ABSTRACT A Silurian–Devonian metamorphic core complex has recently been recognized in northwest Spitsbergen, on the northwest corner of the Barents Shelf at the junction between the Atlantic and Arctic oceans. The associated Keisarhjelmen detachment, a major, ductile-brittle fault zone, is 200–500 m thick and has a map trace >150 km. A top-to-the-north transport direction is parallel to the axis of a large-scale, shallowly north-plunging, detachment corrugation. This detachment zone separates overlying faulted Silurian–Devonian aged cover strata from underlying migmatitic rocks in the core. The detachment shows a diverse array of fault and metamorphic rocks with structural ascent, ranging from sheared migmatite, mylonite, ultramylonite, foliated cataclasite, pseudotachylite, and breccia. Footwall post-kinematic granitic intrusions occurred shortly prior to, and likely during, deposition of the older cover strata. Variably deformed, syn-kinematic granitic sheets and veins within the detachment zone are considered coeval. Thin sections show significant grain size reduction, porphyroclasts, and well-developed composite fault surfaces. Relict garnet sigma porphyroclasts associated with chlorite and sericite indicate retrogression. Feldspar porphyroclasts show significant sericite alteration, undulose extinction and limited recrystallization low in the detachment, and brittle deformation throughout. Quartz deformation textures and grain size vary considerably within and between samples. Deformation during retrogression continued into the brittle realm with the development of thick foliated cataclasites, fault breccias, and local pseudotachylites concentrated at the top of the detachment. Biotite in particular shows grain size reduction, concentration along C-surfaces, and shredding and redistribution, suggesting it played a significant role in both ductile and brittle faulting. Veins, micro-veins, and fluid inclusion planes are ubiquitous throughout the detachment, indicating substantial fault-related fluid flow. Given existing geochronologic and P-T (pressure-temperature) data from the basement rocks of the area, the kinematics, retrogression, and ductile-brittle transition are consistent with exhumation of a core complex developing by orogen-parallel extension associated with transtension during the Late Silurian and Early to Middle Devonian in northwest Spitsbergen. Remaining questions include how this core complex connects with coeval plate-scale strike-slip faults in Svalbard, and its relationship to mainland Norwegian core complexes and Devonian basins to the south.

ABSTRACT Field, microtextural, and geochemical evidence from impact-related melt rocks at the Manicouagan structure, Québec, Canada, allows the distinction to be made between friction-generated (pseudotachylite) and shock-generated melts. Making this distinction is aided by the observation that a significant portion of the impact structure’s central peak is composed of anorthosite that was not substantially involved in the production of impact melt. The anorthosite contrasts with the ultrabasic, basic, intermediate, and acidic gneisses that were consumed by decompression melting of the >60 GPa portion of the target volume to form the main impact melt body. The anorthosite was located below this melted volume at the time of shock loading and decompression, and it was subsequently brought to the surface from 7–10 km depth during the modification stage. Slip systems (faults) within the anorthosite that facilitated its elevation and collapse are occupied by pseudotachylites possessing anorthositic compositions. The Manicouagan pseudotachylites were not shock generated; however, precursor fracture-fault systems may have been initiated or reactivated by shock wave passage, with subsequent tectonic displacement and associated frictional melting occurring after shock loading and rarefaction. Pseudotachylites may inject off their generation planes to form complex intrusive systems that are connected to, but are spatially separated from, their source horizons. Comparisons are made between friction and shock melts from Manicouagan with those developed in the Vredefort and Sudbury impact structures, both of which show similar characteristics. Overall, pseudotachylite has compositions that are more locally derived. Impact melts have compositions reflective of a much larger source volume (and typically more varied source lithology inputs). For the Manicouagan, Vredefort, and Sudbury impact structures, multiple target lithologies were involved in generating their respective main impact melt bodies. Consequently, impact melt and pseudotachylite can be discriminated on compositional grounds, with assistance from field and textural observations. Pseudotachylite and shock-generated impact melt are not the same products, and it is important not to conflate them; each provides valuable insight into different stages of the hypervelocity impact process.

ABSTRACT The early Miocene Markagunt (MGS) and late Oligocene Sevier (SGS) gravity slides in southwestern Utah, USA, exhibit the full range of structural features commonly seen in modern landslides, but on a gigantic scale—they are among Earth’s largest terrestrial landslides. The MGS, discovered in 2013, consists of four distinct structural segments: (1) a high-angle breakaway segment, (2) a bedding-plane segment, (3) a ramp segment where the slide cut up-section and the basal fault “daylighted,” and (4) a former land-surface segment where the upper plate moved at least 35 km over the Miocene landscape. The MGS remained undiscovered for so long precisely because of its gigantic size (>5000 km 2 , >95 km long, estimated volume 3000 km 3 ) and initially confusing mix of extensional, translational, and compressional structures overprinted by post-MGS basin-range tectonism. Preliminary mapping of the SGS, discovered in 2016, shows it to be smaller (>2000 km 2 ) and slightly older than the MGS. Both gravity slides are large contiguous sheets of andesitic lava flows, volcaniclastic rocks, source intrusions, and regional ash-flow tuffs that record southward, gravitationally induced catastrophic failure of the southern flank of the Oligocene to Miocene Marysvale volcanic field. Failure was preceded by slow gravitational spreading accommodated by the Paunsaugunt thrust fault system, which is rooted in Middle Jurassic evaporite-bearing strata at a depth of ~2 km; this thrust system deformed Middle Jurassic through lower Oligocene strata. MGS emplacement is presently constrained between ca. 23 and 21 Ma; SGS emplacement is presently constrained between ca. 25 and 23 Ma. The presence of basal and lateral cataclastic layers, injectites (clastic dikes), pseudotachylyte (frictionite), deformed clasts, and a variety of kinematic indicators suggests that each gravity slide represents a single catastrophic emplacement event from the north to the south; possibly the MGS comprises two gravity slides. The principal zone of failure was in mechanically weak, clay-rich sedimentary strata at the base of the volcanic section. The MGS and SGS are significant because they provide examples of lithified landslide structures so large that they may be mistaken for tectonic features. However, these gravity slides lie at right angles to regional compressional tectonic structures and are cut longitudinally by modern basin-range normal faults, and thus offer compelling case studies for how to differentiate features resulting from surficial verses tectonic processes. Here we offer a history of MGS and SGS discovery, our current understanding of the gravity slides as of late 2018 (which are the focus of ongoing research), and a guide to locations of particularly instructive exposures where we document our conclusions about size, distinctive structural features, emplacement ages, and interpreted emplacement mechanisms.