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The paper is devoted to the integrated analysis of the interdisciplinary problem of the genesis of hydrocarbon potential in suprasubduction provinces. A geodynamic model is proposed for the formation of petroleum pools on the shelves of the Meso-Cenozoic back-arc basins in the western Pacific Ocean. Original data on the tectonics and composition of the mantle peridotites of the Polar Ural ophiolite complexes are given for comparison. These data are used to consider the low-temperature lizardite-chrysotile and high-temperature antigorite types of peridotite serpentinization in the back-arc basins and in the mantle wedges above the subduction zones of the paleozoic Paleouralian ocean. It is established that these processes are responsible for the abiogenic synthesis of hydrogen and methane. Of great theoretical and applied importance are the conclusions concerning the high antiquity of serpentinization as a hydrogen source, the possible relationships of the origin of life and its early evolution with hydrogen emission in the benthal, the predominance of chemoautotrophic prokaryotes with hydrogen exchange among the primary producers of organic matter in anaerobic conditions, and the high probability of naphthidogenesis based on the prokaryotic communities in the Archean and Proterozoic. The origin of hydrocarbons in the framework of the considered model is regarded from a viewpoint of a polygenetic paradigm of oil and gas generation. [ABSTRACT FROM AUTHOR]

Abstract: Extension within the southern Mariana margin occurs both normal to and parallel to the trench. Trench‐normal extension takes place along focused and broad backarc spreading axes forming crust that is passively accreted to the rigid Philippine Sea plate flank to the northwest. To the southeast, trench‐parallel extension has split apart the Eocene‐Miocene forearc terrain accreting new crust diffusely over a 150–200 km wide zone forming a pervasive volcano‐tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates active extension over this forearc region and basement samples date young although waning volcanic activity. Such diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that in a near‐trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid‐ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting. [ABSTRACT FROM AUTHOR]

Abstract: A number of criteria considered diagnostic of double saloon door rifting and seafloor spreading are matched by data from the Japanese Arc. These include: [i)] a pair of terranes, SW and NE Honshu, which rotated in opposite directions from 22–21 Ma to 14–11 Ma; [ii)] rotated terranes which comprise a retro-arc fold/thrust belt attached to an accretionary wedge intruded by a magmatic arc; [iii)] contemporaneous backarc extension from 24 to 21 Ma which is brought to a halt by progressive collision of the Izu-Bonin and Japan Arcs from 15 to 5 Ma; [iv)] isolation of blocks of thicker continental crust by areas of thin continental or oceanic crust, during backarc rifting; [v)] such isolation may be due to simultaneous rifting or to progressively seaward rifts, associated with ridge jumps towards the subduction zone; [vi)] opposite rotations are accommodated by subduction rollback demonstrated by seaward migration of the volcanic front from 30–26 Ma to 16–15 Ma; [vii)] concurrent development of a major arc-orthogonal rift, the Fossa Magna, from 23–18 Ma to 14 Ma, which was thereafter inverted from 15 Ma to the Recent; [viii)] a northeast propagating rift in the northern Japan Basin demonstrated by the relationship of linear magnetic anomalies to the mapped continent ocean boundary. Driving mechanisms for double saloon door tectonics are discussed in relation to various reconstructions of the northwest Pacific. Opposite rotational torques, leading to opposite terrane rotations, may be caused by rollback of a curved trench hingeline, or by the divergent slab sinking forces of the Pacific and Philippine Sea Plates. [ABSTRACT FROM AUTHOR]

Abstract: The Lau Basin and Havre Trough are back-arc basins related to Pacific–Australian plate convergence. Seafloor spreading occurs in the Lau Basin whereas the Havre Trough is in a rifting stage. At present, the spreading propagator''s tip lies at the southern end of the Valu Fa Ridge (VFR) at 22°40′S. Studying this propagation process provides an opportunity to characterize the evolution of rifting to the initiation of seafloor spreading which is fundamental to back-arc basin development. New geophysical data of the southern Lau Basin reveals that as spreading propagates south, it evolves in a discrete style south of 22°40′S. The propagation axis lies along the eastern margin of the basin, where the well defined, linear VFR loses its identifying morphology. Topography in this eastern zone is characterized by grabens separated by short narrow ridges. High backscatter intensity indicates tectonic and magmatic activity in this eastern area. Mantle Bouguer anomalies (MBA) increase southwards from the VFR to form an elevated MBA area extending west from the currently active area. This indicates eastward migration of active rifting, during which the arc crust was extremely thinned. High magnetization is observed in a left-stepping pattern south of the VFR. We interpret this pattern as discrete segments that characterize the initiation of the spreading stage. There is no evidence of a single, continuous spreading axis like that which characterizes the central and northern Lau Basin. The magnetization highs are discrete and are observed in areas where deformation and magmatism are focused. They are offset relative to the VFR, though they generally follow the same north–south trend as the VFR. [Copyright &y& Elsevier]

Abstract: The formation of the Tyrrhenian–Apennines system has been characterized by a complex distribution of major tectonic events, such as opening of basins, migration and distortion of orogenic belts, retreat of trench zones, stopping of consuming processes and starting of new subductions. The various aspects of each event, such as timing, location, dimension and deformation style, impose important constraints on the genetic mechanism. In this work we argue that the numerous constraints implied by all events together may be plausibly and coherently explained as effects of the convergence of the confining Africa, Arabia and Eurasia plates. Crucial evidence for discriminating among the various geodynamic hypotheses so far advanced is provided by the structural/tectonic information recently provided by CROP seismic sections in the study area. The proposed evolutionary history is illustrated by several paleogeographic maps, which provide a more detailed reconstruction than previous attempts. Particular attention is paid to describing the tectonic setting developed since the middle Pleistocene in the Apennines belt and its connection with seismic activity. [Copyright &y& Elsevier]

Dynamic topography provides a measure of stresses within the Earth's interior. Dense slabs induce an upper mantle flow that deflects the surface of the Earth downward above them. By combining a simple theoretical (Stokeslet) model of subduction, gravity modeling, and seismic tomography, I show that a significant fraction (as much as 2000 m) of the topographic variations observed above the Scotia, Mariana, and Hellenic subduction systems appears to be dynamically induced by stresses related to the underlying subduction. [ABSTRACT FROM AUTHOR]

Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best-exposed, longest-lived (160 my), least-tectonized and least-metamorphosed convergent-margin basin complexes in the world. This convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins: a progression from highly extensional (phase 1) through mildly extensional (phase 2) to compressional (phase 3) strain regimes. This trend is largely due to the progressively decreasing age of lithosphere that is subducted, which causes a gradual decrease in slab dip angle (and concomitant increase in coupling between lower and upper plates), as well as progressive inboard migration of the arc axis.This paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin. Phase 1 consists of Late Triassic to Late Jurassic oceanic intra-arc to backarc basins that were isolated from continental sediment sources. New, progressively widening basins were created by arc rifting and sea floor spreading, and these were largely filled with progradational backarc arc-apron deposits that record the growth of adjacent volcanoes up to and above sea level. Inboard migration of the backarc spreading center ultimately results in renewed arc rifting, producing an influx of silicic pyroclastics to the backarc basin. Rifting succeeds in conversion of the active backarc basin into a remnant backarc basin, which is blanketed by epiclastic sands.Phase 1 oceanic arc–backarc terranes were amalgamated by Late Jurassic sinistral strike slip faults. They form the forearc substrate for phase 2, indicating inboard migration of the arc axis due to decrease in slab dip. Phase 2 consists of Early Cretaceous extensional fringing arc basins adjacent to a continent. Phase 2 forearc basins consist of grabens that stepped downward toward the trench, filled with coarse-grained slope apron deposits. Phase 2 intra-arc basins show a cycle of (1) arc extension, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions, followed by (2) arc rifting, characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites. This extensional-rifting cycle was followed by mid-Cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent, caused by a decrease in slab dip as well as a possible increase in convergence rate.Phase 2 fringing arc terranes form the substrate for phase 3, which consists of a Late Cretaceous high-standing, compressional continental arc that migrated inboard with time. Strongly coupled subduction resulted in accretion of blueschist metamorphic rocks, with development of a broad residual forearc basin behind the growing accretionary wedge, and development of extensional forearc (trench–slope) basins atop the gravitationally collapsing accretionary wedge. Inboard of this, ongoing phase 3 strongly coupled subduction, together with oblique convergence, resulted in development of forearc strike-slip basins upon arc basement.The modern Earth is strongly biased toward long-lived arc–trench systems, which are compressional; therefore, evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California. This convergent margin is typical of many others, where the early to middle stages of convergence (phases 1 and 2) create nonsubductable arc–ophiolite terranes (and their basin fills) in the upper plate. These become accreted to the continental margin in the late stage of convergence (phase 3), resulting in significant continental growth. [Copyright &y& Elsevier]