon, 5 km northwest of the alluvial fan-pediment complex, are: (1) a sequence of stream terraces, (2) folded Plio-Pleistocene fanglomerates, and (3) an example of stream capture following a right lateral deflection or offset of several hundred metres. A left step of the Mission Creek fault in Pushawalla Canyon is a probably cause of folds and sporadic uplift that produced the stream terraces. Possible cause of recent stream capture are: (1) juxtaposition of Pushawalla Canyon with a relict canyon moving northwestward along the Mission Creek fault, or (2) right-lateral deflection or offset of Pushawalla Canyon along the fault, with simultaneous headward erosion of a shorter, steep stream flowing toward the Coachella valley. Estimation of a slip rate and identification of paleoseismicity for the San Andreas fault in the Indio Hills is difficult. However, degree of topographic dissection, formation, and preservation of desert pavement, and relative soil profile development suggest that the age of the offset fan may be as old as 70,000 yr but that most likely it is on the order of 20,000 to 30,000 yr. These age estimates for the offset fan indicate a minimum slip rate for the San Andreas fault of 10 to 35 mm/yr, with about 23 to 35 mm/yr the most likely. The pattern of observed offset drainages is complex, but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right-lateral displacement.

Origin and age of the Border Ranges Fault of southern Alaska and its bearing on the Late Mesozoic tectonic evolution of Alaska

Abstract: Local geologic relationships in the western Chugach Mountains, together with regional considerations, suggest that in the Early Cretaceous a subduction zone was formed along the southern edge of the Wrangellia-Peninsular-Alexander composite terrane (Talkeetna superterrane of Csejtey et al., 1982) of southern Alaska. The effects of this event include (1) a shattering of older (pre-Cretaceous) crystalline rocks along a complex fault system, (2) emplacement of a tectonic melange beneath the shattered crystalline rocks, and (3) a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks. This event created a regionally mappable structural contact between broken crystalline rocks and the melange; the Border Ranges fault of MacKevett and Plafker (1974). Regional stratigraphy as well as radiometric ages of rocks predating and postdating the Border Ranges fault appear to bracket the age of the Border Ranges fault and its associated deformational effects to the interval between about 135 and 120 Ma. Further regional tectonostratigraphic associations suggest that the deformation along the Border Ranges fault represents the nucleation of a north and (or) east dipping subduction zone beneath the Talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the Gravina-Nutzotin belt of southeast Alaska. Mismatches in the distribution of different elements of this Early Cretaceous arc-trench system are probably a result of Late Cretaceous or early Tertiary strike slip motion, but Early Cretaceous ridge-trench interaction (suggested by the occurrence of Early Cretaceous near-trench plutons) may have played a role as well. The Talkeetna superterrane is generally thought to be an exotic block that collided with the Cordillera in the middle Cretaceous (Coney et al., 1980). The age data commonly cited as evidence for a middle Cretaceous age for the collision are, however, misleading. A model is proposed here in which the initiation of convergence along the trailing edge of the Talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle Cretaceous deformation recognized along the leading edge of the Talkeetna superterrane (Csejtey et al., 1982) records destruction of a syncollisional flysch basin during the final phase of the collision.

Seismotectonics of southern Iran: The Oman Line

Abstract: Seismotectonic characteristics of the transition area between the Zagros continental collision zone and the Makran ocean-continent convergence zone (the ‘Oman Line’) are carefully examined. A northeast trending zone of earthquakes terminates the Zagros belt of seismicity just west of the Oman Line. This seismic zone extends northeastward beyond the Main Zagros Thrust and coincides with a surface escarpment clearly visible on LANDSAT imagery. Events with accurately determined depths along this seismic zone occurred in the upper 20 km of the crust and possibly show a shallow (<10°) northeastward dip. East of the Oman Line, the Zendan-Minab fault system is relatively aseismic. The Oman Line region may be characterized by underthrusting of a wedge of Arabian shelf edge beneath Iranian crust or by an indentation of Arabia into the Iranian crustal block as a promontory. Available seismological and geological data cannot uniquely distinguish between these two possible tectonic settings. The area located to the northwest of the Oman Line region, now a zone of continental collision, appears to have been characterized by an episode of Neogene subduction. A well-located, intermediate-depth earthquake occurred in this area to the northeast of the Main Zagros Thrust on November 9, 1970. It was accurately located at 107-km depth beneath a line of Quaternary volcanoes. Its focal mechanism may indicate downdip tension, with nodal planes that strike closely parallel to the trend of the Zagros arc. Comparison of this event and other neotectonic features in this part of Iran with those in other active convergent zones suggests that a descending oceanic lithosphere beneath the Zagros volcanic arc may still be attached to the colliding Arabian plate.

Implications of an Elastic Analysis of In Situ Stress Measurements Near the San Andreas Fault

Abstract: Twenty-nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.

Mesoscopic fault array of the northern Umbrian Apennine fold belt, Italy: Geometry of conjugate shear by pressure-solution slip

Abstract: The strata of the northern Umbrian Apennine fold belt are cut by an array of mesoscopic faults that generally display strike- or oblique-slip offset. The majority of these faults have traces less than a few metres long and represent displacements of < 10 cm. Fault surfaces are associated with stylolites and are coated with elongate calcite fibers, suggesting that movement occurred by the mechanism of pressure-solution slip. Crosscutting relationships indicate that faulting occurred before, during, and perhaps after regional folding. The slip on the faults permitted translations of mesoscopic blocks with respect to one another, thereby accommodating regional strain. There is a great range among fault attitudes, but two clusters forming a conjugate set with about a 90° dihedral angle stand out. The mean trend of left-lateral faults of this pair is N72°E, whereas the mean trend of right-lateral faults is N16°W. The bisector between these two fault clusters is about 15° away from the normal to the regional fold axes. This unusual orientation pattern of mesoscopic faults of the study area may indicate that the mechanics of initiating faults in rocks undergoing pressure-solution deformation is different from that in rocks undergoing purely brittle deformation. Alternatively, the fault pattern may indicate that the faults represent slip on pre-existing fractures. If this latter situation is true, the geometry of the fault array may merely reflect the geometry of the pre-existing joint array.

Tectonic evolution of lower Proterozoic rocks, Uinta Mountains, Utah and Colorado

Abstract: Rocks of the early Proterozoic orogenic system that fringes the Archean-aged Wyoming province are exposed along the Uinta fault in the northeastern Uinta Mountains of Utah and Colorado Exposed here are the Red Creek Quartzite, an early Proterozoic-type miogeoclinal metasedimentary sequence more than 4 km thick, and an underlying, newly recognized, Archean gneiss complex more than 27 by old During the Hudsonian orogenic period, the miogeoclinal sequence was emplaced northward over the Archean complex by tectonic translation along a thick mylonite zone in the waning phases of upper amphibolite metamorphism The orogen was disrupted by east-trending block faults with several kilometres displacement during initiation of the Uinta aulacogen and was buried by more than 7 km of middle Proterozoic sediments of the Uinta Mountain Group The middle Proterozoic block faults were reactivated with reversed sense of displacement during the Laramide uplift of the Uinta Mountain block