Peter Copeland

Stratigraphy, structure, and tectonic evolution of the Himalayan fold‐thrust belt in western Nepal

Regional mapping, stratigraphic study, and 40Ar/39Ar geochronology provide the basis for an incremental restoration of the Himalayan fold-thrust belt in western Nepal. Tectonostratigraphic zonation developed in other regions of the Himalaya is applicable, with minor modifications, in western Nepal. From south to north the major structural features are (1) the Main Frontal thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sheet, which consists of Proterozoic to early Miocene, Lesser Himalayan metasedimentary rocks; (3) the Ramgarh thrust sheet, composed of Paleoproterozoic low-grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which consists of medium-grade metamorphic rocks, Cambrian-Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himalayan duplex, which is a large composite antiformal stack and hinterland dipping duplex; and (6) the Main Central thrust zone, a broad ductile shear zone. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene-Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred from the detrital unroofing record. On the basis of 40Ar/39Ar cooling ages and provenance data from synorogenic sediments, emplacement of the Dadeldhura thrust sheet took place in early Miocene time. The Ramgarh thrust sheet was emplaced between ∼15 and ∼10 Ma. The Lesser Himalayan duplex began to grow by ∼10 Ma, simultaneously folding the north limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene-Pliocene time; transport of its hanging wall rocks over an ∼8-km-high footwall ramp folded the south limb of the Dadeldhura synform. Thrusts in the Subhimalayan zone became active in Pliocene time. The minimum total shortening in this portion of the Himalayan fold-thrust belt since early Miocene time (excluding the Tibetan zone) is ∼418–493 km, the variation depending on the actual amounts of shortening accommodated by the Main Central and Dadeldhura thrusts. The rate of shortening ranges between 19 and 22 mm/yr for this period of time. When previous estimates of shortening in the Tibetan Himalaya are included, the minimum total amount of shortening in the foldthrust belt amounts to 628–667 km. This estimate neglects shortening accommodated by small-scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening.

Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet

For providing a square end on a dye spring centre of the type comprising a helical main spring having a wire lacing the turns of which extend between the adjacent helices of the main spring, the invention provides an end adapter defining an annular channel which fits on the terminal helix of the dye spring centre and in which are protrusions of different heights to engage the terminal helix and hold the adapter square.

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr −1 . We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effectoftheevolvingtopographycanexplainafractionofthescatterobservedinthedatabut not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumationarelikely.Ithasbeenarguedthatthesteepphysiographictransitionatthefootof the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

Extension and basin formation in the southern Andes caused by increased convergence rate: A mid‐Cenozoic trigger for the Andes

The southern Andes between 33° and 45°S latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well-described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18°–33°S), leading to the speculation that extension may have characterized the late Oligocene-early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains.

Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models

We present a new geologic map of western Nepal and three balanced regional cross sections in the Himalayan thrust belt. The minimum shortening between the South Tibetan detachment and the Main Frontal thrust is 485–743 km and suggests that total Himalayan shortening may exceed 900 km. All rocks involved in the thrust belt are of upper crustal affinity, implying that a comparable length of Indian lower crust and mantle lithosphere was subducted beneath Tibet. Major structural features are the Subhimalayan thrust system, Lesser Himalayan imbricate zone, Dadeldhura thrust sheet, Lesser Himalayan duplex, Ramgarh thrust sheet, Main Central thrust sheet, and a north-dipping normal-sense shear zone, possibly related to the South Tibetan detachment. These structures are continuous along the entire Nepalese portion of the Himalayan thrust belt. New 40 Ar/ 39 Ar ages from the Ramgarh thrust zone, Greater Himalayan rocks, and the lower part of the Tethyan sequence support a kinematic model in which major thrust systems in Nepal propagated southward from early Miocene time onward. The geometry and kinematic history of the thrust belt in western Nepal are not compatible with recent models for southward ductile extrusion of Greater Himalayan rocks in a mid-crustal channel. Instead, the thrust belt in western Nepal behaved like a typical forward propagating thrust system, involving unmetamorphosed, brittlely deformed rocks in its frontal part and ductilely deformed, higher-grade metamorphic rocks in its hinterland region. Although our results do not support published versions of the channel flow model, they provide additional geological and geo-chronological data that will assist future attempts to develop geodynamic models for the Himalayan-Tibetan orogenic system.

Isotopic Preservation of Himalayan/Tibetan Uplift, Denudation, and Climatic Histories of Two Molasse Deposits

Two distinctive molasse deposits within the Indo-Asian collision zone have been investigated to help understand the post-Oligocene evolution of the Himalaya and southern Tibetan plateau. The Siwalik Group (predominantly fluvial sandstones and siltstones), is widespread throughout the foothills of the Himalaya from Pakistan to eastern India. Paleomagnetic analysis of a measured section in the Bakiya Khola, southeastern Nepal, constrains depositional ages (

Thermal evolution of the Gangdese batholith, southern Tibet: A history of episodic unroofing

t_{dep}

Age, Cooling History, and Origin of Post-Collisional Leucogranites in the Karakoram Batholith; A Multi-System Isotope Study

) to between 10.8 and 4.9 Ma. The average accumulation rate during this interval was 0.4-0.5 mm/yr.