Search

[1] This study reassesses the stratigraphy, sedimentology, and provenance of the Indus Basin sedimentary rocks, deposited within the Indus Tsangpo Suture Zone (ITSZ) during the early phases of India-Eurasia collision. Using field observations, biostratigraphy, and petrographic and isotopic analyses we create a paleo-depositional reconstruction within the paleotectonic setting of the early phases of India-Eurasia collision. We then re-examine existing constraints to the timing of India-Eurasia collision previously interpreted from the earliest occurrence of mixed Indian- and Eurasian-derived detritus in the succession. From mid-Cretaceous to early Paleocene times the Jurutze and Sumda Formations were deposited within an arc-bounded marine basin between the Dras and Kohistan-Ladakh Island arcs. The

The Area Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends ~70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 [+ or -] 1.8 Ma and was intruded into the paleoproterozoic (1799 [+ or -] 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750[degrees]C and 0.7-0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at < 13.2 [+ or -] 1.4 Ma. High-grade metamorphism was followed by postkinematic leucogranite dyke emplacement at 11.6 [+ or -] 0.4 Ma. This integrated data set indicates that high-temperature metamorphism, decompression, and exhumation of the ADM postdates mid-Miocene south directed midcrustal extrusion and is kinematically linked to orogen-parallel extension.

The Insular composite terrane and the Intermontane terrane contract to reveal crustal reflectivity. The reflection images indicate an upper crustal reflectors and a lower crustal ramp with east dips. An early phase of contraction directed from the west caused the two-side Cretaceous Coast belt. Crustal boundinage causes the crust flanking in the east of the Coast Mountains of the Canadian Cordillera to possess a structural character. There is good correlation between structural belts and the crustal reflectivity traversing the southern Coast Mountains.

Images of structures at all levels of the southern Canadian Cordillera, provided by the lithoprobe deep seismic reflection data, reveals a complex and highly reflective crust. The area covered is the interval from the eastern most metamorphic core complexes near Arrow Lakes to the Fraser River. The reflection Moho is the base of the crustal reflectivity. The reflection data indicates that during Cenozoic extension, the Maho and lower crust acted as a zone of detachment. The three major crustal antiforms, namely, the Monashee complex, the Central Nicola horst and the Vernon antiform are also interpreted.

Research investigating the process of thickening of the geological crust of the Himalayan region of Nepal and its consequences is presented. Particular attention is given to the consequence of exhumation of the Central Nepal metamorphic core and to geochronological data.

New and published U-Pb geochronology and 40Ar/39Ar thermochronology from footwall and hanging wall rocks of a segment of the South Tibetan detachment system exposed in the Annapurna area of central Nepal Himalaya bring additional constraints on the timing of metamorphism, crustal thickening, and normal faulting resulting in exhumation of the Himalayan metamorphic core. Early Oligocene crustal thickening led to Eohimalayan kyanite-grade metamorphism between 35 Ma and 32 Ma. The resulting thermal event affected the Early Ordovician augen gneiss (Formation III) and produced kyanite-bearing leucosomes in the upper part of the metamorphic core. This event is linked with underthrusting of the Greater Himalayan metamorphic sequence below the Tethyan sedimentary sequence and the growth of an Oligocene fan structure that has thickened the Tethyan sedimentary sequence to 25 km, thus provoking kyanite-grade melting at deeper structural levels. Early Paleozoic monazite and zircon populations indicate that part of the metamorphism affecting the Himalayan metamorphic core could be pre-Cenozoic. Regional correlations indicate that the Annapurna detachment was active during early Miocene time. A weakly deformed leucogranitic dike intruding into the immediate hanging wall yielded reversely discordant monazite ages between 23 and 22.5 Ma, which suggest that the ductile strain in the Annapurna detachment zone terminated at ca. 22 Ma. On the basis of a 40Ar/39Ar muscovite age, renewed southwest verging deformation (D4) is interpreted to occur at ca. 18 Ma. Rapid exhumation resulting from extensional faulting cooled the entire metamorphic core through the muscovite Ar closure temperature (330°-430°C) between 15 and 13 Ma. Muscovites from the immediate hanging wall of the Annapurna detachment yielded slightly younger ages, between 13 and 11 Ma, testifying to late hydrothermal activity in the Annapurna detachment zone that could be linked with the initiation of brittle faulting associated with the late Neogene Thakkhola graben system. [ABSTRACT FROM AUTHOR]

Eocene extension contributed significantly to the present crustal architecture of the southern Omineca Belt in British Columbia and Washington. High grade gneiss complexes (Valhalla, Okanagan, Kettle-Grand Forks, Monashee, and Priest River) preserve Cretaceous to Eocene deformation superimposed on older structures and have Eocene biotite and muscovite cooling ages. They are juxtaposed by regionally extensive, low- and moderate-angle, ductile and/or brittle normal faults (Valkyr-Slocan Lake, Okanagan Valley, Kettle River, Granby-Greenwood, Columbia River, Standfast Creek (in part), and Newport and Purcell Trench faults) against metamorphosed rocks with a late Paleozoic to Middle Jurassic compressional tectonic history. Some upper plate rocks are overlain by Middle Eocene strata. Upper plate rocks preserve middle Cretaceous and older mica cooling dates indicating that they were less than 300°C in the Eocene, in contrast to lower plate rocks. The complexes have features in common with metamorphic core complexes of extensional origin elsewhere. U-Pb zircon and monazite dates on mylonitic granitic rocks in the footwalls of the Okanagan and Valkyr-Slocan Lake shear zones prove that a significant part of their ductile fabric is related to displacement on Eocene extensional faults. On the eastern side of the Monashee complex, 55 Ma U-Pb zircon and circa 54 Ma Rb-Sr synkinematic muscovite ages demonstrate that the ductile-brittle Columbia River fault is a predominantly Early Eocene normal fault. Contrary to previous interpretations, the circa 162 Ma Galena Bay stock does not intrude footwall mylonites, and therefore the interpretation that at least some of the mylonites are related to Eocene extension is permissible. The distribution of Eocene cooling ages implies that part of the Standfast Creek fault on the eastern boundary of the Clachnacudainn complex is a ductile (+/−brittle) normal fault. Analogous interpretations are made for the Kettle-Grand Forks and Priest River complexes where similar isotopic cooling age patterns prevail. Normal fault systems which bound the metamorphic complexes are fundamental crustal breaks, with displacements of 10-20 and in some cases 40 km, and probably accommodated about 30% extension across the 300 km width of the southern Omineca Belt. Most of the east dipping fault systems were active mainly between 58 and 52 Ma, in contrast to west dipping systems which are 52-45 Ma old, although both systems may have had some younger brittle displacement. Comparison of east-west cross sections with palinspastic restorations implies that the crust was more than 50 km thick prior to extension, that the high grade core complexes were not exposed to erosion prior to the Eocene, and that they were technically denuded and exhumed on Eocene normal fault systems. This extensional model is consistent with known geology, helps to explain several enigmatic geologic relationships, and has important implications for interpreting the pre-Eocene, compressional deformation in the region. [ABSTRACT FROM AUTHOR]

The Valhalla complex, a Cordilleran metamorphic core complex, is a 100 km by 30 km structural culmination within the Omineca belt of southeastern British Columbia. It comprises sheets of granitic orthogneiss ranging in age from 100 to 59 Ma with intervening paragneiss of uncertain age and stratigraphic correlation. The complex is roofed by the ductile Valkyr shear zone and the ductile/brittle Slocan Lake fault zone; the upper plate comprises lower grade metasedimentary rocks intruded by middle Jurassic plutons. The Valkyr shear zone and the Slocan Lake fault zone deform 62 and 59 Ma granitic sheets in their footwalls. The easterly directed Valkyr shear zone is a 2 to 3 km thick zone of distributed ductile strain which is arched over the complex and is exposed around the periphery on the northern, western, and southern margins. The shear zone was active between 59 and 54 Ma under amphibolite facies conditions. The juxtaposition of upper and lower plates with different structural and metamorphic histories indicates that the Valkyr shear zone is a significant structure with large displacement. There is evidence to support an easterly rooting direction consistent with an extensional origin; its surface breakaway is suggested to be west of the Valhalla complex. The Slocan Lake fault zone on the eastern side of the complex is a gently (30°), easterly dipping ductile/brittle normal fault which roots to the east. It was active between 54 and approximately 45 Ma and truncates the Valkyr shear zone. Timing and structural relationships indicate that the Valkyr shear zone and the Slocan Lake fault zone are genetically related. Movement on the ductile Valkyr shear zone, arching of the complex, and displacement on the Slocan Lake fault zone occurred as a continuum in Early to Middle Eocene time. This paper documents the presence of significant Eocene ductile strain in the Valhalla complex and suggests that the role of extension in this region is more profound than had been previously recognized. [ABSTRACT FROM AUTHOR]

The high relief of the Coast Mountains of British Columbia has been produced in part by rapid late Neogene uplift. The relationship of this uplift to the presently diverse and complex plate tectonic regime off the west coast of British Columbia, however, is not clear-cut. To study this problem, the areal patterns, rates, and total amounts of Cenozoic uplift from 50° to 55°N have been determined by using fission track dating of zircon and apatite separated from rocks collected at high and low altitudes along several traverses through the mountains. Accepting closure temperatures of 105°C ± 10°C and 175°C ± 20°C for apatite and zircon, respectively, and a likely closure temperature difference between zircon and apatite of 70° ± 15°C, paleogeothermal gradients have been measured at Kemano ( °C/km 35 Ma ago) and near Ocean Falls-King Island ( °C/km 20 Ma ago). Subcrustal heat flow is suggested to have significantly increased in the late Neogene near King Island. Uplift rates in the axial part of the northern (52°-55°N) Coast Mountains were 0.1-0.2 km/Ma from 25 to 15 Ma ago; this uplift was temporally and tectonically related to part of the subsidence in the adjacent Queen Charlotte basin. Uplift rates increased somewhat in the Late Miocene to about 0.4 km/Ma, probably due to passage of the Anahim 'hot spot' beneath the area. Late Miocene-Recent erosion in the north (52°-55°N) has reduced summit altitudes and been more extensive than farther south. Despite this, relicts of Miocene river valleys and topography are still locally preserved, partly beneath 7-10 Ma basalts. The area of the present southern (50° to 52°N) Coast Mountains was near sea level and experienced very low uplift rates (<0.1 km/Ma) throughout the middle Cenozoic (30-10 Ma), despite its more active volcanism. Late Miocene basaltic lavas that were erupted onto a mature erosion surface near Taseko Lakes were elevated and warped by rapid Pliocene-Pleistocene uplift (>0.5 km/Ma) of the southern Coast Mountains. The profile of this uplift was broadly plateaulike, and this uplift led to the inversion of the Miocene coastal topography. Middle Cenozoic uplift from 52° to 55°N was probably the result of denudation and diminishing uplift after the terminal Paleocene-Eocene orogenic episode which grossly thickened the crust. This middle Cenozoic erosion resulted in the restoration of a normal crustal thickness. Late Neogene uplift in the Coast Mountains was the result of thermal expansion in the mantle caused by a mantle hot spot near 53°N and changes in the geometry of the subducted slab farther south. [ABSTRACT FROM AUTHOR]