Abstract
The channel-flow model for the Greater Himalayan Sequence (GHS) of the Himalayan orogen involves a partially molten, rheologically weak, mid-crustal layer “flowing” southward relative to the upper and lower crust during late Oligocene–Miocene. Flow was driven by topographic overburden, underthrusting, and focused erosion. We present new structural and thermobarometric analyses from the GHS in the Annapurna-Dhaulagiri Himalaya, central Nepal; these data suggest that during exhumation, the GHS cooled, strengthened, and transformed from a weak “active channel” to a strong “channel plug” at greater depths than elsewhere in the Himalaya. After strengthening, continued convergence resulted in localized top-southwest (top-SW) shortening on the South Tibetan detachment system (STDS). The GHS in the Annapurna-Dhaulagiri Himalaya displays several geological features that distinguish it from other Himalayan regions. These include reduced volumes of leucogranite and migmatite, no evidence for partial melting within the sillimanite stability field, reduced structural thickness, and late-stage top-southwest shortening in the STDS. New and previously published structural and thermobarometric constraints suggest that the channel-flow model can be applied to mid-Eocene–early Miocene mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. However, pressure-temperature-time (PTt) constraints indicate that following peak conditions, the GHS in this region did not undergo rapid isothermal exhumation and widespread sillimanite-grade decompression melting, as commonly recorded elsewhere in the Himalaya. Instead, lower-than-typical structural thickness and melt volumes suggest that the upper part of the GHS (Upper Greater Himalayan Sequence [UGHS]—the proposed channel) had a greater viscosity than in other Himalayan regions. We suggest that viscosity-limited, subdued channel flow prevented exhumation on an isothermal trajectory and forced the UGHS to exhume slowly. These findings are distinct from other regions in the Himalaya. As such, we describe the mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya as an atypical example of channel flow during the Himalayan orogeny.
Highlights
The kinematic and metamorphic evolution of the metamorphic core of the Himalayan orogen (Fig. 1), referred to as the Greater Himalayan Sequence (GHS), is the central focus of all models of Himalayan orogenesis (e.g., Grujic et al, 1996; Beaumont et al, 2001; Bollinger et al, 2006; Robinson et al, 2006; Searle et al, 2006; Kohn, 2008; Mukherjee, 2013b; He et al, 2014; Cottle et al, 2015; Frassi, 2015; Montomoli et al, 2015)
Field structural observations, combined with new and previously published thermobarometric and geochronometric constraints indicate that the mid-crustal evolution of the Upper GHS (UGHS) in the Annapurna-Dhaulagiri Himalaya is more favorably explained by the channel-flow model than models based on thrust-stacking mechanisms
Consideration of the rheological properties of the UGHS with a specific focus on the effects of melt volume and channel thickness suggests that channel flow was more limited and subdued in the Annapurna-Dhaulagiri Himalaya than in other regions where melt volume and structural thickness were larger
Summary
The kinematic and metamorphic evolution of the metamorphic core of the Himalayan orogen (Fig. 1), referred to as the Greater Himalayan Sequence (GHS), is the central focus of all models of Himalayan orogenesis (e.g., Grujic et al, 1996; Beaumont et al, 2001; Bollinger et al, 2006; Robinson et al, 2006; Searle et al, 2006; Kohn, 2008; Mukherjee, 2013b; He et al, 2014; Cottle et al, 2015; Frassi, 2015; Montomoli et al, 2015). Many of the geological and geophysical constraints on which the channel-flow model is based (e.g., pressure-temperature [PT] conditions and crustal thicknesses) are derived from the Everest, Sikkim, and Bhutan regions (e.g., Grujic et al, 1996, 2002) of the central-eastern Himalaya (Fig. 1A) In these regions, the channel-flow model provides a robust explanation for the kinematic, dynamic, and temporal evolution of the GHS (e.g., Nelson et al, 1996; Beaumont et al, 2001; Searle and Szulc, 2005; Unsworth et al, 2005; Searle et al, 2006; Streule et al, 2010). Field observations from the Modi Khola and Kali Gandaki valleys (Fig. 1B) reveal geological features of the GHS that are atypical when compared to other Himalayan regions along strike These include reduced volumes of leucogranite and migmatite, an absence of evidence for partial melting within the sillimanite stability field, reduced structural thickness, and late-stage top-SW shortening on the STDS following cessation of top-NE extensional shearing. The presence of such features has significant implications for the rheology of the GHS and raises the question of whether or not the GHS in this region was weak enough for mid-crustal flow
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