Kinematics and Vorticity in Kangmar Dome, Southern Tibet: Testing Midcrustal Channel-flow Models for the Himalaya

Kinematic, kinematic vorticity (Wm), and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the Kangmar gneiss dome, southern Tibet record ductile deformation patterns of a “frozen” segment of a southward flowing midcrustal channel. Microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top-north and top-south shear in garnet zone rocks to dominantly top-north shear in staurolite/kyanite zone and deeper rocks. Kinematic vorticity values indicate an increase in pure shear component with depth from ∼48% pure shear in chloritoid zone rocks through ∼62% in garnet zone to staurolite/kyanite zone rocks to ∼68% pure shear in an orthogneiss, the deepest exposed rocks. Deformation temperatures inferred from grain-scale microstructures and quartz lattice preferred orientations increase from ∼300°C–400°C in chloritoid zone rocks to ≥600°C in the deepest exposed rocks. These temperatures are equivalent to temperatures derived from garnet-biotite thermobarometry, indicating that Wm was recorded during peak metamorphism. This ductile deformation zone was cut by the brittle southern Tibetan detachment system (STDS) that juxtaposed metasedimentary rocks upon the orthogneiss. On the basis of these relations, midcrustal rocks in the core of Kangmar Dome record: (1) general shear (vertical thinning and N–S horizontal extension) with a component of top-north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the STDS, (2) an increase in pure shear with structural depth, a consequence of an increase in lithostatic load, and (3) displacement of the high-temperature shear zone by the brittle STDS. Our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel

Size and Exhumation Rate of Ultrahigh-Pressure Terranes Linked to Orogenic Stage

A growing set of data indicates a stark contrast between the evolution of two types of ultrahigh-pressure (UHP) terranes: large terranes that evolved slowly (over 10–30 Myr), and small terranes that formed and were exhumed on timescales of \u3c 10 Myr. Here we compare the characteristics – area, thickness, formation rate, exhumation rate, age, and tectonic setting – of these two endmember types of UHP terrane worldwide. We suggest that the two UHP terrane types may form during different orogenic stages because of variations in the buoyancy and traction forces due to different proportions of subducting crust and mantle lithosphere or to different rates of subduction. The initial stages of continent collision involve the subduction of thin continental crust or microcontinents, and thus tectonic forces are dominated by the density of the oceanic slab; subduction rates are rapid and subduction angles are initially steep. However, as collision matures, thicker and larger pieces of continental material are subducted, and the positive buoyancy of the down-going slab becomes more prominent; subduction angles become gentle and convergence slows. Assessing the validity of this hypothesis is critical to understanding the physical and chemical evolution of Earth\u27s crust and mantle. Included here is the post-print copy of this article. The final publication is available via ScienceDirect at http://www.sciencedirect.com/science/article/pii/S0012821X1100756

Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

Seismic anisotropy as a constraint on composition in the lower crust

Our current interpretation of the composition of the middle and lower crust comes mainly from seismic observations, yet it remains a challenge to link seismic observations directly to composition. This is because isotropic seismic properties are similar across a range of compositions. Taking anisotropy into account allows for further refinement of our interpretation of composition provided that anisotropy is characterized for candidate rock types. This study uses electron backscatter diffraction (EBSD) measurements of crystallographic preferred orientation of minerals to calculate seismic anisotropy in samples of the Pelona-Orocopia-Rand (POR) schist from the Mojave region of southern California. The goals of this work are to characterize the seismic anisotropy of the POR schist and its relationship to observed lower crustal anisotropy in the region, and to refine predictions of lower crustal composition based on seismic anisotropy

Diapirs as the source of the sediment signature in arc lavas

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 4 (2011): 641-646, doi:10.1038/ngeo1214Many arc lavas show evidence for the involvement of subducted sediment in the melting process. There is debate whether this “sediment melt” signature forms at relatively low temperature near the fluid-saturated solidus or at higher temperature beyond the breakdown of trace-element-rich accessory minerals. We present new geochemical data from high- to ultrahigh-pressure rocks that underwent subduction and show no significant depletion of key trace elements in the sediment melt component until peak metamorphic temperatures exceeded ~1050ºC from 2.7 to 5 GPa. These temperatures are higher than for the top of the subducting plate at similar pressures based on thermal models. To address this discrepancy, we use instability calculations for a non-Newtonian buoyant layer in a viscous half-space to show that, in typical subduction zones, solid-state sediment diapirs initiate at temperatures between 500–850ºC. Based on these calculations, we propose that the sediment melt component in arc magmas is produced by high degrees of dehydration melting in buoyant diapirs of metasediment that detach from the slab and rise into the hot mantle wedge. Efficient recycling of sediments into the wedge by this mechanism will alter volatile fluxes into the deep mantle compared to estimates based solely on devolatilization of the slab.Funding for this work was provided by NSF and WHOI’s Deep Ocean Exploration Institute

Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

India (Tethyan Himalaya Series) in central Myanmar: implications for the evolution of the Eastern Himalayan Syntaxis and the Sagaing transform-fault system

In the Katha Range of central Myanmar, lithologic tracers and pressure-temperature-deformation time data identify Cambro-Ordovician, Indian-affinity Tethyan Himalaya Series, located ∼700 km from their easternmost outcrop in S-Tibet, and ∼450 km from Himalayan rocks in the Eastern Himalayan Syntaxis. Metamorphism began at ∼65 Ma, peaked at ∼45 Ma (∼510°C, 0.93 GPa), and exhumation/cooling (∼25°C/ Myr) occurred until ∼30 Ma in a subduction-early collision tectonic setting. When the Burma microplate-part of the intra-Tethyan Incertus arc-accreted to SE-Asia, its eastern boundary, the southern continuation of the Indus-Yarlung suture (IYS), was reactivated as the Sagaing fault (SF), which propagated northward into Indian rocks. In the Katha rocks, this strike-slip stage is marked by ∼4°C/Myr exhumation/cooling. Restoring the SF system defines a continental collision-oceanic subduction transition junction, where the IYS bifurcates into the SF at the eastern edge of the Burma microplate and the Jurassic ophiolite-Jadeite belts that include the Incertus-arc suture

Subduction factory: 4. Depth‐dependent flux of H 2 O from subducting slabs worldwide

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95328/1/jgrb16727.pd