ABSTRACT Recent seismic reflection data across the Japan Trench show that frontal accretion involves offscraping sediments on top of horsts and scooping-up sediment from grabens. However, seismic profiling does not illuminate the structure within the accretionary prism, and thus the processes of accretion and prism growth are unknown. Key data from scientific drilling at Integrated Ocean Drilling Program Site C0019 that penetrated the prism in the region of large displacement during the 2011 Tohoku earthquake support a model in which frontal accretion occurs by imbricate thrusting, folding, and stacking of thrust sheets that are composed of semicoherent-sediment strata. Using palinspastic restoration techniques, we conclude that out-of-sequence thrusting and duplex development during the underthrusting of horsts can form and displace hanging-wall ramps along the plate-boundary detachment, which helps to explain the formation of some unexpected tectonostratigraphic relations at C0019, such as the emplacement of a thick section of the youngest sediments at the base of the accretionary prism, and numerous juxtapositions of different-age sediments within the basal plate-boundary fault zone.

The Triassic Qiangtang metamorphic belt in central Tibet consists of eclogite- and blueschist-bearing tectonic mélange exposed in an intracontinental setting. Study of the belt has yielded insight about the tectonic history and crustal architecture of the Qiangtang terrane. Weakly deformed mafic blocks exhibiting high-pressure, low-temperature metamorphic mineral assemblages are exposed within a greenschist facies sedimentary-matrix mélange over an east-west distance of ~600 km and north-south distance of ~150 km. Everywhere it has been mapped, the Qiangtang mélange is exposed in the footwalls of Late Triassic–Early Jurassic domal, low-angle normal faults. The hanging walls of these normal faults are composed of late Paleozoic to early Mesozoic metasedimentary and crystalline rocks; similar lithologies are exposed in footwall mélange rocks. High-pressure metamorphism occurred during Middle Triassic time. Regional geological mapping and provenance of detrital zircon crystals from late Paleozoic to Mesozoic supracrustal rocks of the Qiangtang terrane indicate that Qiangtang crust exposed on the north and south sides of the metamorphic belt are of Gondwanan affinity. This is at odds with the widely held belief that the Qiangtang metamorphic belt marks a suture between a Gondwanan affinity southern and/ or western Qiangtang terrane and a Cathayasian affinity northern and/or eastern Qiangtang terrane. Furthermore, metasedimentary rocks exposed within the metamorphic belt yield detrital zircon age probability distributions consistent with derivation from Qiangtang terrane supracrustal strata and Paleo-Tethys affinity rocks exposed north of the Qiangtang terrane. These data, along with the significant north-south surface exposure of the metamorphic belt, suggest that a large part of the lower to middle crust is composed of silica-rich metasedimentary-dominated mélange rock that formed during Middle Triassic southward subduction along the Jinsha suture on the northern edge of the Qiangtang terrane. The implied crustal structure, which is also defined by geophysical experiments, provides an explanation for the apparent deviation from Airy isostasy from the Lhasa terrane to the Qiangtang terrane. We suggest that the replacement or partial incorporation of mafic crystalline middle to lower crust with less dense metasedimentary mélange rock led to a significant contribution of Pratt isostasy across central Tibet.

Abstract This paper summarizes the current knowledge on the nature, kinematics and timing of movement along major tectonic boundaries in the Bohemian Massif and demonstrates how the Variscan plutonism and deformation evolved in space and time. Four main episodes are recognized: (1) Late Devonian–early Carboniferous subduction and continental underthrusting of the Saxothuringian Unit beneath the Teplá–Barrandian Unit resulted in the orogen-perpendicular shortening and growth of an inboard magmatic arc during c. 354–346 Ma; (2) the subduction-driven shortening was replaced by collapse of the Teplá–Barrandian upper crust, exhumation of the high-grade (Moldanubian) core of the orogen at c. 346–337 Ma and by dextral strike-slip along orogen-perpendicular NW–SE shear zones; (3) following closure of a Rhenohercynian Ocean basin, the Brunia microplate was underthrust beneath the eastern flank of the Saxothuringian/Teplá–Barrandian/Moldanubian ‘assemblage’; this process commenced at c. 346 Ma in the NE and ceased at c. 335 Ma in the SW; and (4) late readjustments within the amalgamated Bohemian Massif included crustal exhumation and mainly S-type granite plutonism along the edge of the Brunia indentor at c. 330–327 Ma, and peripheral tectonothermal activity driven by strike-slip faulting and possibly mantle delamination around the consolidated Bohemian Massif's interior until late Carboniferous–earliest Permian times.

Abstract This field guide describes a two-and-one-half day transect, from east to west across southern California, from the Colorado River to the San Andreas fault. Recent geochronologic results for rocks along the transect indicate the spatial and temporal relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-and-thrust belt, and continues westward and structurally downward into the Triassic to Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.

Geophysical and geological observations document that beneath the submerged forearc, processes of sediment subduction and subduction erosion move large volumes of material toward the mantle. The conveying system is the subduction channel separating the upper plate from the underthrusting ocean plate. Globally, the zero-porosity or solid-volume rate at which continental debris is shuttled toward the mantle is estimated to be ∼2.5 km 3 /yr. To deliver this volume, the average thickness of the subduction channel is ∼1.0 km. Some deeply subducted material is returned to the surface of Earth as a component of arc magma or as tracks of high- P/T crustal underplates. But over long periods of time (>50 m.y.), most of the removed material is evidently recycled to the mantle. Applying Cenozoic recycling rates to the past astonishingly implies that since 2.5 Ga a volume of continental crust equal to the standing inventory of ∼6 × 10 9 km 3 has been removed from the surface of Earth. This minimum estimate does not include crustal material recycled at continental collision zones nor reliable estimates of recycling where large accretionary bodies form. The volume of demolished crust is so large that recycling must have been a major factor determining the areal pattern and age distribution of continental crust. The small areal exposure of Archean rock is thus probably more a consequence of long-term crustal survival than the volume originally produced. Reconstruction of older supercontinents is made difficult if not unachievable by the progressive truncation of continental edges effected by subduction zone recycling, in particular by subduction erosion.