Showing papers on "Fault (geology) published in 1981"

Low-angle normal faults in the Basin and Range Province: nappe tectonics in an extending orogen

Abstract: Cenozoic normal fault mosaics bounded beneath by a basal fault in the Basin and Range Province (BRP) have traditionally been described either in terms of large-scale surficial gravity sliding or by some form of in situ lower plate accommodation. I suggest here that these areas may be an extensional analogue to thin-skinned compressional orogens, a process which may even dominate active BRP tectonics.

Cainozoic Evolution of the State of Stress and Style of Tectonism of the Basin and Range Province of the Western United States

Abstract: Cainozoic evolution of the modern plate boundary along the western United States from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western United States. Extensions tectonism responsible for the modern Basin and Range province appears to represent a unique late-stage episode of a much longer period of extension initiated in an ‘in tra -arc ’ setting contemporaneously with calc-alkaline magmatism. Basin-range extension is distinguished from early extension on the basis of angular unconformities, differences in fault trends and spacing, and associated magmatism (basaltic). Prebasin-range extension (i.e. extension preceding the break-up of the region into ranges resembling the modern ones) was under way locally by at least 30 Ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks, by large regions of the crust underlain by passively emplaced subvolcanic batholiths, and by the thickness and distribution of stratigraphic units. Locally, high strain rates that accompanied early extensions of as much as 50-100 % are implied. Data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform WSW -ENE least principal stress orientation in the period ca. 20-10 Ma, during this early extension. The change from early extension to basin-range style faulting of the upper 15 km of crust, which resulted in broadly spaced ranges (25-35 km crest-crest spacing), was time-transgressive and probably not abrupt; locally both types occurred concurrently. Southern Basin and Range block faulting occurred largely in the period 13-10 Ma, in response to a stress field orientated similarly to that responsible for the early extension. In contrast, northern Basin and Range block faulting developed after 10 Ma and continues to the present in response to a stress field orientated approximately 45° clockwise to the earlier stress field. This modern stress field, with a WNW -ESE to E-W directed least principal stress, characterizes the entire modern Basin and Range province and Rio Grande rift region. The 45° change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the San Andreas transform fault. Inclusion of pre-basin-range extension may help resolve the discrepancy between estimates of 15-30% for basin-range block faulting and total extension estimates of 100-300 % for the Basin and Range province.

A reservoir analysis of the Denver earthquakes: A case of induced seismicity

Abstract: Injection of fluid wastes into the fractured Precambrian crystalline bedrock beneath the Rocky Mountain Arsenal near Denver triggered earthquakes in the 1960's. An analysis, based on the assumption that fluid flow in the fractured reservoir can be approximated by flow in a porous medium, is presented. The configuration and hydrologic properties of the reservoir are determined from two lines of evidence: (1) locations of earthquake hypocenters determined by seismic arrays installed at the Arsenal and (2) observed long-term decline in fluid levels in the injection well. Together these two sets of data indicate that a long, narrow reservoir, aligned in the direction N 60/sup 0/W, exists. The reservoir is 3.35 km in width, extends 30.5 km to the northwest and infinitely to the southeast, and spans a depth interval from 3.7 to 7.0 km below land surface. It has a transmissivity of 1.08 x 10/sup -5/ m/sup 2//s and a storage coefficient of 1.0 x 10/sup -5/. Computed pressure buildup along the length of the reservoir is compared with the spatial distribution of earthquake epicenters. The comparison shows that earthquakes are confined to that part of the reservoir where the pressure buildup exceeds 32 bars. This critical value is interpretedmore » as the pressure buildup above which earthquakes occur. The migration of earthquake epicenters away from the injection well, a phenomenon noted by previous investigators, can be accounted for by the outward propagation of the critical pressure buildup. The analysis is extended to examining the effects of rapid flow in fractures opened by high injection pressure. The results show that the effect is confined to a small region within 1 km of the injection well. The existence of a critical pressure buildup above which earthquakes occur is completely consistent with the theory on the role of fluid pressure in fault movement as presented by Hubbert and Rubey.« less

Variable rates of Late Quaternary strike slip on the San Jacinto Fault Zone, southern California

Abstract: Three strike slip displacements of strata with known approximate ages have been measured at two locations on the San Jacinto fault zone. Minimum horizontal offset between 5.7 and 8.6 km in no more than 0.73 m.y. northeast of Anza indicates 8–12-mm/yr average slip rate since late Pleistocene time. Two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the Coyote Creek fault in western Imperial Valley. Horizontal slip of 1.7 m has been calculated for the youngest sediment of Lake Cahuilla since its deposition 275–510 yr B.P. The corresponding slip rate is 2.8–5.0 mm/yr. Right lateral offset of 10.9 m measured on a buried stream channel older than 5060 yr B.P. but younger than 6820 yr B.P. yields average slip rates for the intermediate time periods, 400± to 6000± yr B.P., of 1–2 mm/yr. The average rates of slip for these three time intervals suggest a relatively quiescent period for the San Jacinto fault zone from about 4000 B.C. to about 1600 A.D. To the extent that long-term variations in seismic activity of major strike slip faults elsewhere are known, fluctuations in slip rate for the San Jacinto fault would not appear to be abnormal. If the San Jacinto and adjacent segments of the San Andreas fault alternately assume dominant roles in absorbing motion between the Pacific and American plates, perhaps even more recently than 400 yr ago, the San Andreas fault south of the Transverse Ranges expressed most of the motion but has since become relatively inactive.

A source study of the Thessaloniki (northern Greece) 1978 earthquake sequence

Abstract: Summary The Thessaloniki (northern Greece) earthquake sequence appears to have occurred along faults forming a graben structure. This graben, situated in the border region between the Serbomacedonian massif and the Vardar zone, is bounded to the south-west by clearly exposed north-west striking north-east dipping normal faults. Relative hypocentre determinations, fault-plane solutions, surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes. The source parameters of the mainshock (mb = 6.1, Ms = 6.4) were determined by computing body-wave synthetic seismograms in the time domain and comparing them with the observed. This modelling constrained the orientation of faulting determined by the P-wave fault-plane solution. It also constrained the source depth to 6 |Mp 2 km. Similar depths were calculated by a relative relocation method for the other three large events of the sequence. The dislocation time-function required for the far-field had a total duration of 9 |Mp 1.5 s, the body-wave moment was 5.2 |Mp 1.8 times 1025 dyne cm. For a fault length of 35 km and a fault width of 17 km (both estimated from the aftershock distribution) the static stress drop was found to be 4 bar. An accelerograph record of the mainshock shows two distinct events, 3—4 s apart. These two events were unresolved by the long-period data in the far-field but the unusually long duration of the mainshock time-function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time. Seismic energy released in this fashion can account for waveform complexities observed at some stations.

Present‐day tectonics of the southeastern Caribbean and northeastern Venezuela

Abstract: Four telemetered seismic arrays were operated in northeastern Venezuela during the summer of 1979. An analysis of the data collected has resulted in accurate locations for about 100 microearthquakes and four new focal mechanism solutions. On the basis of these new data, an in-depth analysis of teleseismic data, and geologic evidence, we propose a new plate tectonic framework for the southeastern Caribbean. In this model, underthrusting of the Atlantic seafloor along the Lesser Antilles is extended to the southeast of Trinidad. The subducted slab is shown to dip northwesterly beneath Trinidad and the Caribbean Sea, penetrating depths of at least 150 km. This subduction appears to terminate abruptly in the vicinity of the Los Bajos-El Soldado fault zone, which trends WNW-NW and is located in the Gulf of Paria west of Trinidad. Geologic evidence shows right-lateral strike slip (RLSS) motion in this fault zone, probably initiated in the late Pliocene. This WNW-NW trending fault zone joins up with the E-W trending El Pilar-Casanay fault system in northeastern Venezuela, which also moves right laterally. The El Pilar fault is apparently offset by NNW trending faults that exhibit RLSS motion as well as normal faulting. This complex motion on an en echelon series of faults is interpreted to be the result of WNW motion of South America relative to the Caribbean plate. In our interpretation, the E-W component of relative motion is accommodated by RLSS motion on E-W faults and results in normal faulting on NNW-SSE faults. The smaller component of N-S convergence is reflected by RLSS motion on NNW trending faults and is accommodated by some internal deformation. The timing of the initiation of RLSS motion on the Los Bajos fault zone and the known total amount of displacement on it as well as other geologic indicators lead us to suggest that this complex mode of strike slip motion began only a few m.y. ago, and that the average rate of relative plate motion in this region is probably ≥0.9 ± 0.2 cm/yr.

Algebraic reconstruction of a two‐dimensional velocity inhomogeneity in the High Hazles seam of Thoresby colliery

Abstract: A high proportion of faces started in mechanized coal mines run into underground faults The faults take many forms, from the splitting of a seam through a hidden stress pattern caused by subsidence or folding, to a washout or a vertical throw All faults reduce face output A throw of only 15 m can lead to a face being abandoned Faults of this order cannot be mapped reliably from the surface They may be mapped in an underground seismic surveyRoadways of a mine may give access to fast refracting horizons above and below a coal seam Waveguiding in the plane of the seam, if it occurs, simplifies migration of wave trains reflected from discrete faults The reduction problem involved in a fault mapping is only two-dimensional Given guiding, whether leaky or not, it is possible to map distributed faults of low reflectivity by shooting in transmission across a panel of coalAlgebraic reconstruction techniques are used here to reduce first break times-of-flight through a 425 X 950 m rectangular block of coal into the profile of a velocity inhomogeneity Input data are derived by static correction from hand-picked arrival times The reduction itself is effected using an algorithm which accommodates underground site access restrictions In back projecting first break velocities, a truncated cosine is used to weight the relative contributions of rays passing at different distances from any given mapping pointThe reconstructed velocity field suggests that the coal panel is bisected by a ridge of higher velocity The suspicion of a ridge is reinforced by results of an aberration test based on a standard Huygens-Kirchhoff migration The ridge is found to follow the general line of a system of pillars left in place during the mining of a lower horizon It is concluded that channel waves may be used to map subsidence into old workings underground Coal seams apparently share, with other sedimentary rocks, the property of a pressure-sensitive seismic velocity

Seismic and aseismic deformation associated with the 1952 Kern County, California, earthquake and relationship to the quaternary history of the White Wolf fault

Abstract: Synthesis of geodetic, geologic, and seismic data from the White Wolf fault, California, indicates that the fault separates an area of late Quaternary and continuing rapid uplift in the Tehachapi Mountains and Transverse Ranges from even more rapid subsidence in the southern San Joaquin Valley. On July 21, 1952, rupture of the White Wolf fault produced the ML = 7.2 Kern County earthquake. We used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952–1953 horizontal shear strains to model the measured coseismic vertical displacements, with an elastic dislocation model. A curved fault trace with decreasing fault depth (27 to 10 km from the surface vertically to the base), slip (3 to 1 m), and dip (75° to 20°) from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record, the measured seismic moment, the fault-plane solution, and the pattern of surface rupture. Two short releveled lines near the 1952 epicenter tilted 4 and 17 μrad down to the north from 5–10 years before the earthquake; the preseismic tilts differ significantly from ten other surveys of these lines. Left-lateral fault-crossing shear strain from 0.2–20 years before the quake was two times greater than both preseismic off-fault strains and the post-seismic fault-crossing strains. During the first seven years after the earthquake, aseismic deformation was negligible. From 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone, rising first in the epicentral region and then at the northeast end of the fault. This was unaccompanied by any surface fault slip. Reconstruction of the vertical separation on the White Wolf fault from late Quaternary and late Miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty-fold, from 0.1–0.2 mm/yr to 3–9 mm/yr, between the past 10–15 m.y. and the most recent 0.6–1.2 m.y. We estimate a 170- to 450-yr average recurrence interval for earthquakes on the White Wolf fault with slip equivalent to that in 1952. The 1952 earthquake appears to be characteristic of the Quaternary record of fault displacement in the increase in White Wolf slip toward the San Andreas fault, the ratio of reverse to lateral slip (1.3:1), and the ratio of vertical fault slip to emergence of the hanging wall block (3:1). The >8500-m-deep sedimentary basin on the down-thrown block cannot be explained by repeated slip of the White Wolf fault in an elastic medium.

First manned submersible dives on the East Pacific Rise at 21°N (project RITA): General results

Abstract: A submersible study has been conducted in February–March 1978 at the axis of the East Pacific Rise near 21°N. The expedition CYAMEX, the first submersible program to be conducted on the East Pacific Rise, is part of the French-American-Mexican project RITA (Rivera-Tamayo), a 3-year study devoted to detailed geological and geophysical investigations of the East Pacific Rise Crest. On the basis of the 15 dives made by CYANA in the axial area of the Rise, a morphological and tectonic zonation can be established for this moderately-fast spreading center. A narrow, 0.6 to 1.2 km wide zone of extrusion (zone 1), dominated by young lava flows, is flanked by a highly fissured and faulted zone of extension (zone 2) with a width of 1 to 2 km. Further out, zone 3 is dominated by outward tilted blocks bounded by inward-facing fault scarps. Active or recent faults extend up to 12 km from the axis of extrusion of the East Pacific Rise. This represents the first determination from direct field evidence of the width of active tectonism associated with an accreting plate boundary. Massive sulfide deposits, made principally of zinc, copper and iron, were found close to the axis of the Rise. Other signs of the intense hydrothermal activity included the discovery of benthic fauna of gian size similar to that found at the axis of the Galapagos Rift. We emphasize the cyclic character of the volcanicity. The main characteristics of the geology of this segment of the East Pacific Rise can be explained by the thermal structure at depth below this moderately-fast spreading center. The geological observations are compatible with the existence of a shallow magma reservoir centered at the axis of the Rise with a half-width of the order of 10 km.

Fine-Scale Sonar Study of Tectonics and Volcanism on the Reykjanes Ridge

Abstract: A survey is described using medium and long-range (Gloria) sidescan sonars of the Reykjanes Ridge crest between 58 and 61 degree N. The survey extended from the part of the ridge crest exhibiting a median valley to that with an axial high. An axial zone, about 13 km wide, is dominated by linear volcanic ridges striking 14 degree and believed to form over fissures running approximately normal to the spreading direction. Outside the axial zone the sea-floor morphology is dominated by inward-facing faults striking subparallel to the Reykjanes Ridge axis, and therefore oblique to the spreading direction. The strike of these faults is believed to be controlled by the way in which the strength of the lithosphere increases away from the ridge axis. The faults have an average spacing of 2.6 km and are about 10 km long. This pattern of faulting is essentially the same as that on normal slow-spreading and fast-spreading ridges. The major changes observed going from the SW (median valley) part to the NE (axial high) part of the ridge are decreases in fault throws, fault-block tilts, and heights of volcanic ridges. The origins of these features are discussed qualitatively in terms of a changing temperature structure of the lithosphere along the Ridge axis.

Tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts

Abstract: Paleomagnetic samples of many Deep-Sea Drilling Project (DSDP) basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid-ocean ridges. Rather than arising from peculiarities of the geomagnetic field, the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes. Several lines of evidence support this conclusion: observations of the Basin and Range Province, an extensional tectonic regime occurring in continental crust; observations of the Troodos complex, Cyprus, an oceanic extensional tectonic regime now exposed on land; and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid-oceanic ridges. The tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30° to 50°, although rotations as large as 70° to 90° are not uncommon. The paleomagnetic consequences of these rotations can be calculated for DSDP basalts if the original strike and latitude of a ridge are known. At four DSDP sites for which adequate data are available, namely sites 319, 332, 410, and 417, rotations ranging from 50° to 70° are required to explain the paleomagnetic results. If the original strike and location of the ridge are not known, as is the case for most of the older parts of the Pacific Ocean, the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations. This information, combined with a similar but independent relationship derived from the marine magnetic anomalies, puts severe constraints on the original strike and location of the ridge.

Post-Paleozoic Evolution of the East Greenland Continental Margin

Abstract: The post-Caledonian basins of East Greenland were initiated by rifting in Carboniferous and Permian time. The tectonic activity increased through Mesozoic time and a number of well-dated tectonic phases can be recognized. Active sea-floor spreading finally occurred in late Paleocene time. The Mesozoic rift is orientated north-south. It is cut by major NW-SE cross faults and opened in the Jurassic from the south by progressive downfaulting of blocks. The combined effect of the tectonism and of eustatic sea-level changes was a stepwise basin extension and submergence of the rifted basin. Triassic rift deposition was mainly continental and the Triassic-Jurassic boundary was characterized by a change from arid to humid climate and from continental to marine deposition. During Jurassic time the environments changed from a tidal bay over a shallow siliciclastic shelf sea to a wide muddy shelf. An important tectonic episode at the Jurassic-Cretaceous boundary resulted in fragmentation and collapse of the shelf by antithetic block-faulting, and coalescent submarine fans were formed along the fault scarps. The Aptian sea transgressed an irregularly erosional relief surface and the remaining part of Cretaceous time was characterized by outer shelf and slope mudstones interrupted by deposition of coarser elastics during periods of block faulting. The Tertiary rocks comprise pre-drift sediments overlain by 1-7 km of late Paleocene basaltic lavas extruded immediately prior to active spreading. Subsequent subsidence of the shelf led to accumulation of 2-8 km of post-basaltic sediments offshore whereas the land area was uplifted 1-2 km. Initiation of spreading along the Kolbeinsey Ridge during late Oligocene time was accompanied by renewed tectonism within the middle part of the margin. Finally the shelf was characterized by strong progradation during Miocene time.

Structure and distribution of fault rocks in the Alpine Fault Zone, New Zealand

Abstract: Summary The Alpine Fault Zone, which is the dominant strand in a dextral system of transform faults along the boundary between the Indo-Australian and Pacific plates, forms a pronounced lineament about 1 km in width. Within the fault zone, the cross-strike passage from cataclasite through augen mylonite (both derived from largely granitoid basement to the NW) to mylonites derived from the high-grade Alpine Schists on the upthrown south-eastern side, is thought to reflect the original distribution of fault rocks with depth below near-surface gouge zones. Structural data suggest that the fault rocks had their fabrics impressed during the late Cenozoic phase of oblique compression when the Southern Alps were ‘ploughed-up’ along the fault. Mylonitic foliation indicates a dip approaching 50°SE for the fault zone at depth, while penetrative stretching lineations plunge in a direction sub-parallel to the present-day interplate slip vector, consistent with dextral-reverse-oblique shear across the fault zone. Pseudotachylyte friction-melt is fairly widely distributed and can be found cutting all other fault rock types apart from gouge. Models for the evolution of the fault zone are considered.

San Luis Obispo Transform Fault and Middle Miocene rotation of the Western Transverse Ranges, California

Abstract: A fault-disrupted, linear belt of lower middle Miocene pyroclastic and volcanic rocks was erupted 15–17 m.y. B.P. along a trend 140 to 200 km long in the southern Coast Ranges and along the northern flank of the western Transverse Ranges of California. Offset segments of the belt are coincident with the Oceanic-West Huasna, Santa Maria River-Little Pine, and Lompoc-Solvang fault zones. The volcanic rocks thin eastward and also vary in thickness along strike, the latter reflecting the undulation of ‘porpoising’ highs and lows of basins. To account for the linear distribution of over 3000 km2 of bimodal volcanic rock types, it is proposed that they were intruded, extruded, and ejected into a continental submarine margin environment along a feature here named the San Luis Obispo transform (SLOT). The fault was near the mid-Miocene continental margin and evidently joined the early mid-Miocene Mendocino triple junction with the crest of the East Pacific Rise. Subduction of the Farallon Plate had stopped along the adjacent continental margin before Miocene magmatism in this region. The inferred position of the East Pacific Rise crest was too far to the south, and the inferred position of the Mendocino triple junction was too far to the north between 15 and 20 m.y. B.P. to have been directly related to the Miocene volcanic event near 35°N latitude at that time. A part of the western Transverse Ranges was transported northwestward immediately following Miocene volcanism. Earlier movement is possible, but evidence is inconclusive. Microplate transport was concurrent with the development of pull-apart structures and the ≃12- to ≃16-m.y.-old period of volcanism along the southern margin of the microplate within the Los Angeles basin. Variable paleomagnetic directions that display clockwise deflections in the Oligocene (e.g., Morro Rock-Islay Hill Complex) and Miocene igneous rocks (e.g., western Transverse Ranges) in the vicinity of SLOT can be accounted for by extension and rotational tectonics during pull-apart development, rotation of blocks between faults during the northwestward transport of the western Transverse Ranges along SLOT, and by rotation and possible northward translation of a part of the western Transverse Ranges.

Imbricate low-angle faulting in uppermost Franciscan rocks, South Yolla Bolly area, northern California

Abstract: The South Yolla Bolly area of northern California contains the intersection of the Franciscan complex, the Klamath Mountains, and the Great Valley sequence. Four distinct lithic units, among them the South Fork Mountain Schist, as well as several subunits, have been discerned in the Franciscan rocks of this area. These units are composed mostly of coherent or semi-coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange, and are arranged in largely upright, parallel imbricate fault slices. Four major low-angle faults have been clearly recognized. Paleontologic data indicate that at least one fault places older material over younger. Lithic units exhibit various degrees of differences in metamorphic mineral assemblages, textural grade, and amount of internal deformation across the faults, with more intensely deformed and metamorphosed material occupying successively higher structural positions. These contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and/or after metamorphism. Original bedding and compositional layering in all of the coherent units generally parallel the thrust faults. Two fault-bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert, and further overlain by mudstone and graywacke or their metamorphic equivalents. The faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock – mafic-crust interface, perhaps as discrete underthrust slices. The distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material.

Thrust and extensional faulting under the Chilean coast: 1965,1971 Aconcagua earthquakes

Abstract: Summary. Earthquakes under coastal northern Chile, from 18° S to 33° S occur at two very different depths. The shallower interplate events have the typical thrust mechanisms associated with subduction of the Nazca Plate under South America. The deeper (close to 70 km) events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab. We study in detail a. couple of Ms∼ 7.5 events that took place about 60 km north of Valparaiso, Chile. The 1971 July 9 event was a shallow thrust interplate event at 40 km depth, while the 1965 March 28 earthquake had a normal fault mechanism and a depth of 72 km. We carry out a detailed analysis of long period P-and surface waves observed in the WWSSN network of stations in order to improve fault plane solutions and depth determinations. For the 1971 event we find a seismic moment of 5.6 ∼ 1027 dyne cm from both P-and surface waves. The source area for this event is relatively small compared to that of other underthrust events with similar moments. Assuming a circular fault shape and a rigidity of 7 ∼ 1011 dyne cm−2 we find a stress drop of 38 bar. The 1965 event is more complex; seismic moments of 1.8 and 1.0 ∼ 1027 dyne cm are found from surface and body waves respectively. The stress drop estimated from P-waves and a circular fault model is close to 91 bar. The existence of earthquakes at two different depths near the Chilean coast may be interpreted in terms of a double layered Benioff zone, as has been found in Honshu, Kuriles or central Peru. Unfortunately the hypo-central determinations of events in this region are not precise enough to separate the two layers. An alternative method to demonstrate the existence of a double Benioff zone is to look for compressional events associated with the upper layer of the down-going slab. A search among available fault plane solutions is ambiguous; three shallow coastal events are found in Stauder's catalogue with horizontal pressure axes normal to the trench, but they might be intra-continental plate events.

The Eagle Pass detachment, southeastern Arizona: Product of mid-Miocene listric(?) normal faulting in the southern Basin and Range

Abstract: Mid-Miocene low-angle normal faulting is an integral part of the regional tectonic strain of the southern Basin and Range. The distinguishing characteristic is the presence of “detachments” — allochthons of moderately to steeply dipping hanging-wall rocks resting in very low angle fault contact on structurally deeper, generally older, footwall rocks. Rotation of detachment strata was accomplished by imbricate listric(?) normal faulting and coordinated tear faulting. Regional structural domains, each thousands of kilometres in area, contain detachments marked by uniform sense of detachment-strata rotation. The Eagle Pass detachment, 70 km northeast of Tucson, Arizona, reveals informative structural details of parts of the system. Steeply southwest-dipping mid-Miocene volcanic and sedimentary rocks, several thousands of metres in thickness, rest in very low angle (10°±) fault contact on Precambrian granitic basement. Detachment strata were faulted into position by NE-directed translation, accompanied by rotation of strata to steep SW dips. The detachment fault which underlies the 25 km2 detachment has the form of a large northeast-trending mullion trough with relief of at least 500 m. Along the keel of the detachment, Precambrian quartz monzonite was transformed into a hydrothermally altered selvage of microbreccia. The steep (± 80°) northwest margin of the mullion trough served as a tear fault along which quartz monzonite was crushed, volcanics were dragged, and fanglomerate was locally sheared into flattened-pebble conglomerate. Detachments like that at Eagle Pass resemble slump blocks on a grand scale. They are elements of a regional system of extensional deformation which coincides with the belt of Cordilleran metamorphic core complexes. Mid-Miocene detachment faulting post-dates the formation of exposed mylonitic tectonites of core complex affinity, but the direction of translation of the detachments is parallel to penetrative lineation in nearby and/or underlying tectonite. Moreover, in southeastern Arizona examples, the sense of translation of detachments matches the sense of simple-shear movements disclosed by tectonite fabrics. We speculate that large crustal displacements achieved during extension-induced formation of tectonite produced a geometric and/or dynamic condition which strongly influenced subsequent detachment faulting. Consequently, with renewed(?) or continued(?) extension in mid-Miocene time, deep upper-crustal slumping was triggered and detachment rocks were translated along listric(?) normal faults in the direction and sense of the earlier displacements which had accommodated the formation of tectonite.

Attenuation of P and S waves in a magma chamber in Long Valley Caldera, California

Abstract: Shallow earthquakes around the southwest boundary of Long Valley caldera, west of the Hilton Creek fault, are characterized by lack of S-waves at regional seismic network stations to the northwest, north and northeast, and P-waves for these same station-event combinations are deficient in frequencies higher than about 2-3 Hz. Earthquakes east of the Hilton Creek fault and southeast of the caldera have normal P- and S-wave signatures at the same stations. These effects are explained by propagation through a magma chamber in the south-central part of Long Valley caldera, at depth greater than 7-8 km.

Centrifuge modeling of fault propagation through alluvial soils

Abstract: The behavior of alluvial deposits subjected to fault movements in the underlying bedrock is of major concern for critical structures located within fault zones. An understanding of fault propagation through soils would assist in design of such structures, but could also be utilized in geological interpretation of fault displacement history. On the premise that alluvial fault morphology contains shear patterns characteristic of modes and rates of fault displacements, a study was undertaken involving centrifugal and numerical models of reverse faulting. This paper describes the centrifuge model testing performed to backup simultaneous numerical studies (Geognosis Report, 1980), which will be described elsewhere. A comprehensive model test series under earth gravity conditions (1g) involving reverse and normal faulting under different angles has recently been undertaken by Cole (1979). However, model tests performed under 1g-conditions are limited to rather thin soil layers because of their inability to simulate realistic gravity stress conditions. Furthermore, it is not possible to simulate faulting fast enough to include inertial effects with such models.

Tertiary extension north of the Las Vegas Valley shear zone, Sheep and Desert Ranges, Clark County, Nevada

Abstract: Detailed mapping reveals the presence of high-angle extensional faults and low-angle gravity slides on the west side of the Sheep Range. Three major high-angle faulting events each account for 20° of eastward rotation and accommodate extension between the Las Vegas Range and the Desert Range. Low-angle faults represent surficial slides in response to topography produced by extension on the high-angle faults. Faulting took place during the Miocene, synchronously with deposition of the Horse Spring Formation and with displacement on the Las Vegas Valley shear zone. The extension in the Sheep Range took place without volcanism, intrusion, or metamorphism of the Paleozoic sedimentary rocks. Offset thrust faults suggest that the area west of the Sheep Range extended almost 100% during the Miocene, while the corresponding area south of the Las Vegas Valley shear zone did not extend significantly. The shear zone bounded the extending terrane on the south, acting as a transform fault. This extension west of the Sheep Range may in part balance that mapped by Anderson (1971) in the Eldorado Mountains. The Las Vegas Valley shear zone and the Lake Mead fault system may have acted together to compensate for areas of localized extension between the Colorado Plateau and the vicinity of the Specter Range.

Continental underthrusting beneath the Southern Alps of New Zealand

Abstract: Both the inferred thermal regime and the variation in crustal thickness beneath the Southern Alps suggest that the mode of crustal shortening changes substantially along the length of the Alpine fault. North of Mt. Cook, crustal shortening is being accommodated predominantly by uplift and erosion of Pacific plate continental crust, as postulated in the Wellman-Adams tectonic model. However, south of Mt. Cook, the absence of high heat flow and a large crustal root that is not in isostatic equilibrium may have resulted from underthrusting of as much as 100 km of Indian plate continental crust.

Experimental high temperature and high pressure faults

Abstract: Deformation experiments on Hale albite rock have produced faults at 5 to 15 kb confining pressure and 700° to 1125°C, when hydrolytic weakening is suppressed by either the absence of water or by low pressure. The faults are characterized by: 1) an angle of about 45° to σ1, 2) very little gouge, and 3) several percent permanent strain before failure. Temperature dependent friction is believed to allow frictional sliding and faulting at stresses below those predicted by the Coulomb failure criterion, but above those of hydrolytically weakened crystal plastic flow. High confining pressure, low friction and reduced tensile stress concentrations may allow the fault to propagate as a shear rather than a tensile crack.