摘要:

The combination of detailed topographic leveling on the southwest segment of the El Asnam thrust fault with existing seismic and geologic data implies that the geometry of this fault involves shallow dipping flats and steep ramps. The fault appears to be growing along strike toward the southwest end, where the main shock initiated in 1980. From a depth of about 10 km, the main thrust appears to ramp to the basement‐Cenozoic cover interface on a plane striking N40°E and dipping 50°–55° to the northwest. Along the southwest segment where folding has not yet developed, the thrust continues steeply through the Cenozoic cover to the near surface where it flattens, causing normal faulting. Along the central and northeast segments, which display a more evolved fold structure, the deep thrust probably flattens at a depth of 5–6 km, into a decollement along the Cenozoic‐Jurassic interface before ramping to the surface. The Sara El Marouf and Kef El Mes anticlines have thus formed as fault propagation folds. Normal faults at Beni Rached probably branch with the thrust to maintain kinematic compatibility between the deep ramp and decollement. The greater separation (∼7 km) between the normal faults at Beni Rached and the thrust where it crosses Oued Cheliff than along the southwest segment (∼1 km) reflects the greater depth of the ramp to flat bend. We infer that the September 9, 1954, earthquake activated only the central deep segment of the main thrust together with the Beni Rached normal faults, while that of October 10, 1980, activated the whole system of flat decollements, ramp thrusts and compatibility normal faults. Further complexities of the faulting in map view are related to changes of strike of the thrust (in particular north of

ISSN:0278-7407    

DOI:10.1029/91TC01449

年代:1992

数据来源: WILEY

摘要:

Seismic reflection data from the south central Canadian Cordillera covering the interval from the easternmost metamorphic core complexes near Arrow Lakes to the Fraser River fault system along the Fraser River reveal a highly reflective and complex crust. The base of the crustal reflectivity, interpreted as the reflection Moho, is clearly delineated by a continuous sharp boundary that is essentially planar and slopes uniformly over a distance of 250 km from about 12.0 s in the east to about 10.5 s in the west. This virtual lack of relief at the base of the crust contrasts sharply with surface structures that involve 25 km or more of structural relief. Some of these surface structures can be readily correlated to structures that are outlined by the reflection data and that can be followed into the middle and lower crust. Even though part of this area was subjected to large amounts of Eocene extension, the crust is not divisible into transparent upper and reflective lower layers as it is in parts of the U.S. Cordillera. Three structural culminations, the Monashee complex, the Vernon antiform, and the Central Nicola horst, are interpreted on the basis of the reflection configuration and the surface geological relationships to have formed initially during Jurassic to Eocene compression and then to have been modified and exposed during early and middle Eocene extension. An example of a compressional structure observed on the profiles is the Monashee decollement, which can be traced from the surface westward into the lower crust. Extension is manifested along a variety of normal faults, including the regionally extensive low angle Okanagan Valley‐Eagle River fault system, moderately dipping faults such as the Columbia River and Slocan Lake faults, and high‐angle faults such as the Quilchena Creek and Coldwater faults. Both Jurassic to Eocene compressional shear zones and early to middle Eocene extensional shear zones are listric into the lower crust or Moho under the Intermontane b

ISSN:0278-7407    

DOI:10.1029/91TC02332

年代:1992

数据来源: WILEY

摘要:

Knowledge of the magnitude of differential stress and strain rate during the formation of mylonitic shear zones in metamorphic core complexes provides constraints on the mechanical behavior of the middle continental crust during extension. We analyzed the differential flow stress during the mylonitization of quartzofeldspathic rocks in the Whipple Mountains, California, using grain‐size piezometers and kinetic laws for grain growth. Mylonitic gneisses collected from two widely separated transects have grain sizes that cluster in a range from 32 to 61 µm. Analysis of grain growth kinetics indicates that mylonitization of the gneisses continued during cooling to temperatures ≤500°C, compatible with estimates from two‐feldspar thermometry. Quartz grain‐size piezometers suggest that the mylonitization occurred under differential stresses (σ1–σ3) of ∼40–150 MPa, or maximum shear stresses of 20–75 MPa. Extrapolation of quartzite flow laws to 500°C indicates that the mylonitization occurred at strain rates faster than 10−14s−1. These estimates suggest that the mylonitic zone within the Whipple Mountains had an effective viscosity of the order of 1018±4−1020±4Pa s. These low viscosities and rapid strain rates, combined with seismic reflection data showing that continental crust is layered, suggest that more realistic physical models of extension of the continental lithosphere should treat the lithosphere as a heterogeneous distribution of high‐viscosity regions se

ISSN:0278-7407    

DOI:10.1029/91TC01291

年代:1992

数据来源: WILEY

摘要:

Central Japan is situated on the inflection point of the bow‐shaped Japanese islands. Numerous NW‐SE trending active faults, arranged in parallel at intervals of 20 to 80 km are found in this area. These active faults are more than 30 km long with shattered zones from 30 to 300 m wide. Several active faults constitute a given block boundary, which serves as the dividing line for one of the four blocks that make up central Japan. The block boundaries require careful study since numerous historical earth‐quakes have occurred along these lines. Offset measurements of basement rocks, created during the Quaternary period due to left‐lateral faulting, amount to 1 to 7 km. Gravity lineaments, which link points of sudden change and saddles of Bouguer anomalies, are clearly found along the block boundaries. The NW‐SE trending active faults appearing on the ground surface are associated with motions of the block boundaries. Block rotational movement, caused by left‐lateral faulting, plays an important role in the crustal deformation of central Japan. Rotational angles of the blocks calculated from the amount of displacement of basement rocks, initiated during the Quaternary period, are estimated to be 3° to 7° in a clo

ISSN:0278-7407    

DOI:10.1029/91TC01511

年代:1992

数据来源: WILEY

摘要:

Normal faults were measured in three selected areas of the late Tertiary and Pleistocene lava pile in Iceland. The arithmetic mean throw of 315 faults is 8.2 m, the minimum being 0.5 m and the maximum 150 m. About 93% of the faults have throws of less than 20 m. The mean strike of the faults is N31°E, but the mode is N45°E. The faults dip from 42° to 89°, nearly 80% dip between 65° and 79°, and the mean dip is 73°. Fault breccia is common and varies in thickness from 0.05 m to 2 m. There is a positive linear relation between breccia thickness and throw, the linear correlation coefficient being 0.78. It is proposed that the normal faults in Iceland form by two basic mechanisms. One is the development of faults from sets of inclined joints in the lava pile. The initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1° for every 150–170 m depth. The inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints. The other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures. The great majority of fractures within the active rift zone are tension fractures, but when they attain the length of several hundred meters they normally change into normal faults. It is concluded that normal fault development in Iceland commonly starts at depths of about 1 km in

ISSN:0278-7407    

DOI:10.1029/91TC01536

年代:1992

数据来源: WILEY

摘要:

Exploration in the Rukwa rift, using gravity and seismic reflection surveys, fieldwork, and drilling has defined the structure and stratigraphy of the basin in greater detail than any other part of the Western rift. The stratigraphy comprises Precambrian basement, Karroo sandstones, shales and coals, upper Miocene red beds, and Miocene‐Recent lacustrine and fluvial sediments. During Miocene‐Recent rifting the greatest sediment input apparently came from axial fluvial systems flowing from the northwest and southeast. The southwestern area experienced alternating shallow lacustrine and fluviodeltaic conditions during the Miocene‐Recent. Cenozoic age rift structures have a dominant NW‐SE and a subordinate N‐S trend. The NW‐SE trend tends to follow a Precambrian basement and later Karroo structural trend. NE‐SW seismic lines indicate up to 10 km extension of the Tertiary section in a direction oblique to the probable E‐W regional extension direction. In the southeastern portion of the basin both Karroo and Tertiary‐Quaternary sediments expand into the Lupa fault zone, reaching thicknesses of up to 3 and 7 km, respectively. Tertiary‐Quaternary sediment thicknesses decrease northwestward, accompanied by a decrease in the amount of extension and a broadening of the basin as extension is transferred to the L

ISSN:0278-7407    

DOI:10.1029/91TC02102

年代:1992

数据来源: WILEY

摘要:

Regional tectonic features of the western Canadian Cordillera can be interpreted in terms of Middle Jurassic accretion of a single composite superterrane (Stikinia + Wrangellia + Alexander) to ancestral North America. Closure of the intervening Cache Creek‐Bridge River ocean moved the continental edge to a new position west of the accreted superterrane. The Coast Belt is primarily a succession of superimposed Middle Jurassic to early Tertiary magmatic arcs related to prolonged subduction of Pacific Ocean lithosphere beneath the new North American margin. Discrete magmatic pulses, separated by lulls or periods of relative quiescence, successively overprinted the new margin. The Insular and Intermontane superterranes, previously viewed as widely separate entities prior to mid‐Cretaceous time, were already amalgamated before an initial Middle Jurassic overprint produced an ancestral Coast Belt. Late Jurassic to Early Cretaceous rift‐related(?) marine basins, mid to Late Cretaceous compressional structures, and early Tertiary extensional features coincident with the Coast Belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary, subduction‐related Andean magma

ISSN:0278-7407    

DOI:10.1029/91TC02183

年代:1992

数据来源: WILEY

摘要:

Oblique continental convergence across the Alpine fault since the mid‐Miocene has loaded the Australia plate with the leading edge of the Pacific plate and formed a foreland basin. The basin occurs mostly offshore beneath the continental shelf, but remnants of the marine basin fill overlie Ordovician basement in a narrow coastal strip 10–20 km wide in Westland, between the Tasman Sea and the Alpine fault. The results of fission track analyses of apatite and zircon separates from this basement, integrated with the regional structure and stratigraphy, indicate that the foreland basin formerly extended across the whole of Westland and that its succession increased in thickness eastward toward the Alpine fault, reaching a maximum of about 4 km. Continuing covergence across the main thrust (Alpine fault) has caused its footwall to thicken by reverse fauting in a transitional zone 10–20 km wide immediately west of the mountain front (Southern Alps). Consequently, the inner margin of the foreland basin has been inverted and the succession almost completely eroded. In the southern part of Westland, upper crustal shortening in the Australia plate amounted to about 2 km; associated uplift started about 5 m.y. ago and involved only one major fault‐bounded block. In northern Westland, upper crustal shortening amounted to about 12 km; two major fault‐bounded blocks were involved, and uplift was underway by 10 Ma. Assuming a preuplift geothermal gradient of 25°C/km, the long‐term rate of uplift ranged between 0.8 and 2.0 mm/yr in southern Westland and between 0.4 and 1.2 mm/yr in different parts of north

ISSN:0278-7407    

DOI:10.1029/91TC02362

年代:1992

数据来源: WILEY

摘要:

Recent marine and onland geophysical and geological investigations along the northern Caribbean plate boundary call for a review of its tectonic interpretation, in the light of a new compilation of the seismological data avalaible from Cuba to Puerto Rico. We show that the shallow seismicity in the northeastern Caribbean is concentrated along an east‐west trending lineament corresponding to the trace of the major strike‐slip fault system. The most intense seismicity is located around restraining bends such as southern Cuba and northern Hispaniola. The stress and strain distribution deduced from focal mechanisms and microtectonic analysis lead us to infer a small N‐S convergence component associated with the major eastward strike‐slip motion of the Caribbean plate versus North America. Earthquake distribution and focal mechanisms suggest the existence of a lithospheric slab inherited from the frontal subduction under the Lesser Antilles diping down under Puerto Rico and eastern Hispaniola. We propose a model in which this slab is disconnected from the Atlantic oceanic lithosphere by transcurrent faulting along the plate b

ISSN:0278-7407    

DOI:10.1029/91TC02364

年代:1992

数据来源: WILEY

摘要:

The Salmon River suture zone in west central Idaho juxtaposes volcanic arc rocks of the Wallowa terrane directly against cratonic North America. Detailed metamorphic studies along a 10 km traverse perpendicular to the suture indicate that the arc and two crystalline fragments thrust upon it each record different pressure‐temperature (P‐T) histories. From lowest to highest structural level: the Wallowa terrane shows only subgreenschist metamorphism, the Rapid River plate (RRP) records unroofing and cooling from ∼8 kbar and 550°C to 6 kbar and 475°–500°C, and the Pollock Mountain plate (PMP) shows evidence for polymetamorphism and records burial and heating paths to final equilibration conditions of 9–11 kbar and 600°–625° C. Ar‐Ar hornblende ages combined with the P‐T data suggest that currently exposed levels of the RRP and PMP were juxtaposed against one another at 15–20 km depth at or prior to 118 Ma, indicating that 10–20 km of uplift, and hence also the onset of collision‐related metamorphism, occurred before ∼118 Ma. Correlation of the metamorphic and age data with geometric constraints from the initial Sr 0.706 line and the dimensions of the RRP and PMP permit construction of large‐scale retrodeformable sections of the west side of the suture from Late Jurassic through Late Cretaceous time. The abrupt nature of the Sr 0.706 line implies that the arc‐continent boundary extends vertically through most of the crust, which requires sharp downwarping of the arc lithosphere in order to account for the PMP metamorphic data. Narrow zoned overgrowths on PMP garnets record this burial event and require initially rapid (≥3 km/m.y.) uplift rates in order to be preserved. We suggest that the onset of rapid uplift resulted from the separation of the negatively buoyant lithospheric root from the downwarped arc, allowing buoyant rise of fragments of thickened crust. Detachment of the root is suggested to change the environment of crustal shortening from one in which footwalls of thrusts or shear zones sink to one in which hanging walls rise. This mechanism represents an alternative to cessation of shortening or onset of tectonic denudation as an explanation for the transition from burial to uplift of high‐pressure metamorphic terrains. Subsequent uplift appears to have been slow and to have occurred in a hinged fashion such that mineral and whole rock ages decrease systematically towards the suture zone. The consumption of lithosphere during ≥40 km of shortening between two crustal blocks implies that the Salmon River suture is the trace of an intracontinental subduction zone. Burial and collision apparently began before about 130 Ma, and thus any precollision strike‐slip faulting or tectonic escape of intervening terranes was likely accomplished

ISSN:0278-7407    

DOI:10.1029/91TC02418

年代:1992

数据来源: WILEY