Kyle P. Larson

Defining the Himalayan Main Central Thrust in Nepal

An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1–3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2–3 km thick, bounded by an upper thrust, MCT-2 (= Vaikrita thrust), and a lower thrust, MCT-1 (= Munsiari thrust). Some workers define an ‘upper Lesser Himalaya’ thrust sheet that shows similar P–T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (<5 Ma) Th–Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (early–middle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal

Greater Himalayan sequence rocks exposed in the Manaslu–Himal Chuli Himalaya can be separated into distinct upper and lower parts. Deformation recorded in both parts occurred at temperatures ranging between ∼450 °C and ∼640 °C and is characterized by almost equal coaxial and noncoaxial components. Across the upper Greater Himalayan sequence, peak metamorphic temperatures are essentially isothermal, whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars/km. In the lower Greater Himalayan sequence, however, both metamorphic pressure and temperature decrease with structural depth. The abnormal pressure gradient in the upper Greater Himalayan sequence is attributed to ∼50% vertical thinning during southward displacement, while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the Main Central thrust shear zone and the progressive incorporation of more rock into the Greater Himalayan sequence. Deformation in the upper portion of the Greater Himalayan sequence was characterized by extending flow, i.e., extension in the direction of flow, whereas deformation in its lower portion was characterized by compressing flow, i.e., compression in the direction of flow. Extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands, while compressing flow is emblematic of displacement and distortion in orogenic foreland regions. The transition between the upper and lower parts of the Greater Himalayan sequence therefore represents a fundamental transition between hinterland-style deformation, involving processes such as lateral midcrustal flow, and foreland-style deformation, involving critical-taper thrust-fold wedge development.

Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal: implications for the structural framework of central Nepal

Abstract: In the Dhaulagiri region of central Nepal quartz-rich specimens sampled from the Greater Himalayan sequence yield well-defined quartz c-axis fabrics with a dominant top-to-the-SW sense of shear. These fabrics reflect pervasive crystal-plastic deformation extending more than 8 km structurally below previously mapped locations of the Main Central thrust fault. Quartz c-axis fabric opening angles suggest deformation temperatures of c. 500 ± 50 °C within the lower portion of the Greater Himalayan sequence and up to c. 670 ± 50 °C within the migmatitic upper portion. These temperatures generally plot within error of garnet–biotite temperature estimates for metamorphic assemblages and are interpreted to reflect Tertiary deformation during extrusion of the mid-crust. The quartz c-axis data, and a new, detailed tectonostratigraphic map, constrain the position of the Ramgarh thrust in the Dhaulagiri region to be coincident with the Main Central thrust at the base of the pervasively deformed exhumed mid-crustal core. Mean kinematic vorticity numbers (Wm) measured in specimens sampled from the lower portion of the Greater Himalayan sequence range between 0.49 and 0.80 (c. 66–41% pure shear) with an average value of 0.67 (c. 53% pure shear). These data indicate that extrusion of the mid-crust was facilitated by a significant component of pure shear strain.

Extrusion vs. duplexing models of Himalayan mountain building 3: duplexing dominates from the Oligocene to Present

The Himalaya is a natural laboratory for studying mountain-building processes. Concepts of extrusion and duplexing have been proposed to dominate most phases of Himalayan evolution. Here, we examine the importance of these mechanisms for the evolution of the Himalayan crystalline core via an integrated investigation across the northern Kathmandu Nappe. Results reveal that a primarily top-to-the-north shear zone, the Galchi shear zone, occurs structurally above and intersects at depth with the Main Central thrust (MCT) along the northern flank of the synformal Kathmandu Nappe. Quartz c-axis fabrics confirm top-to-the-north shearing in the Galchi shear zone and yield a right-way-up deformation temperature field gradient. U-Pb zircon dating of pre-to-syn- and post-kinematic leucogranites demonstrates that the Galchi shear zone was active between 23.1 and 18.8 Ma and ceased activity before 18.8–13.8 Ma. The Galchi shear zone is correlated to the South Tibet detachment (STD) via consistent structural fabrics, lithologies, metamorphism, and timing for four transects across the northern margin of the Kathmandu Nappe. These findings are synthesized with literature results to demonstrate (1) the broad horizontality of the STD during motion and (2) the presence of the MCT-STD branch line along the Himalayan arc. The branch line indicates that the crystalline core was emplaced at depth via tectonic wedging and/or channel tunnelling-type deformation. We proceed to consider implications for the internal development of the crystalline core, particularly in the light of discovered tectonic discontinuities therein. We demonstrate the possibility that the entire crystalline core may have been developed via duplexing without significant channel tunnelling, thereby providing a new end-member model. This concept is represented in a reconstruction showing Himalayan mountain-building via duplexing from the Oligocene to Present.

Midcrustal discontinuities and the assembly of the Himalayan midcrust

Detailed quartz lattice preferred orientation (LPO) data define two structural discontinuities in the exhumed high-grade metamorphic core of the Himalaya exposed in the upper Tama Kosi region of east central Nepal. The structures are marked by abrupt breaks in a general trend of up structural section increasing quartz LPO-defined deformation temperatures. Deformation associated with the upper structural discontinuity, which occurs within sillimanite grade rocks, is postpeak metamorphism in both the hanging wall and the footwall. New geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 Ma, indistinguishable from previously published ages for the footwall. Movement across this structure represents Early Miocene strain localization and thickening in the Himalayan midcrust. Movement across the lower discontinuity, which occurs between staurolite and kyanite grade rocks, appears to be synmetamorphic with material in its footwall at approximately 10 Ma, but postpeak metamorphism for material in its hanging wall. This movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core. The recognition of two thrust-sense discontinuities in the exhumed Himalayan core in the Tama Kosi region is consistent with other similar structures recognized along the Himalaya. The widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large, hot orogens.

Lateral extrusion, underplating, and out-of-sequence thrusting within the Himalayan metamorphic core, Kanchenjunga, Nepal

Integrated pseudosection modeling and monazite petrochronology of paragneiss from the Kanchenjunga region of northeastern Nepal reveal the presence of cryptic tectonometamorphic discontinuities within the Himalayan metamorphic core. These new data outline a series of thrust-sense structures that juxtapose rocks that generally record a protracted history of early Eocene to latest Oligocene−early Miocene (ca. 41−23 Ma) prograde metamorphism and lateral extrusion against others that typically record short prograde (between 2 and 7 m.y.) and retrograde (between 3 and 6 m.y.) histories variably spanning the middle Oligocene to the middle Miocene (ca. 31−12 Ma). Retrograde metamorphism in the hanging wall of the thrust faults is typically coeval with prograde metamorphism in the footwall, indicating that overthrusting/underthrusting accommodated crustal shortening and drove metamorphic processes. The structures and juxtaposed panels were cut by early Miocene (ca. 20−18 Ma) out-of-sequence thrusting coincident with the previously mapped High Himal thrust. The resulting kinematic model for the evolution of the Himalayan metamorphic core in the Kanchenjunga area demonstrates that the Himalayan metamorphic core was dominated by underplating from at least the Oligocene through to present, and that the internal structure of the exhumed metamorphic core is significantly more complex than has been documented previously.

The southern termination of the Western Main Ranges of the Canadian Rockies, near Fort Steele, British Columbia: stratigraphy, structure, and tectonic implications

Abstract The Wild Horse River map-area is situated near the Crowsnest Pass cross-strike discontinuity, which marks the southern termination of the Western Main Ranges subprovince of the Southern Canadian Rocky Mountains. The Crowsnest Pass cross-strike discontinuity is a transverse northeast-trending tectonic feature marked by profound changes in structural style and in Paleozoic stratigraphy between the Laurentia craton and the Cordilleran miogeocline. Cambrian and Ordovician strata in the Wild Horse River map-area accumulated on the northwest side of the discontinuity in two contrasting tectonostratigraphic domains. One domain consists of an incomplete stratigraphic succession that was deposited on the Windermere high, a high-standing fault block within the Paleozoic miogeocline that was intermittently tilted eastward. The other domain contains a thicker more complete shaly succession that accumulated adjacent to the margin of the Laurentia craton. This latter succession contains local intercalations of diatreme-related volcanic rocks, the emplacement of which may have been controlled by basement faults that defined the margin of the miogeocline. The east verging Lussier River thrust fault separates the two domains. The dominant structures east of it are the Porcupine Creek anticlinorial fan structure and the east-verging Ruault Lake thrust fault. The Ruault Lake fault has been synformally backfolded in the west flank limb of the Porcupine Creek fan structure; both it and the fan structure are truncated by the out-of-sequence Lussier River fault. All convergence-related structures are cut and thermally overprinted by approximately 111 Ma granitic stocks that date all the thrusting and folding as pre-Albian. The transverse Boulder Creek fault, which branches from the Lussier River fault, may be a dextral tear fault that was reactivated as a down-to-the-north normal fault during Tertiary displacement along the Southern Rocky Mountain Trench fault.

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

The recent identification of multiple strike-parallel discontinuities within the exhumed Himalayan metamorphic core has helped revise understanding of convergence accommodation processes within the former mid-crust exposed in the Himalaya. While the significance of these discontinuities to the overall development of the mountain belt is still being investigated, their identification and characterization has become important for potential correlations across regions, and for constraining the kinematic framework of the mid-crust. The result of new phase equilibria modelling, trace element analysis, and high-precision Lu-Hf garnet dating of the metapelites from Likhu Khola region in east-central Nepal, combined with previously published monazite petrochronology data confirms the presence of one of such cryptic thrust-sense tectonometamorphic discontinuities within the lower portion of the exhumed metamorphic core and provides new constraints on the P-T estimates for that region. The location of the discontinuity is marked by an abrupt change in the nature of P-T-t paths of the rocks across it. The rocks in the footwall are characterized by a prograde burial P-T path with peak metamorphic conditions of ~ 660 °C and ~ 9.5 kbar likely in the mid-to-late Miocene, which are overlain by the hanging wall rocks, that preserve retrograde P-T paths with P-T conditions of > 700 °C and ~ 7 kbar in the early Miocene. The occurrence of this thrust-sense structure that separates rock units with unique metamorphic histories is compatible with orogenic models that identify a spatial and temporal transition from early midcrustal deformation and metamorphism in the deeper hinterland to later deformation and metamorphism toward the shallower foreland of the orogen. Moreover, these observations are comparable to those made across other discontinuities at similar structural levels along the Himalaya, confirming their importance as important orogen-scale structures. This article is protected by copyright. All rights reserved.

Defining shear zone boundaries using fabric intensity gradients: An example from the east-central Nepal Himalaya

Uranium-lead (U-Pb) zircon geochronology, whole-rock geochemistry, and petrographic observations indicate that specimens from a suite of variably deformed granite and orthogneiss from the Okhaldungha region of east-central Nepal share a common origin. Microtextural characterization and quartz crystallographic fabric preferred orientation analyses of these same specimens outline a strain gradient that marks the location of a shear zone boundary. The location of this boundary, at the base of the orthogneiss, coincides with one of the interpreted locations of the Main Central thrust, though it cannot be uniquely identified as such. This study provides the first steps toward empirical constraints on the location and geometry of thrust structures in the region, helping to clarify the complex local kinematic framework. These methods not only help in assessing orogenic models of the Himalaya, but may also be applied to investigating other orogenic systems where the potential location of shear structures is contested.

COOLING, EXHUMATION AND KINEMATICS OF THE KANCHENJUNGA HIMAL, FAR EAST NEPAL

New single crystal 40Ar/39Ar and apatite fission track ages from the Kanchenjunga region of far east Nepal yield insight into the timing of assembly of the Himalayan mid-crust and the mechanisms that controlled its exhumation. The 40Ar/39Ar data are compared with new U(Th)/Pb zircon and monazite intrusive crystallization ages and existing metamorphic monazite ages from across the study area to test for internal consistency and potential excess Ar contributions. This new dataset, which significantly enhances the density and spatial coverage available from the region, shows that inferred thrust-sense discontinuities within the now-exhumed former midcrustal rocks exposed therein must have ceased motion by ca. 12 Ma. Furthermore, the spatial distribution of ages across the Kanchenjunga region, older ages (ca. 12 – 16 Ma) to the south and north and younger ages (ca. 8 Ma) in the middle portion of the transect, is compatible with simulations of tectonic-enhanced exhumation above a developing duplex system in nearby Bhutan.