摘要:

Seismic modeling and geometric analysis provide clues for identification of multiple generations of cross‐cutting planar rotational faults in seismic reflection profiles. Many elements characteristic of this structural geometry are apparent, although contorted, at shallow levels in unmigrated and migrated synthetic seismograms. Structural features potentially signaling an earlier generation of rotational faulting within a tilt‐block terrain include first‐generation fault segments, intrablock terminations of horizons, steep dips of prefault strata, anomalously high fault‐strata intersection angles, “composite” basin geometry, truncations of early synextension sedimentary wedges, and anomalous crustal thinning. Evaluation of published profiles from the Bay of Biscay and Galicia Bank indicates that the crosscutting “domino” fault model probably is not applicable in these areas as previously suggested. While the model may be appropriate for the basement structure of Spring Valley, Nevada, seismic eviden

ISSN:0278-7407    

DOI:10.1029/92TC01969

年代:1993

数据来源: WILEY

摘要:

The Patagonian orocline is the 90° bend in the southernmost Andes between 50°S and 56°S. Paleomagnetic and structural data indicate that the orocline is, at least in part, the product of tectonic rotation. Recent field work in the Beagle Channel region of southernmost Chile provides evidence for widespread left‐lateral strike‐slip faulting in the internal zones of the mountain belt. Both arms of the Beagle Channel are interpreted to be left‐lateral strike‐slip faults based on detailed study of mesoscale strike‐slip faults (Riedel shears) observed in coastal outcrops. Although much of the evidence indicates Cenozoic brittle strike‐slip faulting, other fabric data, including vertical foliation zones containing horizontal quartz stretching lineations and ductile left‐lateral kinematic indicators, suggest that Mesozoic ductile strike‐slip or oblique‐slip shearing also occurred. The implication is that the mid‐Cretaceous Andean orogeny involved the transpressional inversion of the Rocas Verdes marginal basin and that transpression has been the dominant deformational regime in the region for the last 120 Ma. Regional left‐lateral strike‐slip faults are now recognized in all lithotectonic provinces of the southernmost Andes. A statistical study of regional lineament trends using aerial photographs and satellite imagery suggests that many unstudied lineaments are also strike‐slip faults. A new model is proposed that integrates the development of strike‐slip faulting and the structural evolution and uplift of the southernmost Andes with the rotational development of the orocline. The Patagonian orocline appears to be the product of broad interplate shearing accommodated by strike‐slip faulting, block rotation, and contraction and is pro

ISSN:0278-7407    

DOI:10.1029/92TC01790

年代:1993

数据来源: WILEY

摘要:

The northern part of the Philippine fault zone, in Luzon, corresponds to a complex braided system of left‐lateral strike‐slip faults. The NW oriented main active branch, which emerges from the Philippine Sea, splits into an array of north striking splays responsible for the tectonic evolution of the Central Cordillera. This complex fault pattern has favored local stress field variations. Strike‐slip basins have evolved in this framework along or in the vicinity of the main splays of the Philippine fault. Apart from the classical pull‐apart tectonics on a releasing fault termination, overlap, or bend, we describe other mechanisms such as the strike‐slip tilting or the warping of a strip limited by two strike‐slip faults. The strike‐slip basins are good recorders of the evolution of the Philippine fault system. Those located along the north striking cordilleran faults individualized in late early Pliocene to Pleistocene time when the present‐day Philippine fault initiated, but their main control is the fault shape acquired in upper middle Miocene time. The recent tectonic evolution of the fault system is best recorded in central Luzon, where the active basins trace an asymmetrical V shape, with the longest branch trending NE parallel to the East Luzon Trough, and the shortest one trending NW related to the Philippine fault. Both the fault pattern and the basin distribution demonstrate the influence of the Benham Rise in the tectonic evolution of Luzon. The structural setting is interpreted as the result of an early Miocene collision event between the Benham Rise and the eastern margin of Luzon, and subsequent inception of the NW striking strand of the Philippine fault. The present locations of the basins result from the interaction between the structural heritage and the present‐day regi

ISSN:0278-7407    

DOI:10.1029/92TC01968

年代:1993

数据来源: WILEY

摘要:

The Hsüehshan Range, exposed in the northern and central Taiwan slate belt, is a fault‐bounded structural high cored by biotite grade slates and metasandstones. Syntectonic overgrowths in pyrite pressure shadows indicate that much of the eastern Hsüehshan Range experienced coaxial strain histories and that finite strain magnitudes generally increase toward the hinterland. Near the eastern boundary of the Hsüehshan Range, however, pressure shadows record noncoaxial strain histories consistent with a top‐to‐the‐east sense of shear along a steep NW dipping shear zone. This noncoaxiality is attributed to SE directed backthrusting on the Lishan fault, which separates the higher‐grade, Eo‐Oligocene rocks of the Hsüehshan Range from the lower‐grade Miocene rocks of the Backbone Range. Because it is bounded to the east by the SE‐vergent Lishan fault and to the west by a series of NW‐vergent thrusts (e.g., the Chüchih fault), the Hsüehshan Range is envisaged as a pop‐up structure. Strain magnitudes measured from pressure shadows in the coaxial part of the range are consistently lower than those predicted by steady state wedge models that assume all deformation is accommodated by penetrative strain. Departure from the model predictions is attributed primarily to strain localization along discrete fault surfaces (e.g., the Lishan fault). The Hsüehshan Range tapers in width to the south; thus the pop‐up may be buried or die out to the south wh

ISSN:0278-7407    

DOI:10.1029/92TC01711

年代:1993

数据来源: WILEY

摘要:

Paleomagnetic results have been obtained from a suite of Paleozoic samples from Sierra de Almeida within the high eastern portions of the Atacama desert. Characteristic directions are discussed for two sequences of pre‐Silurian lavas of probable Cambro‐Ordovician age, the Late Cambrian Choscas pluton, three Late Ordovician‐Early Silurian plutons, the Devonian‐Carboniferous Lila Formation and late Paleozoic volcanic units of the Pular and Cas Formations. The Choschas pluton and one lava series yield similar northerly and shallow directions which for the presence of reversals and their concordance are suggested to represent early Paleozoic Arequipa plate directions. Directions in the three Late Ordovician‐Early Silurian plutons pass a tilt test using an overlying erosional unconformity, and these also include reversals. Together with directions from a second lava series (a roof pendant in an Early Silurian pluton) these define poles compatible with Silurian Gondwana results from Africa and Australia. In situ directions from the basal red beds of the Devonian Lila Formation are inconsistent with Devonian Gondwana or stable South American Poles, but (like the Devonian strata of the Appalachians) they are consistent with a tilt‐corrected overprint of Kiaman Superchron age. These results, together with previous results from the late Paleozoic Cas and Pular Formations are discordant from the Gondwana path only for the latest Cambrian‐earliest Ordovician. The discordance in paleomagnetic data, together with regional geologic constraints, can be explained by a model in which the Arequipa block, representing a paraautothonous finger of Gondwana (like Japan or the Iberian Peninsula) rotated about a nearby pole but was then resutured during the Silurian. Such a scenario resolves much of the discrepancies in the models which have emerged from Peru, Chile,

ISSN:0278-7407    

DOI:10.1029/92TC00619

年代:1993

数据来源: WILEY

摘要:

A regional reanalysis of the Otago Schist shows that ductile mesoscopic structures can be interpreted as elements of a single, progressive, heterogeneous, noncoaxial, Jurassic deformation related to juxtaposition of the Caples and Torlesse terranes. Stretching lineations are parallel, oblique, and perpendicular to the orogen in various parts of Otago, but changes in trend are generally unrelated to the terrane boundary. Structural geometry and rare shear criteria indicate that the Caples terrane overthrust the Torlesse terrane from the south and west, but precise transport directions are not known, being model‐dependent. Macroscopic recumbent folds may be present in the schist, but mainly in the form of partially developed nappe structures truncated by zones of retransposed foliation and/or high strai

ISSN:0278-7407    

DOI:10.1029/92TC01563

年代:1993

数据来源: WILEY

摘要:

The Tongbai area of the eastern Qinling belt in China includes granulite‐grade metamorphic assemblages (Qinling Complex) which were previously regarded as Archean to early Proterozoic in age and belonging to the southern margin of the North China plate (craton). Our petrological and geochemical data characterize these rocks as two‐pyroxene granulites and garnet granulites which formed at temperatures of 757°–840°C and pressures of about 9.5 kbar and are now found as xenoliths in granodioritic gneisses. The protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro. The207Pb/206Pb ratios derived from evaporation of single zircons yield ages of 470±20 and 470±14 Ma, respectively, for the basic granulites which we interpret to reflect the time of protolith emplacement. These are intruded by a 435±14 Ma granodioritic gneiss post‐dating granulite formation. A metaquartzite sample contains detrital zircons as old as 2555±8 Ma. Two samples of granitoid gneiss from the Tongbai Complex S of the Qinling granulites have single‐zircon207Pb/206Pb evaporation ages of 776±8 and 746±10 Ma, respectively, and document late Proterozoic igneous activity. We suggest that the Qinling granulites document an important and hitherto unknown phase of early Silurian crustal thickening following subduction and continental collision and that both the Qinling and Tongbai Complexes were part of the southern margin of the North China craton prior to this event and record late Proterozoic

ISSN:0278-7407    

DOI:10.1029/92TC01788

年代:1993

数据来源: WILEY

摘要:

Paleozoic carbonates of cratonic eastern North America comprise the footwall of the Appalachian‐Ouachita fold‐and‐thrust belt and contain a layer‐parallel shortening (lps) fabric that is preserved by mechanically twinned calcite. Shortening directions are generally parallel to the Appalachian‐Ouachita thrust‐transport direction in carbonates of the thrust belt proper (restored width ∼400 km) and within carbonates up to 1700 km into the foreland, giving a pre‐thrusting sedimentary prism ∼2100 km wide through which compressive orogenic stresses were transmitted. The shortening strain magnitudes (<6%) and the inferred calcite twinning differential stress magnitudes (<90 MPa) decrease exponentially away from the orogenic front. Calcite twinning strain patterns in other adjacent tectonic provinces, such as the Grenville, Laramide, Keweenawan Rift, and Newark Basin, are distinct from the twinning strains preserved in the cratonic Paleozoic carbonates. (Appalachian orogen, far‐field stresses, calci

ISSN:0278-7407    

DOI:10.1029/92TC01106

年代:1993

数据来源: WILEY

摘要:

A perplexing variety of crustal models have been proposed for the region beneath the southern Appalachian overthrust. Determining the true nature of crustal composition and structure in this region carries important scientific and economic implications. To evaluate the numerous models, compressional wave velocities and densities were measured on 30 rock samples collected within the Grandfather Mountain window in the Blue Ridge of North Carolina. The window breaches high‐grade crystalline rocks, exposing Grenville gneisses, Late Proterozoic metasedimentary and metavolcanic rock, and Cambrian quartzite, phyllite, and dolostone that are likely to underlie the regional Blue Ridge‐Piedmont thrust. Measured velocities range widely from 4.21 km s−1for Cambrian phyllite to 7.47 km s−1for tectonized Cambrian Shady Dolomite at 200 MPa. Velocity anisotropy is as great as 47% in the phyllite. The samples exhibit a broad range in densities as well: from 2500 kg m−3in fractured quartzite to 3000 kg m−3in metabasalts. Synthetic reflection seismograms were generated using these physical properties to consider the origin of subthrust reflection events. In addition to the much‐publicized interpretation of Paleozoic sedimentary rocks residing beneath the crystalline sheet, modeling reveals that the reflections could also be explained by metamorphosed Paleozoic strata, by the regional Late Proterozoic metavolcanic and metaclastic rift sequence, and by pervasive shear zones (i.e., mylonites) within compositionally homogeneous gneisses. We interpret field data to strongly favor mylonitic gneisses as the subthrust reflectors. Still, surface data remain sufficiently ambiguous that the question will not be definitively settled without contine

ISSN:0278-7407    

DOI:10.1029/92TC01831

年代:1993

数据来源: WILEY

摘要:

The western Paleozoic and Triassic belt in the southern Klamath Mountains contains several convergent‐margin tectonic assemblages that show great variation in lithology and structural patterns. Many preserve evidence of intermediate‐ to high‐pressure facies‐series metamorphism consistent with a subduction and/or accretionary origin prior to regional mid‐Jurassic magmatism and shortening. Retention of a pre‐Middle Jurassic record is revealed by new40Ar/39Ar data from the eastern Hayfork terrane chert‐argillite mélange, which document unexpectedly old cooling ages of 346–360 Ma for micas and amphiboles in a variety of blocks within mélange matrix. These Early Mississippian cooling ages and petrologic criteria indicate that the blocks probably originated as olistoliths derived from an exhumed Central Metamorphic belt in the late Paleozoic. Minor low‐temperature discordance of the block ages suggests low‐grade reheating between about 240 and 280 Ma, coincident with an age on eastern Hayfork matrix of 263 Ma that reflects Early Permian or younger mélange formation. The mineral spectra indicate regional temperatures have not significantly exceeded ∼300°C since the Mississippian. This Early Permian accretionary history may only be preserved in the southern Klamath Mountains where mid‐Jurassic granitoids are sparse. We suggest that a late Paleozoic eastern Hayfork accretionary complex was constructed proximally to denuded Eastern Klamath forearc basement in an environment analogous to the modern

ISSN:0278-7407    

DOI:10.1029/92TC01325

年代:1993

数据来源: WILEY