Anatexis, cooling, and kinematics during orogenesis: Miocene development of the Himalayan metamorphic core, east-central Nepal

Abstract

The exposed mid-crustal rocks of the Himalayan orogen provide a natural laboratory for constructing the kinematic evolution of the midcrust during a large-scale continental collision. Kinematic models provide testable, geometrically valid, internally consistent, integrated solutions for diverse geological data from deformed regions. We investigated the Tama Kosi region of east-central Nepal with structural, geochemical, and geochrono logical methods to refine a detailed kinematic model for the Miocene Epoch, during which the mid-crust was pervasively deformed, translated southward, and progressively stacked via basal accretion. Geochemical and U-Pb zircon data demonstrate that two similar orthogneiss bodies were derived from different protoliths, one formed through vapor-absent melting at 1940 ± 16 Ma and the other via vapor-present melting at 1863 ± 14 Ma, respectively, indicating that they do not reflect structural repetition. In situ Th-Pb monazite petrochronology from the Mahabharat Range links the orogenic foreland to the exposed mid-crust of the High Himalaya via a coeval, protracted metamorphic growth-crystallization and/or recrystallization record spanning late Eocene or early Oligocene to early Miocene. Differential cooling of white mica, evidenced by 40 Ar/ 39 Ar cooling ages across the studied area, may outline a previously unrecognized out-of-sequence thrust, the occurrence of which is coincident with the location of a sharp break previously recognized from quartz crystallographic fabric deformation temperatures. Together with previous work, these data form the basis for a new, internally consistent kinematic model for rocks of the Tama Kosi region during the Miocene Epoch that tracks the transition from distributed ductile deformation in the mid-crust to deformation along discrete surfaces during their exhumation.

Preservation of the Early Evolution of the Himalayan Middle Crust in Foreland Klippen: Insights from the Karnali Klippe, West Nepal

“…In this scenario, out‐of‐sequence deformation brought northern and structurally deeper GHS rocks to their current position, such that the two regions represent significantly different across‐strike positions in the middle crust, despite their geographical proximity. Out‐of‐sequence thrusting isolating foreland metamorphic rocks has also been suggested north of the Kathmandu klippe in central Nepal (Figure a; Johnson et al, ; Larson et al, ), in far‐eastern Nepal (Figure a; Schelling & Arita, ), in Bhutan (Figure a; Grujic et al, , ; Kellett et al, ; Warren et al, ), and in Arunachal Pradesh (Figure a; Warren et al, ). Out‐of‐sequence thrusting of hinterland GHS on top of foreland GHS is also predicted by thermal‐mechanical numerical models containing an embedded weak upper layer in the upper crust (e.g., model HT‐111; Jamieson et al, ).…”

“…Black polygon in the Karnali klippe indicates the map area of Figure . The locations of other areas relevant to this study are indicated by circled numbers: 1—Dadeldhura klippe (Antolín et al, ); 2—Gurla Mandhata (Murphy & Copeland, ); 3—Humla Karnali (Braden et al, ; Yakymchuk, ); 4—Dolpo synclinorium (Cannon & Murphy, ); 5—Mugu Karnali (Iaccarino, Montomoli, Carosi, Massonne, & Visonà, ; Montomoli et al, ); 6—Lower Dolpo (Carosi et al, , ; Gibson et al, ); 7—Annapurna (Brown & Nazarchuk, 1993; Godin, ; Godin, Brown, & Hanmer, ; Hurtado et al, ); 8—Jajarkot klippe (Soucy La Roche et al, ); 9—Kathmandu klippe (Johnson et al, ; Larson et al, ); 10—Ama Drime (Groppo et al, ; Kellett et al, ); 11—far‐eastern Nepal (Schelling & Arita, ); 12— Bhutan (Grujic et al, , ; Kellett et al, ; Warren et al, ); 13—Arunachal Pradesh (Warren et al, ). (b) Cross section A‐A′ illustrating the relationship between the GHS in the homoclinal slab and the GHS in the Karnali klippe (modified from Soucy La Roche et al, ).…”

The Himalaya in 3D: Slab dynamics controlled mountain building and monsoon intensification

“…16 to ca. 12 Ma (e.g., Carrapa et al, 2016;Kellett et al, 2013), and to the south a contemporaneous outof-sequence thrust system occurs (e.g., Grujic et al, 2011;Larson et al, 2016). The apparently restricted range of these systems could indicate that they respond to the relatively large magnitude burial and subsequent uplift associated with the final slab detachment, as in the Chemenda group modeling.…”

Cooling, exhumation, and kinematics of the Kanchenjunga Himal, far east Nepal

“…Where identified, these structures typically occur as either late Oligocene to early Miocene reverse sense shear zones that drive metamorphism in the overridden footwall [ Montomoli et al ., , ] or as postpeak metamorphic, middle to late Miocene out‐of‐sequence structures [ Grujic et al ., ; Warren et al ., , ]. While most of the observed structures are related to deformational processes that occurred during the assembly and/or initial exhumation of the middle crust [ Larson et al ., ; Cottle et al , ], there is some indication that ductile deformation associated with these faults and other major structures in the Himalaya may have occurred at temperatures as low as ~450–500°C [ Law et al ., ; Larson and Cottle , ; Larson et al ., ]. Such temperatures approach the effective range of common mineral thermochronometers.…”