摘要:

The Gangdese batholith, southern Tibet, was part of an Andean‐type arc at the southern margin of Asia prior to the collision of India and Asia at approximately 50 to 40 Ma. Fission‐track and40Ar/39Ar analyses of 28 rocks from 10 Gangdese granitoid plutons along an ∼250 km length of the batholith in the Lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks. These data suggest a range of ages for these plutons of 94 to 42 Ma, with the majority being of Tertiary age. The postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling, dropping many tens of degrees in a few million years. Conversely, these plutons all experienced intervals during which they cooled very slowly or not at all; these slow‐cooling intervals lasted from 5 to 50 million years. Our data indicate that since the collision between India and Asia began, response to continued convergence has been quite variable in even this relatively small area. The data reported here are consistent with a recently proposed model of Oligo‐Miocene crustal shortening along the Gangdese Thrust system in

ISSN:0278-7407    

DOI:10.1029/94TC01676

年代:1995

数据来源: WILEY

摘要:

Nineteen40Ar/39Ar and 22 Rb‐Sr mineral ages are presented for metamorphic rocks and granites collected from the northern part of the Nanga Parbat‐Haramosh Massif (NPHM) and adjacent parts of the Kohistan arc, western Himalaya. The majority of these ages are interpreted as exhumation‐related cooling ages, and they therefore place important constraints on the exhumation history of the region. The40Ar/39Ar and Rb‐Sr mica cooling ages for areas of the NPHM to the north of the Indus River lie in the range 2–8 Ma and are similar to previously published ages from the Indus gorge itself. Cooling histories for these areas show little systematic variation from east to west, suggesting that the basement may have been exhumed as a relatively rigid block, after the main stages of folding. Mica ages increase dramatically from the NPHM westward into the Kohistan arc, where ages lie in the range 13–30 Ma. The contrast in both cooling ages and the age differential between mineral pairs between the two terranes indicates rapid cooling of the NPHM in the last 10 m.y. compared with the Kohistan arc. The relationship between rates of cooling and exhumation is poorly constrained in regions of rapid uplift, where the characteristics of the evolving geotherm are not well understood. However, the results are consistent with an anomalously rapid exhumation history of the NPHM during the last 10 m.y. compared with the adjacent Kohistan arc. The interpretation of some of the ages documented in this study is problematical. Two biotite samples from contrasting lithologies within the NPHM have yielded higher87Sr/86Sr ratios than the corresponding whole rocks, but the cause of this isotopic disequilibrium remains unclear. Other biotite samples from the region have consistently yielded younger Rb‐Sr ages than40Ar/39Ar ages, which may be a reflection of either preferential leaching of87Sr during fluid infiltration on a regional scale or the presence of excess argon. Alternatively, the relative ages indicate slightly lower closure temperature for strontium diffusion in biotite than for argon diffusion in biotite where samples have undergone

ISSN:0278-7407    

DOI:10.1029/94TC02906

年代:1995

数据来源: WILEY

摘要:

The Upper Lahul region in the NW Himalaya is located in the transition zone between the High Himalayan Crystalline (HHC) to the SW and the Tethyan Zone sedimentary series to the NE. The tectonic evolution of these domains during the Himalayan Orogeny is the consequence of a succession of five deformation events. An early Dl phase corresponds to synmetamorphic, NE verging folding. This deformation created the Tandi Syncline, which consists of Permian to Jurassic Tethyan metasediments cropping out in the core of a large‐scale synformal fold within the HHC paragneiss. This tectonic event is interpreted as related to a NE directed nappe stacking (Shikar Beh Nappe), probably during the late Eocene to the early Oligocène. A subsequent D2a phase caused SW verging folding in the HHC. This deformation is interpreted as contemporaneous with late Oligocene to early Miocene SW directed thrusting along the Main Central Thrust. In the Tethyan Zone, a D2b phase is marked by a decollement thrust, a system of reverse faults, and gentle folds, associated with SW directed tectonic movements. This deformation is related to an imbricate structure, characteristic of a shallow structural level, and developed in the frontal part of a nappe affecting the Tethyan Zone units of SE Zanskar (Nyimaling‐Tsarap Nappe). A later D3 phase generated the Chandra Dextral Shear Zone (CDSZ), a large‐scale, ductile, dextral strike‐slip shear zone, located in the transition zone between the HHC and the Tethyan Himalaya. The CDSZ most likely represents a part of a system of early Miocene extensional and/or dextral, strike‐slip shear zones observed at the HHC‐Tethyan Zone contact along the entire Himalaya. A final D4 phase induced large‐scale doming and NE vergin

ISSN:0278-7407    

DOI:10.1029/94TC02455

年代:1995

数据来源: WILEY

摘要:

Dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear. Each class of model predicts a different incremental strain history that should be recorded in the footwall. In the Tauern Window (eastern Alps), postmylonitic structures in the footwall of the Brenner Line normal shear zone predominantly consist of closely spaced, steep, west down and east down microfaults. Formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear. In contrast, formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes. The field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed. However, both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion‐lined microfaults with the macroscopic west down and east down faults, obtaining density data for the inclusions, and correlating these data with previously obtained geochronologic data. The results indicate that the west down structures formed at depths of 10–20 km and temperatures>450°C in the mid to late Oligocene and that the east down structures formed at 2‐ to 10‐km depth and temperatures of 300 ± 50°C in the mid‐Miocene. These data support the hypothesis that a "rolling hinge" was present in the footwall of the Brenner Line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear. The depths at which west down and east down faulting occurred, coupled with the angle of dip of the Brenner Line, yield a minimum lateral displacement on the fault of 15–26 km. Approximately coeval ductile shearing and brittle faulting at depths of 15–20 km and temperatures in excess of 400°C may reflect local variations in strain rate as the footwall rocks entered the zone of rolling

ISSN:0278-7407    

DOI:10.1029/94TC03085

年代:1995

数据来源: WILEY

摘要:

The upper Pennine nappes in the western Alps are cut by structures that locally place European basement rocks over those of the Piemonte zone, which represent the sutured remains of Alpine ocean crust. In the Täschalp area of Switzerland, the Mischabel “backfold” and associated SE verging shearing carries gneisses of the Grand Saint Bernard nappe over eclogitic material of the Zermatt‐Saas zone. Greenschist‐facies assemblages are developed locally in the foot wall and hanging wall of the Mischabel structure as well as in the sheared rocks of the thrust zone. Rb‐Sr,40Ar/39Ar, and U‐Pb studies of white micas, amphiboles and sphenes show that greenschist assemblages developed at circa 44 Ma in the Zermatt‐Saas zone of the footwall and at circa 40 Ma in the thrust zone. Rb‐Sr data from the literature document muscovite and biotite ages in strongly foliated Grand Saint Bernard gneisses of the hanging wall of circa 36 and 32 Ma, respectively. These are thought to indicate that deformation on the Mischabel structure continued for at least 5 m.y. Rapid cooling inferred for the Zermatt‐Saas ophiolites in the mid‐Eocene may document either the tectonic emplacement of the ophiolite over the underlying Monte Rosa massif or their extensional unroofing. It is not possible to determine with certainty whether the Mischabel structure is extensional or contractional on a crustal scale. The age of cooling and deformation reported here show that it is not possible that the Mischabel structure was responsible for the exhumation of the high‐pressure assemblages in the Zermatt‐S

ISSN:0278-7407    

DOI:10.1029/94TC02017

年代:1995

数据来源: WILEY

摘要:

At the junction between an oceanic transform fault and the adjacent ridge segments, there is normally a curved fabric. The Tröllaskagi peninsula is located just south of the junction between the Kolbeinsey ridge and the Tjörnes fracture zone and offers a rare opportunity for studying the three‐dimensional tectonics of the associated curved fabric. For the purpose of elucidating its infrastructure, we measured the attitudes of tens of lava flows and several hundred dykes, faults, and mineral veins in 28 profiles in the northern half of the Tröllaskagi peninsula. At sea level the general dip of the lava pile is 6°–14°, but in the northernmost part of the peninsula the dip is 10°–36°, with an average of 22°. Most dykes are basaltic and strike north or NNE. They are exceptional as regards thickness, frequency of multiple dykes and crustal dilation. The average thickness, 5.9 m, and a crustal dilation of 28% in a 4.5‐km‐long profile are the highest figures reported from regional dyke swarms in Iceland. Most faults are normal faults. The curved fabric is limited to the northernmost part of the Tröllaskagi peninsula and consists of two main sets of fractures. One set includes north and NNE striking faults, mineral veins, and dykes. The other set consists of NW striking (oblique) faults, mineral veins, and tilted lava flows. A boundary element model indicates that the abnormal lava dip and the NW striking (oblique) extensional structures in the northernmost part of the peninsula are attributable to the stress field associated with the junction between the Kolbeinsey ridge and the Tjo

ISSN:0278-7407    

DOI:10.1029/94TC02904

年代:1995

数据来源: WILEY

摘要:

Mariscal Mountain anticline is a major Laramide asymmetric fold located along the frontal margin of the Cordilleran orogenic belt in the Trans‐Pecos region of southwest Texas. Exposed within the limbs and nose of the anticline are concordant mafic and felsic intrusions whose emplacement, based largely on their arcuate outcrop geometry, has been interpreted to predate folding. Paleomagnetic fold tests on samples from large (≥ 30 m thick) and thin (≤ l m thick) gabbro sills are negative at the 99% confidence level. Baked sandstones adjacent to the thick sill give sample directions identical to those of the gabbro, whereas sample directions from a site well removed from thermal effects of the gabbro are scattered. Results of the contact tests are consistent with a thermoremanent origin for the gabbro sill magnetization. The negative fold test indicates that sill intrusion at Mariscal Mountain anticline postdated Laramide folding. Two40Ar/39Ar analyses of plagioclase from the gabbro give discordant age spectra but yield an inverse correlation age of 37.0 ± 1.3 Ma (2σ). This date is similar to those from rocks of similar composition elsewhere in Trans‐Pecos Texas, but significantly younger than whole rock and pyroxene K‐Ar dates of Late Cretaceous and early Eocene age commonly cited for the Mariscal Mountain gabbro. The paleomagnetic and geochronologic data indicate that Laramide deformation had clearly ceased by late Eocene/early Oligocene time, consistent with absence of contractional deformation in middle Eocene to lower Oligocene volcanic and sedimentary rocks elsewhere in the region. Although paleomagnetic data from the gabbro sills may not adequately average secular variation, the in situ group mean direction (D= 348.6°,I= 50.9°,k= 65.1, α95= 4.3°,n= 18 sites) is indistinguishable from expected mid‐Tertiary reference directions. This indicates no evidence of significant vertical axis rotation in the Mariscal Mountain area. Analysis of paleomagnetic data from studies elsewhere in the Big Bend region indicates that significant problems exist with most data sets used to argue for the existence of local vertical axis rotations. Thus we conclude that existing paleomagnetic evidence regarding the timing and magnitude of rotations associated with Basin and Range exten

ISSN:0278-7407    

DOI:10.1029/94TC03089

年代:1995

数据来源: WILEY

摘要:

The Taiwan Slate Belt can be subdivided into two structural domains based on fold and cleavage geometries and finite and incremental strain histories. The two belts correspond generally to two topographic ranges in Taiwan: the Hsüehshan Range and the Backbone Range. Rocks in the Hsuehshan Range are characterized by upright, symmetric folds and a steeply SE dipping, axial planar pressure solution cleavage. Synkinematic fibers in pressure shadows are typically straight, parallel to cleavage, and plunge downdip, indicating that the rocks experienced a coaxial strain history. In contrast, rocks in the Backbone Range are characterized by inclined, asymmetric folds that verge toward the foreland and display a moderately SE dipping, axial planar pressure solution cleavage. Synkinematic fibers are straight and parallel to the downdip stretching lineation inXYsections. InXZsections, however, the fibers typically display a clockwise curvature (viewed to the NE), indicating a noncoaxial strain history. We propose that the different strain histories are due, at least in part, to differences in the original tectonostratigraphic settings of the strata in the two ranges. Strata in the Hsüehshan Range are interpreted to have been deposited in a half‐graben associated with an east dipping normal fault. The strata of the Backbone Range, in contrast, are considered to have been deposited on the unfaulted Chinese continental margin. We suggest that the basement horst on the east side of the half‐graben functioned as a steep‐faced, rigid backstop that concentrated pure shear deformation throughout most of the Hsüehshan Range; shear strains were apparently limited to zones near the base of the sedimentary sequence and adjacent to the backstop. The noncoaxial strains characteristic of the Backbone Range are interpreted to be the result of a regional‐scale shear zone that formed as Chinese basement and its sedimentary cover were underthrust southeastward beneath the

ISSN:0278-7407    

DOI:10.1029/94TC02451

年代:1995

数据来源: WILEY

摘要:

Cretaceous thrust structures are found along the front of the Verkhoyansk miogeoclinal fold belt along the eastern boundary of the Siberian platform in northeast Asia. The Verkhoyansk thrust front is subdivided into a number of segments, each of which has its own thrust system geometry. Balanced cross sections have been constructed for each segment on the basis of the structural study of surface geology and available seismic and drilling data. Distinctions between the segments are also expressed in gravity anomalies and modern topography. Analysis of vitrinite reflectance shows that folding of the Verkhoyansk thrust front was initiated during sedimentation as early as in the Late Jurassic. This period marks the beginning of the collision between the Siberian continent and the Kolyma‐Omolon superterrane, now located 500 km to the east of the Verkhoyansk thrust front. Deformation of the thrust front ended by the late Late Cretaceous; erosion of the frontal anticlines began in the early Late Cretaceous. The frontal thrust structures formed in the Late Cretaceous were rejuvenated during a middle to late Pleistocene reactivation, which produced the modern mountain topography. The least amount of erosion of the Verkhoyansk thrust front, 840 m, is observed in its central part, in the Kuranakh segment. To the north and south the erosion increases to 1500 m and 2100 m, respectively. The general configuration of the Verkhoyansk fold belt and its frontal structures are defined by the geometry of Devonian rift‐related structures on the eastern Siberian platform and the principal direction, approximately east–west, of Late Cretaceous compressional str

ISSN:0278-7407    

DOI:10.1029/94TC03088

年代:1995

数据来源: WILEY

摘要:

Three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of Nova Scotia, eastern Canada. The profiles cross the East Coast Magnetic Anomaly (ECMA), which parallels much of the margin of eastern North America south of the Grand Banks and which is usually associated with the transition from continental to oceanic crust. Studies to the south of the work reported here suggest that the ECMA may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high‐velocity lower crustal layer. The seismic data show that seaward dipping reflections (SDR) continue northward into the study area and support the correlation between the SDR unit and the presence of a well‐developed ECMA. Magnetic modeling confirms this association, although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture. Just north of the study area the ECMA diminishes and is no longer well developed. The SDR unit also terminates and it is not observed over most of the Nova Scotian margin. If our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north. The association of the transition with significant changes in the prerift fabric of the adjacent continental crust, in the trend of synrift extensional structures, and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced. We suggest that these clues are consistent with small‐scale convection as a mechanism for delivering large melt volumes to crustal depths during ri

ISSN:0278-7407    

DOI:10.1029/94TC03090

年代:1995

数据来源: WILEY