Dawn A. Kellett

Miocene structural reorganization of the South Tibetan detachment, eastern Himalaya: Implications for continental collision

In the eastern Himalaya (Bhutan), there are two distinct top-down-to-the-north segments of the South Tibetan detachment system. The outer segment is a diffuse ductile shear zone preserved as klippen in broad open synforms. New age constraints show that it was active until at least ca. 15.5 Ma and cooled by ca. 11.0 Ma, as constrained by sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology of magmatic zircon and 40 Ar/ 39 Ar thermochronology of muscovite in ductilely deformed leucogranite sills. The inner segment is a ductile shear zone active at least until ca. 11.0 Ma (constrained by SHRIMP U-Pb geochronology of magmatic zircon) and overprinted by more recent brittle faulting. These age constraints indicate that ductile deformation continued on the South Tibetan detachment more recently in the eastern Himalaya than in central and western parts of the orogen. These improved constraints on timing of South Tibetan detachment segments allow for a more detailed reconstruction of continental collision in the eastern Himalaya in which the outer South Tibetan detachment segment was abandoned in the mid-Miocene and passively transported southward in the hanging wall of the Main Himalayan thrust (the basal detachment of the orogen), while top-to-the-north ductile to brittle shearing continued on the inner South Tibetan detachment segment. Hinterland stepping of the South Tibetan detachment to maintain an orogenic critical taper (frictional wedge model) is a possible mechanism for this tectonic reorganization of the South Tibetan detachment during the Miocene. However, our data combined with published geochronologic data for the eastern Himalaya demonstrate that foreland translation and exhumation of a midcrustal dome (viscous wedge model) is the more tenable mechanism.

New insight into the South Tibetan detachment system: Not a single progressive deformation

[1] Low-angle normal faults (LANF), typically regarded as accommodating crustal or lithospheric extension, may also form during lithospheric shortening. The best-studied system of syn-contractional LANFs is the South Tibetan detachment system, a network of low-angle normal sense faults and shear zones that formed coevally with and parallel to south-vergent thrusts during lithospheric shortening accompanying development of the Himalayan orogen. In the eastern Himalaya, there are several across-strike exposures of the South Tibetan detachment system. We present new structural and thermometry data from the eastern Himalaya that demonstrate that the South Tibetan detachment system cannot have formed as a single progressive structure. We characterize and distinguish two distinct structural and tectonic components within the currently recognized system: (1) an extensive diffuse, sheared layer that formed the boundary between strong upper crust and weak, southward-flowing middle crust, and (2) a network of brittle-ductile LANFs that locally exhume, partly excise and overprint the earlier mylonite zone at the topographic break between the Himalayan orogen and the Tibetan plateau. The sheared layer, not a LANF, formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the Himalaya at the base of klippen of upper crustal rocks preserved in Bhutan, along the crest of the Himalaya where it has been excised and exhumed by the brittle-ductile extrusion LANFs, and bounding the cores of the North Himalayan gneiss domes. Citation: Kellett, D. A., and D. Grujic (2012), New insight into the South Tibetan detachment system: Not a single progressive deformation, Tectonics, 31, TC2007, doi:10.1029/2011TC002957.

Pre-Miocene deformation of the Himalayan superstructure, Hidden valley, central Nepal

Abstract: Throughout the Himalaya, the Tethyan sedimentary sequence forms the detached carapace, or superstructure, to exhumed mid-crustal rocks. In central Nepal, low metamorphic-grade Cambrian to Jurassic rocks of this carapace preserve five phases of deformation, structurally dominated by second phase (D2) north-verging back folds that oppose the vergence of the orogen. The folds are overprinted and cross-cut by the c. 22 Ma South Tibetan detachment system and the Manaslu leucogranite. New structural mapping in Hidden valley provides further constraints on the character of D2: megascopically, large asymmetric north-verging folds define D2; microscopically, it is characterized by an axial-planar cleavage. Balanced cross-sections and bed-length restoration of F2 folds indicate a minimum of 32–38% shortening and 180% thickening during D2. These data indicate that north-verging folds played a significant role in pre-Miocene crustal thickening of the Himalayan superstructure. The formation of these folds is compatible with wedge extrusion or channel flow of the underlying mid-crustal rocks, whereby their geometry reflects early coupling between the upper and middle crust.

Eocene deep crust at Ama Drime, Tibet: Early evolution of the Himalayan orogen

Granulitized eclogite-facies rocks exposed in the Ama Drime Massif, south Tibet, were dated by Lu-Hf garnet geochronology. Garnet from the three samples analyzed yielded Lu-Hf ages of 37.5 ± 0.8 Ma, 36.0 ± 1.9 Ma, and 33.9 ± 0.8 Ma. Eclogitic garnet growth is estimated at ca. 38 Ma, the oldest age for burial of the lower Indian crust beneath Tibet reported from the central-eastern Himalaya. Granulite-facies overprinting followed at ca. 15–13 Ma, as indicated by U-Pb zircon ages. Unlike ultrahigh-pressure eclogites of the northwest Himalaya, the Ama Drime eclogites are not characteristic of rapid burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having already reached at least ∼60 km thickness by the late Eocene.

Direct shear fabric dating constrains early Oligocene onset of the South Tibetan detachment in the western Nepal Himalaya

A newly identified and dated segment of the South Tibetan detachment in the Karnali klippe, western Nepal Himalaya, constrains initiation of mid-crustal tectonically driven exhumation to the early Oligocene. The folded top-to-the-northeast high-temperature (∼600 °C) shear zone separates amphibolite-facies rocks with a ca. 36–30 Ma prograde metamorphic history in the footwall from weakly to non-metamorphosed upper crustal rocks in the hanging wall. In situ dating of syn-kinematic–post-metamorphic peak monazite indicates that the base of the shear zone was active from ca. 30–29 to <24 Ma, and a post-deformation muscovite cooling age implies that ductile shearing had ceased by ca. 19 Ma. Deformation along the South Tibetan detachment in western Nepal was thus synchronous with thrust-sense shearing along the lower boundary of a zone of migmatitic rocks, compatible with tectonic models involving mid-crustal channelized flow during the Oligocene. Along with other published data from the Himalayan range, this suggests that the South Tibetan detachment actively exhumed the middle crust for almost 20 m.y.

Diachronous deformation along the base of the Himalayan metamorphic core, west-central Nepal

Geologic mapping combined with microstructural and geochronologic analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style despite variability in the timing of metamorphism and deformation. The Main Central thrust zone at the base of the Himalayan metamorphic core varies in thickness, tectonostratigraphy, and metamorphic gradient along the 250 km of strike length studied. In situ U-Th/Pb geochronology of monazite sampled from an along-strike transect at the top of the high-strain zone records Eocene−Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The timing of prograde and retrograde metamorphism and the muscovite 40 Ar/ 39 Ar dates gradually decrease along strike from northwest to southeast. This age trend is punctuated by an abrupt ∼3−8 m.y. decrease in the age of prograde and retrograde metamorphism and muscovite 40 Ar/ 39 Ar dates near the Marsyandi River in central Nepal. Quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high-strain zone document similar flow style at ∼440 °C in central Nepal. Muscovite 40 Ar/ 39 Ar ages, interpreted to approximate the age of deformation at this structural level, decrease from ca. 7 to 4 Ma along strike from northwest to southeast. Diachronous deformation and metamorphism along strike in west-central Nepal demonstrate the necessity of incorporating more than a single transect into tectonic models. Along-strike tectonometamorphic variability in west-central Nepal spatially corresponds to faults in the Indian basement bounding the subsurface Faizabad ridge, highlighting the possible influence of inherited basement faults on the geometry of the basal Himalayan detachment, the Main Himalayan thrust, as well as the tectonic evolution of the structurally overlying Himalayan metamorphic core. This study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along-strike diachroneity of deformation in the Himalaya.

The age of salinic deformation constrained by 40Ar/39Ar dating of multiple cleavage domains: Bathurst Supergroup, New Brunswick Appalachians

In the New Brunswick Appalachians, polydeformed felsic volcanic rocks of the Bathurst Supergroup record four cleavage-forming tectonic events, of which D1 and D2 record subduction-related underplating of buoyant elements of the Tetagouche backarc basin and subsequent collision between composite Laurentia and the Gander margin, respectively, during the Salinic orogenic cycle. Here we present an integrated approach to dating multiple cleavage domains in which we performed step heat 40Ar/39Ar analyses, microstructural observations and mineral-chemistry analysis on a suite of samples, as well as in situ 40Ar/39Ar analysis on two samples from the suite with clear S1 and S2 cleavage relationships. We use this dataset to characterize the complex relationships between S1 and S2 white mica between samples at different structural settings across the supergroup. We refine the timing of S1 white mica growth, and hence M1-D1 of the Salinic cycle in the Bathurst Supergroup to ca. 452 to 437 Ma, and S2 white mica growth (also D2) to ca. 427 to 418 Ma. New and published data indicate local thermal resetting of white mica at ca. 411 Ma, which we interpret to indicate a buried intrusion, likely a component of the Central plutonic belt. Delineation of the two white mica age components, particularly within a single sample, was informed by both spatially-controlled analysis (in situ 40Ar/39Ar), and the higher age precision of step heat 40Ar/39Ar analyses.

Northern provenance of the Gondwana Formation in the Lesser Himalayan Sequence: constraints from 40Ar/39Ar dating of detrital muscovite in Darjeeling-Sikkim Himalaya

Single-grain 40Ar/39Ar dates are reported for detrital white mica from low-grade meta-arkose samples from the Gondwana Formation in the Darjeeling-Sikkim Himalaya. The majority (61%) of single grains from five samples yielded dates of ~480 Ma. The remaining grains yielded dates between ~590 and 1700 Ma, and a few grains were younger than 480 Ma. These data suggest that the source of the Gondwana sedimentary rocks in the Himalaya lay to the north, in contrast to the previously held view that it was in the south. This also indicates that the Cambro-Ordovician granites, extensively present in the Himalayan metamorphic core, the Greater Himalayan Sequence, were exposed at the surface in the Permian in a mountain range to the north of the Permian continental rifts. Preservation of the original muscovite ages and the absence of evidence of a thermal overprint during the Himalayan orogenesis indicate that the burial temperatures of the Gondwanan sedimentary rocks, located at the base of the Lesser Himalayan Sequence, did not exceed 400 °C.

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe, west Nepal

Funding information Division of Earth Sciences, Grant/Award Number: EAR-1119380; Ontario Council on Graduate Studies, Council of Ontario Universities, Grant/Award Number: Ontario Graduate Scholarship; Geological Society of America, Grant/Award Number: Graduate Student Research Grant; Natural Sciences and Engineering Research Council of Canada, Grant/Award Number: Alexander Graham Bell Canada Graduate Scholarship, Discovery Grant; National Science Foundation, Grant/Award Number: EAR-1119380