Abstract Tephra beds are considered to be potential failure planes for submarine landslides. Here, we report on an example of a coarse-ash/lapilli-tuff bed influencing translational slides. The studied mass-transport deposit (MTD) is intercalated in the Pleistocene forearc basin fill exposed in east-central Japan. This MTD consists of stacked siltstone blocks resulting from repeated imbricate thrusts branching from the décollement. The basal slide plane is located immediately below a pumice-rich coarse ash/lapilli-tuff bed. The material comprising the slip zone is injected into the overlying coarse-ash/lapilli-tuff bed, suggesting an upwards escape of excess porewater that resulted from elevated pore pressure. To explain this mode of occurrence, we propose that the detachment preferentially occurred at the top and base of the coarse-ash-tuff-rich interval which appears to have been stronger relative to the adjacent silt-dominated interval. The pumiceous coarse-ash and lapilli-tuff bed behaved as a rigid plate on top of the high-pore-pressure slip zone, which sustained the translational slide on the gentle continental slope. Therefore, in translational submarine landslides, the preferential formation of a slide plane is caused by differing frictional resistances in the layered sediments.

Abstract The overwhelming documentation of coarse-grained alluvial fans dominated by mass flow contrasts with the scarce accounts of finer grained, traction-dominated alluvial fans. To fill this gap, we present sedimentological and architectural data from a set of sand-rich, streamflow-dominated Pleistocene fans flanking the eastern Upper Valdarno Basin, Italy. The routing of sand-rich sediment resulted from the fast, intense weathering of the feldspar-rich, carbonate-deprived sandstone bedrock underlying the fan catchments. Although capable of entraining large boulders, high water discharge sustained tractional reworking along the proximal facies belts, hindering mass flow. The medial facies belts have a channelized, braided planform and are dominated by processes hardly distinguishable from those characterizing fluvial environments. Along the distal facies belts, extensive overbank tracts are composed of terminal splays and crevasse lobes, the spatial arrangement of which controlled the evolution of through-going tributary channels connected to the axial basin drainage. This study focused on the sedimentary processes, stratal architecture and morphodynamics of the alluvial fans and considered the effect of bedrock inheritance on their development. The results underline how granulometry and mass flow dominance are not distinctive of alluvial fan sedimentation per se and indicate how the critical detection of piedmont, radial palaeomorphology is crucial in the identification of ancient alluvial fans.

The Upper Cretaceous to Paleocene(?) Canaan Peak Formation of southwestern Utah consists of approximately 100 m of cobble conglomerate and subordinate sandstone. Coarse-grained lithofacies include massive to very crudely stratified pebble to cobble conglomerate (Gm) and trough cross-stratified conglomerate (Gt). Minor associated lithofacies include trough (St) and planar (Sp) cross-stratified sandstone and scour-fill sandstone (Ss). Gravel deposition occurred during high-discharge periods within a Scott/Donjek-type perennial braided fluvial system on longitudinal bars (Gm) and sinuous-crested transverse bars (Gt), and as a product of longitudinal bar-top and interbar channel scour filling (Gt). Sand accumulated under lower flow-velocity conditions through migration of interbar channel dunes and transverse bars (St/Sp) and development of scour and fill deposits (Ss). Clast imbrication and trough-axis orientation measurements indicate east to north-east paleoflow directions. Gravel-sized clasts are predominantly resistant lithologies including Upper Precambrian-Cambrian quartzite, fossiliferous Paleozoic chert, and some silicified middle Jurassic volcanic rocks; less resistant Paleozoic limestone clasts are only locally abundant. Canaan Peak Formation detritus was derived from erosion of highlands created by Cretaceous Sevier-style thrust-fault development to the west in southeastern Nevada and western Utah and distributed across an extensive gravel-dominated braidplain complex. However, it is not possible to determine whether Canaan Peak deposition records active uplift or is postorogenic. Sediment was transported eastward a minimum distance of 45 to 60 km and was subject to continuous high-energy reworking within the Canaan Peak fluvial system, resulting in destruction of all but the most stable clast lithologies.

Pressure-temperature (P-T) paths observed in pelitic schists on either side of the Main Mantle Thrust in northern Pakistan record the dynamics of the collision between the Kohistan Island-Arc and Indian plate. Geothermometry studies, mineral reaction textures, and thermodynamic modeling of zoned garnets suggest that the rocks in the Kohistan Arc and the Nanga Parbat–Haramosh Massif experienced different pressure-temperature histories as a result of imbrication of these two terranes during thrusting. Rocks in the Kohistan Arc followed decreasing pressure-temperature paths, with early garnet growth occurring at high pressures (9.5 kbar) and later garnet growth at lower pressures (8.5 kbar). Conversely, rocks in the Nanga Parbat–Haramosh Massif record an increasing P-T path history. The early P-T history within the massif was at low pressures (4.0 kbar) and low temperatures (450°C). Later, both pressure and temperature increased to a maximum of 7.5 kbar and 580°C. The contrasting P-T paths observed within these two terranes provide evidence for overthrusting of the Kohistan Arc over the Nanga Parbat–Haramosh Massif along the Main Mantle Thrust.

India collided with the northern Kohistan/Asian plate at about 55 Ma. Subsequently, Asia has overridden India, developing a wide range of thrust slices at the top of the Indian plate. Balanced sections in the imbricated sedimentary cover of the Indian plate indicate a minimum displacement of more than 470 km since collision. This requires the Kohistan region to the north to be underlain by underthrusted middle to lower Indian crust, the internal ductile deformation and thickening of which accounts for the main overall crustal thickening beneath Kohistan. In the Besham area of north Pakistan, a stratigraphy can be documented for the northern part of the Indian plate that includes basement sequences of quartzo-feldspathic gneisses of the Besham Group, and of Precambrian schists of the Tanawal Formation intruded by the Swat-Mansehra granite. The basement rocks are unconformably overlain by carbonate-rich Paleozoic sedimentary rocks. Sedimentary rocks of both the basement and cover sequences were metamorphosed at an early stage of the Himalayan deformation during tectonic burial associated with crustal thickening. Structures just south of the suture related to this crustal thickening include a sequence of ductile mylonites thickened by thrust-related folding, a folded thrust stack involving basement rocks imbricated with cover strata, and late cross-folds. Much of the thickening of the Indian plate in the footwall of the Main Mantle Thrust can be related to the necessary changes in thrust wedge shape as it climbs through the crust.

The West African orogens (Mauritanides, Bassarides, Rokelides) record a locally complex, polyphase, tectonothermal evolution. The earliest event corresponded to westward rifting of a continental fragment from the West African craton at ca. 700 Ma. This led to development of a rift-facies lithotectonic succession that included sedimentary units and intracontinental igneous sequences. Rifting must have been of limited extent because a western ensialic arc began to develop as early as 680 Ma. The associated convergence culminated in an episode of folding and metamorphism at ca. 650 Ma (Pan-African I orogenesis) in the Mauritanides and Bassarides. Following widespread deposition of Late Proterozoic glacial and flyschoid sediments, a second tectonothermal event (most penetratively developed in the Rokelides) occurred between ca. 575 and 550 Ma (Pan-African II orogenesis). This may have resulted, in part, from collision of the Guyana craton (South America) and southern portions of the West African craton. The West African orogens were marked by general tectonic quiescence throughout most of early and middle Paleozoic time. Late Paleozoic (ca. 300 Ma) collision of Gondwanaland and Laurentia resulted in eastward translation of previously tectonized Mauritanide units over their foreland, and emplacement at highest structural levels of previously imbricated nappes, which include sequences with poorly defined palinspastic origins. The late Paleozoic (Hercynian) transport was largely intracontinental, and only westernmost portions of the exposed Mauritanides record a penetrative tectonothermal overprint. The polyphase tectonothermal evolution of the West African orogens complicates definition of tectonostratigraphic terranes. Three preliminary and likely composite terranes are outlined. These include: (1) sequences related to the initial Late Proterozoic rifting; (2) ensialic arc sequences and their continental hosts (with uncertain affinity to the West African craton); and, (3) westernmost, variably retrogressed and complexly imbricated continental units with poorly defined palinspastic affinities.