Mary S. Hubbard

Structural evolution and vorticity of flow during extrusion and exhumation of the Greater Himalayan Slab, Mount Everest Massif, Tibet/Nepal: implications for orogen-scale flow partitioning

Abstract The Greater Himalayan Slab (GHS) is composed of a north-dipping anatectic core, bounded above by the South Tibetan detachment system (STDS) and below by the Main Central thrust zone (MCTZ). Assuming simultaneous movement on the MCTZ and STDS, the GHS can be modelled as a southward-extruding wedge or channel. New insights into extrusion-related flow within the GHS emerge from detailed kinematic and vorticity analyses in the Everest region. At the highest structural levels, mean kinematic vorticity number (Wm) estimates of 0.74–0.91 (c. 45–287fb3e69cure shear) were obtained from sheared Tethyan limestone and marble from the Yellow Band on Mount Everest. Underlying amphibolite-facies schists and gneisses, exposed in Rongbuk valley, yield Wm estimates of 0.57–0.85 (c. 62–357fb3e69cure shear) and associated microstructures indicate that flow occurred at close to peak metamorphic conditions. Vorticity analysis becomes progressively more problematic as deformation temperatures increase towards the anatectic core. Within the MCTZ, rigid elongate garnet grains yield Wm estimates of 0.63–0.77 (c. 58–447fb3e69cure shear). We attribute flow partitioning in the GHS to spatial and temporal variations that resulted in the juxtaposition of amphibolite-facies rocks, which record early stages of extrusion, with greenschist to unmetamorphosed samples that record later stages of exhumation.

Metamorphic constraints on the thermal evolution of the central Himalayan Orogen. Discussion

Recent studies that integrate conventional thermobarometry of pelitic mineral assemblages with thermodynamic modeling of garnet zoning reveal complex Tertiary P -T paths for the Greater Himalayan metamorphic sequence in the central Himalaya. Viewed in light of our current understanding of the structural evolution of the Himalaya, these data provide insights into the relations between tectonic and thermal processes during orogenesis. In this paper, we present an interpretive model for tectonothermal evolution of the Greater Himalaya in the central part of the range. This model involves: (1) middle Eocene—early Oligocene burial to depths of more than 30 km during the early stages of collision between India and Asia; (2) early—late Oligocene uplift and cooling; (3) late Oligocene heating and renewed burial synchronous with the early stages of anatectic melting and leucogranite plutonism; (4) latest Oligocene-middle Miocene rapid uplift and continued leucogranite production associated with ramping on the structurally lower Main Central Thrust and tectonic denudation on structurally higher low-angle detachment systems; and (5) middle Miocene—Recent rapid cooling during the final stages of uplift to the surface.

Lateral displacement during Neogene convergence in the western and central Alps

Although major deformation in the western Alps is clearly the result of north-northwest-directed thrust tectonics, there is evidence suggesting that orogen-parallel deformation may have been important in the late tectonic history of the western Alps. The authors propose that this northeast-trending deformation includes (1) southwest-directed normal-fault movement along the Simplon line; (2) a diffuse zone of northeast-striking dextral strike-slip deformation along the Rhone Valley in Switzerland, between the Mont Blanc and Aiguilles Rouges massifs, and through the Belledonne massif; and (3) southwest-directed thrusting in the Embrunais-Ubaye and Digne nappe systems of southeastern France. The correlation of these three broad regions of deformation is based on similar amounts of minimum displacement, consistent kinematics, and timing of deformation.

Progressive localization of deformation during exhumation of a major strike-slip shear zone: Norumbega fault zone, south-central Maine, USA

Detailed structural studies along with 40Ar/39Ar thermochronology provide important constraints on the temporal,spatial and structural evolution of a major orogen-parallel strike-slip shear zone, the Norumbega fault zone in Maine. Detailed structural analysis in south-central Maine reveals two very different styles of dextral noncoaxial deformation, a wide zone (>25 km) of heterogeneously distributed ductile shear structures, and a relatively narrow zone (∼1 km) of lower greenschist facies high-strain mylonitization. 40Ar/39Ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following Middle Devonian amphibolite facies metamorphism. The wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature (post-amphibolite facies metamorphism but pre-regional cooling below 320°C) Late Devonian to Early Carboniferous transcurrent tectonism. The narrow zone of dextral shear represents both a younger (latest Carboniferous) and significantly cooler (<320°C) deformational event occurring at much higher structural levels. A general model of increasingly narrow, but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations.

Constraints on unroofing rates in the high Himalaya, eastern Nepal

Thermobarometric data for samples across the Main Central thrust zone in eastern Nepal show an inversion in temperature but not in pressure. These data have been interpreted to represent a portion of the paleogeotherm at the time of Main Central thrust deformation. A 40Ar/39Ar age on hornblende (closure temperature (Tc)=500±50°C) constrains the timing of this deformation to be ∼21±0.2 Ma. The 40Ar/39Ar ages of other minerals (muscovite, Tc=350°C, age (t)=12.0±0.2 Ma; K-feldspar, Tc=220°C, t=8.0±0.2 Ma) from the same location further constrain the cooling history of this region. Together the geochronologic and thermobarometric data yield an average unroofing rate of 1.2±0.6 mm/yr for the High Himalaya of eastern Nepal. Simple thermal models show that these geochronologic and thermobarometric data are consistent with a wide range of different initial geotherms, applied boundary conditions and magnitude of radiogenic heat production. The variation through time of the unroofing rates can only be poorly constrained, however. The unroofing histories were found to be largely insensitive to the details of the assumed initial geotherm, fairly sensitive to the magnitude of radiogenic heat production, and extremely sensitive to the nature of the boundary conditions applied below the fault zone. This study underscores the difficulty in constraining uplift histories on the basis of cooling rates even when thermobarometric data are available to supplement geochronologic constraints on the cooling history of the region.

Tectonic exhumation of the Nanga Parbat massif, northern Pakistan

Abstract Structural analysis in the Nanga Parbat region of northern Pakistan indicates that extensional deformation has been, in part, responsible for the exhumation of gneissic rocks of the Indian plate basement complex that dominates the Nanga Parbat massif. This massif has been mapped as a syntaxis at the boundary between the Indian and Asian plates in the northwest Himalaya and has received recent attention for evidence of rapid exhumation within the last 10 Ma. Field work along the Main Mantle Thrust (MMT) and within the Indian plate rocks in the area between Babusar Pass and Toshe Gali, southwest of the peak of Nanga Parbat, provided us with evidence from shear fabrics and a shallowly WSW-plunging stretching lineation for a dominant phase of extensional deformation. During this deformation rock of the Kohistan sequence and the Indian plate cover sequence moved to the WSW relative to rocks of the Indian plate basement including the Nanga Parbat gneiss. A post-metamorphic ductile shear fabric pervades the MMT contact, the Indian plate cover sequence below the MMT, but decreases in intensity in the Indian plate basement and the Nanga Parbat gneiss. Using 39 Ar/ 40 Ar geochronology of hornblendes we have constrained the age of pre-deformational, amphibolite facies metamorphism to be ≤ 40–56 Ma. Published cooling ages from the Babusar Pass region further document that the extensional deformation was completed by ∼ 20 Ma. We interpret this ductile, extensional shear as the mechanism responsible for significant tectonic exhumation of the gneissic and granitic rocks of the Nanga Parbat massif.

Great Plains Workshop held to prepare for USArray deployment

Relative to most parts of North America, the Great Plains region, which is bordered by the Rocky Mountain Front on the west and the Mississippi River on the east, has been understudied in terms of the structure, formation, and evolution of the underlying crust, mantle, and core. The anticipated arrival of the USArray portable seismic stations, which will cover the entire United States regardless of surface geology and tectonic activities, and the deployment of the accompanying flexible array stations and the permanent seismic stations in this area, will fill this gap and address numerous problems related to the structure and dynamics of the Earth. Detailed information about USArraycan be found at http://www.earthscope.org/usarray/. To maximize the effectiveness of the upcoming USArray, formulate cooperative studies, and identify geologic targets for detailed studies using the flexible array stations of USArray, a pre-EarthScope Great Plains workshop was recently hosted by Kansas State Universitys Department of Geology The workshop brought together about 40 geoscientists with interests ranging from surface processes to mantle dynamics, from about 25 institutions. Participants discussed scientific objectives related to USArrays Great Plains coverage, with an emphasis on future collaborations to maximize our understanding of the geology of the Great Plains region, from the Earths surface to the core-mantle boundary. This will lead to a better understanding of the geologic development of cratonic regions, and provide valuable data for integrated studies of continental lithosphere and deep Earth structure over a wide range of scales.