Kinematics and vorticity in Kangmar Dome, southern Tibet: Testing midcrustal channel flow models for the Himalaya

Abstract

[1] Kinematic, kinematic vorticity (W m ), 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 W m 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 topnorth 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. Citation: Wagner, T., J. Lee, B. R. Hacker, and G. Seward (2010), Kinematics and vorticity in Kangmar Dome, southern Tibet: Testing midcrustal channel flow models for the Himalaya, Tectonics, 29, TC6011,

Subduction‐ and exhumation‐related structures in the Cycladic Blueschists: Insights from south Evia Island (Aegean region, Greece)

“…This extruding slice was subjected to ductile thinning (Figure 14c) and transport‐parallel elongation coeval with decompression of the Cycladic Blueschists and underthrusting of the Basal unit (Figure 16b). A similar deformation pattern has also been documented in other zones of ductile extrusion [e.g., Xypolias and Koukouvelas , 2001; Law et al , 2004; Wagner et al , 2010].…”

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

“…Rather than trying to apply a conventional view of a LANF in an extensional setting in which high temperature mylonites are exhumed in the footwall of an ongoing normal fault detachment, progressively rotated into a sub‐horizontal position, and overprinted by brittle deformation [e.g., Mancktelow , 1990], we find evidence of a more complex sequence of deformation stages for the formation of LANFs in contractional settings such as the STDS in the Himalaya. We propose (Figure 10) that the broad, sub‐horizontal layer of top‐to‐the‐north ductile shear at the top of the GHS sometimes referred to in the literature as the “lower” component of the STDS and exposed at the base of the Bhutan klippen (outer STDS), at Dzakaa Chu, along the crest of the Himalaya (e.g., Lhotse detachment, Everest; Annapurna detachment, central Nepal) and in the cores of some of the North Himalayan gneiss domes (e.g., Kangmar Dome) [e.g., Burchfiel et al , 1992; Lee et al , 2000; Grujic et al , 2002; Searle et al , 2003; Cottle et al , 2007; Kellett et al , 2010; Searle , 2010; Wagner et al , 2010] is an extensive, diffuse, sheared layer at the boundary between mid‐ and upper crust that deformed as the result of south‐directed mid‐crustal flow [ Beaumont et al , 2001; Jamieson et al , 2004] during the Early Middle Miocene.…”

The South Tibetan detachment system facilitates ultra rapid cooling of granulite‐facies rocks in Sikkim Himalaya

“…Nelson et al . ; Hauck et al ., ; Lee et al ., ; Wagner et al ., ] potentially connecting further north to the basal detachment of a pre‐Miocene south‐vergent fold and thrust belt [e.g. Ratschbacher et al ., ].…”

“…Geophysical and geologic data indicate that the deepest components of the STDS sole out into the upper-to mid-crust [e.g. Nelson et al 1996;Hauck et al, 1998;Lee et al, 2000;Wagner et al, 2010] potentially connecting further north to the basal detachment of a pre-Miocene south-vergent fold and thrust belt [e.g. Ratschbacher et al, 1994].…”