Magmatic rocks and depositional setting of associated volcaniclastic strata along a north‐south traverse spanning the southern Tien Shan and eastern Pamirs of Kyrgyzstan and Tajikistan constrain the ...tectonics of the Pamirs and Tibet. The northern Pamirs and northwestern Tibet contain the north facing Kunlun suture, the south facing Jinsha suture, and the intervening Carboniferous to Triassic Karakul–Mazar subduction accretion system; the latter is correlated with the Songpan‐Garze–Hoh Xi system of Tibet. The Kunlun arc is a composite early Paleozoic to late Paleozoic‐Triassic arc. Arc formation in the Pamirs is characterized by ∼370–320 Ma volcanism that probably continued until the Triassic. The cryptic Tanymas suture of the southern northern Pamirs is part of the Jinsha suture. A massive ∼≤227 Ma batholith stitches the Karakul–Mazar complex in the Pamirs. There are striking similarities between the Qiangtang block in the Pamirs and Tibet. Like Tibet, the regional structure of the Pamirs is an anticlinorium that includes the Muskol and Sares domes. Like Tibet, the metamorphic rocks in these domes are equivalents to the Karakul–Mazar–Songpan‐Garze system. Granitoids intruding the Qiangtang block yield ∼200–230 Ma ages in the Pamirs and in central Tibet. The stratigraphy of the eastern Pshart area in the Pamirs is similar to the Bangong‐Nujiang suture zone in the Amdo region of eastern central Tibet, but a Triassic ocean basin sequence is preserved in the Pamirs. Arc‐type granitoids that intruded into the eastern Pshart oceanic‐basin–arc sequence (∼190–160 Ma) and granitoids that cut the southern Qiangtang block (∼170–160 Ma) constitute the Rushan‐Pshart arc. Cretaceous plutons that intruded the central and southern Pamirs record a long‐lasting magmatic history. Their zircons and those from late Miocene xenoliths show that the most distinct magmatic events were Cambro‐Ordovician (∼410–575 Ma), Triassic (∼210–250 Ma; likely due to subduction along the Jinsha suture), Middle Jurassic (∼147–195 Ma; subduction along Rushan‐Pshart suture), and mainly Cretaceous. Middle and Late Cretaceous magmatism may reflect arc activity in Asia prior to the accretion of the Karakoram block and flat‐slab subduction along the Shyok suture north of the Kohistan‐Ladakh arc, respectively. Before India and Asia collided, the Pamir region from the Indus‐Yarlung to the Jinsha suture was an Andean‐style plate margin. Our analysis suggests a relatively simple crustal structure for the Pamirs and Tibet. From the Kunlun arc in the north to the southern Qiangtang block in the south the Pamirs and Tibet likely have a dominantly sedimentary crust, characterized by Karakul–Mazar–Songpan‐Garze accretionary wedge rocks. The crust south of the southern Qiangtang block is likely of granodioritic composition, reflecting long‐lived subduction, arc formation, and Cretaceous‐Cenozoic underthrusting.
A 20-km-long seismic line characterises the crustal reflection pattern of the easternmost Dabie Shan, the archetypal ultrahigh-pressure (UHP) orogen of eastern China. The weak- to non-reflective ...upper crust (5
s two-way travel time (TWT); ∼15
km depth) is interpreted to comprise UHP rocks thrust over lithologically similar but non-UHP crust. The tectonic contact, although not imaged as a distinct reflector, is probably outlined by the rather abrupt change to diffuse but strong reflectivity within the mid to lower crust. Thus, the seismic pattern of the upper crust implies that mafic, oceanic crust does not constitute a significant proportion. The middle to lower crust (5–10
s TWT; ∼15–33
km depth) probably represents cratonal Yangtze basement, unaffected by the UHP metamorphism. The prominent lowermost reflectors (10–12
s TWT; ∼33–40
km depth) are interpreted to trace the Moho, excluding the presence of a crustal root inherited from the UHP orogeny. A tomographic P-wave velocity model for the uppermost crust (<700
m) traces shallowly W-dipping sedimentary rocks east and UHP gneisses west of the Cenozoic Tan Lu fault which is imaged to dip steeply eastward. The UHP rocks exhibit little lateral and vertical velocity variations (<10%), reflecting grossly homogeneous, gneissic lithology. Hundred-metre-scale velocity variations, however, may trace distinct large-scale structures, e.g. folds, known from outcrops and maps.
The largest tract of ultrahigh‐pressure rocks, the Dabie‐Hong'an area of China, was exhumed from 125 km depth by a combination of normal‐sense shear from beneath the hanging wall Sino‐Korean craton, ...southeastward thrusting onto the footwall Yangtze craton, and orogen‐parallel eastward extrusion. Prior to exhumation the UHP slab extended into the mantle a downdip distance of 125–200 km at its eastern end, whereas it was subducted perhaps only 20–30 km at its far western end ∼200 km away. Structural reconstructions imply that the slab was >10 km thick. U/Pb zircon and 40Ar/39Ar geochronology indicate that exhumation up to crustal depths occurred diachronously between 240 and ∼225–210 Ma, reflecting a vertical exhumation rate of >2 mm/yr. The upper boundary of the slab is the Huwan shear zone, a normal‐sense detachment that reactivated the plate suture. The lower boundary is represented by the Lower Yangtze fold‐thrust belt. NW‐trending stretching lineations, NE‐vergent, WNW‐ESE trending folds, dominant top‐NW shear, and conjugate, but overall asymmetric, shear band fabrics, document that exhumation was accomplished by updip and orogen‐parallel extrusion accompanied by layer‐parallel thinning. The orientation and shape of the folds, and a change from SE to SW flow directions, imply that the slab rotated clockwise about a western pivot during exhumation; this rotation was likely caused by the eastward increasing depth of subduction mentioned above, combined with a possible marginal basin and a weak eastern plate boundary. Exhumation of the slab produced considerable shortening in the Lower Yangtze fold‐thrust belt, perhaps producing the foreland orocline.
Xenoliths of subducted crustal origin hosted by Miocene ultrapotassic igneous rocks in the southern Pamir provide important new information regarding the geological processes accompanying tectonism ...during the Indo-Eurasian collision. Four types have been studied: sanidine eclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite), felsic granulites (garnet, quartz, sanidine and kyanite), basaltic eclogites (omphacite and garnet), and a glimmerite (biotite, clinopyroxene and sanidine). Apatite, rutile and carbonate are the most abundant minor phases. Hydrous phases (biotite and phengite in felsic granulites and basaltic eclogites, amphiboles in mafic and sanidine eclogites) and plagioclase form minor inclusions in garnet or kyanite. Solid-phase thermobarometry reveals recrystallization at mainly ultrahigh temperatures of 1000–1100°C and near-ultrahigh pressures of 2·5–2·8 GPa. Textures, parageneses and mineral compositions suggest derivation of the xenoliths from subducted basaltic, tonalitic and pelitic crust that experienced high-pressure dehydration melting, K-rich metasomatism, and solid-state re-equilibration. The timing of these processes is constrained by zircon ages from the xenoliths and 40Ar/39Ar ages of the host volcanic rocks to 57–11 Ma. These xenoliths reveal that deeply subducted crust may undergo extensive dehydration-driven partial melting, density-driven differentiation and disaggregation, and sequestration within the mantle. These processes may also contribute to the alkaline volcanism observed in continent-collision zones.
New SHRIMP and TIMS zircon ages,
40Ar/
39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze ...craton that was subducted to ultrahigh pressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex, intruded over a 10-Myr interval between 137 and 126 Ma, that accommodated ∼100% N–S stretching of the pre-existing collisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishable zircon populations. The population of Triassic zircon ages ranges from ∼219 to ∼245 Ma, leading us to question the prevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation.
Tectonic models for the evolution of the Tibetan plateau interpret observed east-west thinning of the upper crust to be the result of either increased potential energy of elevated crust or geodynamic ...processes that may be unrelated to plateau formation. A key piece of information needed to evaluate these models is the timing of deformation within the plateau. The onset of normal faulting has been estimated to have commenced in southern Tibet between about 14 Myr ago and about 8 Myr ago and, in central Tibet, about 4 Myr ago. Here, however, we report a minimum age of approximately 13.5 Myr for the onset of graben formation in central Tibet, based on mineralization ages determined with Rb-Sr and 40Ar-39Ar data that post-date a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, support models that relate normal faulting to processes occurring beneath the plateau. Thinning of the upper crust is most plausibly the result of potential-energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath Tibet. This is supported by recent geophysical and geological data, which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle.
Analysis of brittle microstructures reveals three Tertiary deformation events in the Western Carpathians: (1) Late Paleogene bedding‐parallel extension; (2) Oligocene (?) to middle Miocene NNE‐SSW ...compression and ESE‐WNW extension; (3) post‐mid‐Miocene NW‐SE extension. These regional deformations, applied to crustal blocks in the intra‐Carpathian region, resulted from forces which were induced by the coupled plate tectonic processes of subduction retreat beneath the Carpathian arc and lateral extrusion from the Eastern Alps toward the Carpathian region. Material flow from the Alps spread toward east and NE, guided by sinistral strike‐slip zones along the NE trending continental margin. Subduction retreat was terminated by mid‐Miocene “soft” collision of the Inner Western Carpathians with the European foreland. Ongoing subduction retreat beneath the Eastern Carpathians caused post‐mid‐Miocene back arc extension forming the Pannonian basin. Extension spread into the Western Carpathians by reactivating the extrusion‐related strike‐slip faults as normal faults.
Recoiling daughters of
α
-decaying U and Th impurities in mica and other minerals produce localised lattice damage: alpha-recoil tracks. The age of a sample can be calculated from the number of ...tracks per unit volume
(
N
ART
)
. To this end, the mica is etched and the etch pits at the sites of recoil-tracks are counted under the optical microscope. Because the measured track densities increase with etching time,
N
ART
is calculated from the fitted regression line. A number of problems inherent in this approach are overcome by the etch–anneal–etch and the mirror-image methods. The track densities determined with these methods are independent of etching time. Although both methods need improvement, they hold the potential of a precise determination of
N
ART
from a single measurement in future recoil-track dating.
Hot and Dry Deep Crustal Xenoliths from Tibet Hacker, Bradley R.; Gnos, Edwin; Ratschbacher, Lothar ...
Science (American Association for the Advancement of Science),
03/2000, Letnik:
287, Številka:
5462
Journal Article
Recenzirano
Anhydrous metasedimentary and mafic xenoliths entrained in 3-million-year-old shoshonitic lavas of the central Tibetan Plateau record a thermal gradient reaching about 800° to 1000°C at a depth of 30 ...to 50 kilometers; just before extraction, these same xenoliths were heated as much as 200°C. Although these rocks show that the central Tibetan crust is hot enough to cause even dehydration melting of mica, the absence of hydrous minerals, and the match of our calculated P-wave speeds and Poisson's ratios with seismological observations, argue against the presence of widespread crustal melting.
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