How does the mid-crust accommodate deformation in large, hot collisional orogens? A review of recent research in the Himalayan orogen

Abstract

The presence of hot, weak crust is a central component of recent hypotheses that seek to 35 explain the evolution of continent-continent collisions, and in particular may play an 36 important role in accommodating the >3000 km of convergence within the Himalaya-37 Tibetan collision over the last ~55 Myr. Models that implicate flow of semi-viscous 38 midcrustal rocks south toward the front of the Himalayan orogen, 'channel flow', are able 39 to account for many geologic observations in the Himalaya, while alternative models of 40 collision, particularly 'thrust-wedge taper', demonstrate that much of the observed 41 geology could have formed in the absence of a low-viscosity mid-crustal layer. Several 42 recent studies, synthesized here, have prompted a shift from initial assumptions that 43 channel flow and thrust-wedge taper processes are by definition mutually exclusive. 44 These new studies reveal the presence of several tectonometamorphic discontinuities in 45 the midcrust that appear to reflect a continuum of deformation in which both channel-46 and wedge-type processes operate in spatially and temporally distinct domains within the 47 orogen, and further, that the system may migrate back and forth between these types of 48 behavior. This continuum of deformation styles within the collisional system is of crucial 49 importance for explaining the evolution of the Himalayan orogen and, hence, for 50 understanding the evolution of Earth's many continent-continent collision zones.

The P–T–t evolution of the exhumed Himalayan metamorphic core in the Likhu Khola region, East Central Nepal

“…(, ) and is thus compatible with the models in which material moves through the orogen through time and is subject to varying deformation styles (e.g. Cottle et al., ; Jamieson et al., , ; Larson & Cottle, ; Larson et al., , , ; Montomoli et al., ; Parsons et al., ; Rolfo et al., ; Wang et al., ). This interpretation correlates well with those made in the adjacent Tama Kosi region (Larson & Cottle, ; Larson et al., ) and nearby Nylam transect (Wang, Rubatto, et al., ; Wang, Zhang, et al., ; Wang et al., ) where movement along a thrust‐sense discontinuity at the same structural level is interpreted to have buried the footwall, driving prograde metamorphism, as the hanging wall was overthrust and brought (with paired erosion) toward the surface (e.g.…”

“…() and Cottle et al. () the P–T–t data obtained from the Likhu Khola region are compared with the expected P–T–t paths extracted from model data (Jamieson, Beaumont, Nguyen, & Grujic, ; Jamieson et al., ) and other studies (Caddick et al., ; Kellett et al., ; Kohn, ; Kohn et al., ; Larson et al., ) in Figure b,c. The clockwise retrograde P–T–t paths obtained from the three specimens structurally above the discontinuity that outline prograde metamorphism between c .…”

“…(a) Interpreted P–T–t paths from the study area. (b and c) Comparison of interpreted P–T–t paths with the predicted model paths for Central HMC and Lower HMC from various studies (modified after Cottle et al., )…”

“…Understanding the structural deformation and metamorphic processes associated with this large‐scale convergence requires an integrated approach including investigation of fault kinematics, spatial‐temporal constraints on metamorphism and deformation, and integration of large‐scale orogen dynamics (e.g. Cottle, Larson, & Kellett, ; Larson & Cottle, ; Montomoli, Carosi, & Iaccarino, ; Wang et al., ; Warren et al., ). When interpreted individually, results from one field of study may point to a unique conclusion, but may not provide an explanation that is consistent with other data sets (e.g.…”

“…During the last two decades, disagreement over the models of development of the Himalayan metamorphic core (HMC), a package of pervasively deformed and metamorphosed rocks that record mid‐crustal deformation and metamorphism during the Cenozoic evolution along the Himalaya (Cottle et al., ; From, Larson, & Cottle, ), has led to the proliferation of numerous orogenic models explaining the various data sets collected. For example, end member critical‐taper wedge models (Hodges, Parrish, & Searle, ; Kohn, ; Robinson, DeCelles, & Copeland, ), channel flow models (Beaumont & Jamieson, ; Beaumont, Jamieson, Nguyen, & Lee, ; Beaumont, Jamieson, Nguyen, & Medvedev, ; Godin, Grujic, Law, & Searle, ; Grujic, Hollister, & Parrish, ; Grujic et al., ; Hodges, , ; Jamieson, Beaumont, Medvedev, & Nguyen, ); tectonic wedging models (Webb, Schmitt, He, & Weigand, ; Webb, Yin, Harrison, Célérier, & Burgess, ) and “hybrid models” (Cottle et al., ; Larson, Ambrose, Webb, Cottle, & Shrestha, ; Larson, Gervais, & Kellett, ) have all attempted to explain the kinematics and structural geometries of the HMC. Many of these models, though they may reflect different processes, have characterized the high‐grade rocks within the HMC as a continuous unit.…”