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Describes the geologic relationships and lithologic units of southwestern and southeastern Tibet, China using field mapping. Two major episodes of Tertiary crustal shortening along the Indus-Tsangpo suture in the Yalu River valley in Tibet; Correlation of the structures in Tibet's Kailas and Zedong areas; Assessment of the creation of the thrust system.

Studies the relationship between leucogranites and the Qomolangma detachment in the Rongbuk Valley, south Tibet. Timing and slip and magnitude of displacement on the primary fault systems that bound the range; Age of the Rongbuk granite; Geologic sketch map of Himalaya; Alternating periods of shortening and extension in the Himalaya since the early Miocene.

Focuses the use of the ion microprobe in detecting age differences in lead and thorium isotopes in rocks. Analysis and profiling age depth of thorium and lead in monazites; Obtaining continuous, high-temperature thermal history information.

States that the two parallel belts of Miocene granite along the Himalaya, differ in age, petrogenesis, and emplacement style. Test suggestion by the authors; Details on the geologic petrologic framework; Use of a thermal model; Results and discussion on the testing.

The geologic map pattern of the Qiangtang terrane in central Tibet defines a >600-km-long and up to 270-km-wide cast-plunging structural culmination. It is characterized by early Mesozoic blueschist-bearing mélange and upper Paleozoic strata in the core and mainly Triassic-Jurassic strata along the limbs. In the western Qiangtang terrane (∼84°E), the culmination is unconformably overlain by weakly deformed mid-Cretaceous volcanic flows and tuffs. Along the Bangong suture to the south (32°N, 84°E), mid-Cretaceous nonmarine red beds and volcanic rocks lie unconformably on Jurassic suture zone rocks. These relationships demonstrate that west-central Tibet was above sea level during the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous time. Growth of the Qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early Mesozoic mélange and upper Paleozoic strata during the Early Cretaceous Lhasa-Qiangtang collision. The north-south width of the inferred thrust sheet provides a minimum slip estimate of ∼150 km at 84°E, decreasing eastward to ∼70 km at 87°E. Paleogene deformation in the Qiangtang terrane is characterized by widely distributed, mainly north-dipping thrust faults that cut Eocene-Oligocene red beds and volcanic rocks in their footwall. Along the Bangong suture, the north-dipping Shiquanhe-Gaize-Amdo thrust system cuts 64 and 43 m.y. old volcanic tuffs in its footwall and accommodated >40 km of post-mid-Cretaceous shortening. The Tertiary south-dipping Gaize-Siling Co backthrust bounds the southern margin of the Bangong suture and marks the northernmost limit of mid-Cretaceous marine strata in central Tibet Cretaceous deformation and denudation in central Tibet is attributed to northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture. This model implies that (1) Cretaceous strata along the Bangong suture and in the northern Lhasa terrane were deposited in a flex ural foreland basin system and derived at least in part from the Qiangtang terrane, and (2) the central Tibetan crust was thickened substantially prior to the Indo-Asian collision. Although its magnitude is poorly known, Tertiary shortening in the Qiangtang terrane is more prevalent than in the Lhasa terrane; this difference may be attributed to the presence of underthrust mélange in the deeper central Tibetan crust, which would have made it weaker than the Lhasa terrane during the Indo-Asian collision. [ABSTRACT FROM AUTHOR]

Presents a study on the zoning and age patterns of zircons from three samples from the eastern Blue Ridge near the Georgia-North Carolina border. Why each sample was selected; Geologic setting of the southern Appalachian Blue Ridge province; What the backscattered-electron (BSE) zoning patterns of almost all grains indicate.

Tectonic deformation can influence spatiotemporal patterns of erosion by changing both base level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, we quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. Our work shows a twofold increase in erosion coefficients between basins within 15 km of major faults compared to those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. This work demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models. [ABSTRACT FROM AUTHOR]

Zircons entrained in mantle-derived magmas offer a unique opportunity to identify cryptic magmatic episodes in the deep crust and thus to image lithospheric thickening and crustal evolution. We investigated zircon xenocrysts from mantle-derived ultrapotassic rocks in southern Tibet to evaluate their potential as a probe of crustal evolution. Similar age (Proterozoic-Paleozoic) distributions of these zircons and those in the Lhasa terrane detrital spectra demonstrate the continental origin of xenocrysts with high U/Yb. Time-progressive variations in zircon εHf(t) reveal three major magmatic pulses ca. 90, 50, and 20 Ma, suggesting significant crustal growth in the Lhasa terrane at those times. This is consistent with major mantle inputs previously documented from surface rocks in the Lhasa terrane. Increasing DyN/YbN and U/Yb since ca. 55 Ma are interpreted to reflect progressive crustal thickening in response to the India-Asia convergence. Zircon xenocrysts with varying U-Pb ages and heterogeneous Hf isotopes indicate assimilation of Lhasa terrane crust in the genesis of ultrapotassic magmas. [ABSTRACT FROM AUTHOR]

Detailed field mapping, geochronologic and thermochronologic analyses, and petrologic investigations conducted along the southern segment of the late Cenozoic Kongur Shan extensional system provide new information on the Cenozoic tectonic evolution of the eastern Pamir at the western end of the Himalayan-Tibetan orogen. Field relations and cooling-age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of eastwest extension along the southern Kongur Shan extensional system, from 20 km or less along the Muztaghata massif in the north, to <3 km along the Tashkorgan fault in the south. These results, in conjunction with previously published work on the northern segment of the Kongur Shan extensional system, show a general southward decrease in east-west extension along the entire length of the extensional system, consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the Pamir. Petrologic data, 40Ar/39Ar cooling ages, and monazite Th-Pb ages from schists and gneisses in the footwall of the southern Kongur Shan normal fault along the Muztaghata massif record two tectonic events that immediately preceded late Miocene initiation of east-west extension: (1) high-grade schists and gneisses experienced upper amphibolite facies metamorphic conditions (9--10 kbar, 700--750 °C) dated as late Oligocene to middle Miocene by in situ ion-microprobe analyses of monazite inclusions in garnet; and (2) high-grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late Miocene with 40Ar/39Ar biotite cooling ages of 7.5--9 Ma. Rapid exhumation was accommodated in part by the east-west--striking, south-dipping, Shen-ti normal fault. Field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the Central Pamir gneiss domes, and the antiformal Muztaghata massif is the eastward continuation of the Sares dome of the Central Pamir. These observations suggest that the antiformal gneiss domes of the Central Pamir have not been offset across the Karakorum rightslip fault from the Qiangtang anticlinorium in Tibet. Instead, we propose that the development of the Central Pamir gneiss domes may have been related to Oligocene-Miocene northward underthrusting and thickening of crust beneath the Pamir. [ABSTRACT FROM AUTHOR]