ABSTRACT This field trip examines Paleoproterozoic basement, Neoproterozoic metasedimentary strata, and crosscutting Mesozoic intrusive rocks at Frazier Mountain, Placerita Canyon, and Limerock Canyon in the western San Gabriel Mountains block, California. We present new U-Pb zircon geochronology results that constrain the Proterozoic through Cretaceous tectonic and magmatic history. The excursion ends in San Antonio Canyon in the eastern San Gabriel Mountains where several large rock avalanche deposits are sourced from distinct basement rocks. 10 Be surface exposure ages and post-infrared infrared stimulated luminescence burial ages demonstrate late Pleistocene to Holocene movements for these landslides.

Abstract Himalayan high-pressure metamorphic rocks are restricted to three environments: the suture zone; close to the suture zone; and (mostly) far (>100 km) from the suture zone. In the NW Himalaya and South Tibet, Cretaceous-age blueschists (glaucophane-, lawsonite- or carpholite-bearing schists) formed in the accretionary wedge of the subducting Neo-Tethys. Microdiamond and associated phases from suture-zone ophiolites (Luobusa and Nidar) are, however, unrelated to Himalayan subduction–collision processes. Deeply subducted and rapidly exhumed Indian Plate basement and cover rocks directly adjacent to the suture zone enclose eclogites of Eocene age, some coesite-bearing (Kaghan/Neelum and Tso Morari), formed from Permian Panjal Trap, continental-type, basaltic magmatic rocks. Eclogites with a granulite-facies overprint, yielding Oligocene–Miocene ages, occur in the anatectic cordierite ± sillimanite-grade Indian Plate mostly significantly south of the suture zone (Kharta/Ama Drime/Arun, north Sikkim and NW Bhutan) but also directly at the suture zone at Namche Barwa. The sequence carpholite-, coesite-, kyanite- and cordierite-bearing rocks of these different units demonstrates the transition from oceanic subduction to continental collision via continental subduction. The granulitized eclogites in anatectic gneisses preserve evidence of former thick crust as in other wide hot orogens, such as the European Variscides.

Abstract Reconstruction of the protolith lithostratigraphy of amphibolite-facies metasedimentary rocks of the Greater Himalayan Series (GHS) in Nepal documents a single, long-lived passive-margin succession that was deposited along the northern margin of the Indian Craton. In the Langtang area, Paleoproterozoic gneisses are unconformably overlain by a succession of upper Neoproterozoic–Ordovician fluvio-deltaic quartzite, basinal pelite and psammitic beds that grade upsection into micaceous semipelite and pelite. U–Pb zircon geochronology yields maximum depositional ages between c. 815 and 460 Ma for the GHS in Langtang. Regional variations in the composition and thickness of the GHS along the length of the Himalaya are attributed to siliciclastic depocentres centred on Zanskar in northern India, Langtang and Everest in central to western Nepal, which contrast with coeval marine carbonate shelf deposition in the Annapurna region. The protolith lithostratigraphy documented for Langtang provides a coherent framework for interpreting subsequent Cenozoic Himalayan deformation, specifically the homogeneously distributed layer-normal shortening (i.e. flattening) and layer-parallel stretching (i.e. transport-parallel stretching) that characterizes the GHS. Within the context of a single protracted northern Indian marginal sedimentary succession, the distinction between the Lesser, Greater and Tethyan Himalaya is structural rather than lithostratigraphic in origin.

ABSTRACT The Nordaustlandet terrane of Svalbard plays a critical role in evaluating strike-slip displacements in the Caledonian orogen. Comparison of Silurian and Tonian magmatism in Nordaustlandet, East Greenland, and the Pearya terrane on Ellesmere Island provides a means to evaluate models of large-scale versus minimal displacements. Augen gneiss with an emplacement age of 972 ± 5 Ma demonstrates that the Tonian granite suite is coeval with the calc-alkaline Kap Hansteen volcanic rocks. Zircon from Silurian leucosomes, leucogranites, and granites is dominated by xenocrystic components, making it analytically difficult to isolate magmatic versus inherited age domains. Zircon systematics from a relatively undeformed Silurian granite (431 ± 5 Ma) resemble those of similar granites interpreted to be Tonian in age (e.g., Kontaktberget granite). Assuming less deformed granites of Nordaustlandet are Silurian, synemplacement or syntectonic deformation of Tonian augen gneiss and volcanic rocks is no longer required. Tonian magmatic rocks of Svalbard share a common origin with basement rocks of the Pearya terrane within a continental arc system, but are distinctly older than Tonian igneous and metamorphic rocks of East Greenland. Migmatite complexes and granite intrusions in Nordaustlandet, with ages ranging from 440 to 425 Ma, are coeval with granites in East Greenland that record the combined effects of subduction beneath Laurentia and mid-crustal melting. Migmatites in East Greenland are juxtaposed with low grade rocks by syn-contraction normal faults whereas migmatites show gradational contacts into lower grade rocks on Nordaustlandet. These differences in basement age and structural setting preclude proximity of the Nordaustlandet terrane with East Greenland during the 440–400 Ma continent-continent collision phase (Scandian) of the Caledonian orogen. Similarly, differences in depositional, magmatic, and metamorphic history between the Pearya terrane basement and Nordaustlandet terrane argue against simple offset of crustal fragments. The Pearya and Nordaustlandet terranes likely were not involved in the main phase of crustal thickening directly related to collision of Baltica and Laurentia, but rather resided on a convergent boundary north of the Scandian continent-continent collision zone, consistent with models of Gee and Teben’kov (2004) and Johansson et al. (2005).