A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China

On 12 May 2008, a magnitude 7.9 earthquake ruptured the Longmen Shan margin of the eastern Tibetan plateau. This event occurred within the context of long-term uplift and eastward enlargement of the plateau. The area has numerous geological features not typical of active convergent mountain belts, including the presence of a steep mountain front (>4 km relief) but an absence of large-magnitude low-angle thrust faults; young high topography (post ca. 15 Ma) and thickened crust but low global positioning system (GPS) shortening rates (<3 mm/yr); and no coeval foreland subsidence. In our interpretation, crustal thickening beneath the eastern Tibetan plateau occurred without large-scale shortening of the upper crust but instead is caused by ductile thickening of the deep crust in a weak (low-viscosity) layer. Late Cenozoic shortening across the Longmen Shan could be as little as 10-20 km, with folding and faulting mainly accommodating differential surface uplift between the plateau and the Sichuan Basin. The earthquake of 12 May probably reflects long-term uplift with slow convergence and right-slip, of the eastern plateau relative to the Sichuan Basin. GPS-determined rates in the vicinity of the 12 May event suggest an average recurrence interval of ∼2,000-10,000 yr

Crustal development within a retreating subduction system: The Hellenides

In retreating subduction systems, where the subduction rate is faster than the convergence rate between the upper and lower plates, the processes by which the upper plate crust is constructed have not been well understood. From our studies in the Hellenides, which formed above a retreating slab, we conclude that the external part of the Cenozoic Hellenide orogen was constructed from rocks derived from the subducting plate at least at two crustal levels. The upper crustal level within the external Hellenides consists of west-vergent thrust sheets emplaced progressively from east to west along a regional décollement from ca. 35 Ma to present. These thrust sheets consist of Mesozoic and Cenozoic strata that have been stripped from their underlying basement to form the Hellenides. The middle and lower crustal layer consists of slices of continental crust detached from the downgoing slab at depth and accreted below the upper crustal thrust sheets. These accreted slices represent ~35% (or less) of the crust belonging to the subducting lithosphere; the remainder of the crust appears to be subducted with the slab. While the process of slab rollback may be continuous at depth, the episodic detachment of crustal slices guarantees that rollback is step-like in time at the crustal level. As the subducted lithosphere rolled back beneath the Hellenides, it passed progressively from east to west through the region occupied by present-day lower crust and mantle, where there is a well-defined Moho. Any irregularities that may have been present at the base of the accreted slabs have been smoothed by processes that remain to be determined. © 2018 The Authors

Mesozoic sequence of Fuerteventura (Canary Islands): Witness of Early Jurassic sea-floor spreading in the central Atlantic

The Fuerteventura Jurassic sedimentary succession consists of oceanic and elastic deposits, the latter derived from the southwestern Moroccan continental margin. Normal mid-oceanic-ridge basalt (N-MORB) flows and breccias are found at the base of the sequence and witness sea-floor spreading events in the central Atlantic. These basalts were extruded in a postrift environment (post-late Pliensbachian), We propose a Toarcian age for the Atlantic oceanic floor in this region, on the basis of the presence higher up in the sequence of the Bositra buchi filament microfacies (Aalenian-Bajocian) and of elastic deposits reflecting tectono-eustatic events (e.g,, late Toarcian to mid-Callovian erosion of the rift shoulder). The S-l sea-floor oceanic magnetic anomaly west of Fuerteventura is therefore at least Toarcian in age. The remaining sequence records Atlantic-Tethyan basinal facies (e.g., Callovian-Oxfordian red clays, Aptian-Albian black shales) alternating with elastic deposits (e.g., Kimmeridgian-Berriasian periplatform calciturbidites and a Lower Cretaceous deep-sea fan system). The Fuerteventura N-MORB outcrops represent the only Early Jurassic oceanic basement described so far in the central Atlantic. They are covered by a 1600 m, nearly continuous sedimentary sequence which extends to Upper Cretaceous facies

Evolution and dynamics of the Cenozoic tectonics of the South Balkan extensional system

The South Balkan extensional system consists of normal faults and associated sedimentary basins within southern Bulgaria, Macedonia, eastern Albania, northern Greece, and northwestern Turkey. Extensional tectonism began during the final convergence across the Vardar, Intra-Pontide, and Izmir-Ankara suture zones, where oceanic regions closed between continental Europe and continental fragments that make up the Pelagonian, Sakar, and western Anatolian tectonic units. Earliest extension of latest Cretaceous-middle Eocene age appears to have occurred within a regional convergent tectonic setting and may be related to an increase in gravitation potential energy within a thickening continental lithosphere. Following diachronous closure across the suture zone, from the middle Eocene to late Oligocene, the transition from a regionally convergent to a regionally extensional tectonic setting occurred and was associated with abundant magmatism and formation of sedimentary basins. Extension was associated with lithospheric thinning probably related to changes in geometry of the subducted slab, dynamics of the mantle wedge, and beginning of slab rollback along the Hellenic subduction zone. A short period of local and diachronous (?) shortening (during latest Oligocene-early Miocene time) occurred in the Thrace basin of northwestern Turkey and in some basins in western Bulgaria and eastern Macedonia. Regional extension began in middle Miocene time and was related to the regional extensional tectonic setting that has dominated the Aegean extensional region to the present. Trench rollback was the dominant dynamic process, but during late Miocene time it was modified by the formation of the western part of the North Anatolian fault zone that partially decoupled the South Balkan extensional system from the Aegean extensional region. During late Cenozoic time, east-west-striking normal faults and associated sedimentary basins in the eastern part of the South Balkan extensional system propagated westward in tandem with westward migration of north-south-striking normal faults and sedimentary basins from western Bulgaria into eastern Albania. This migration was caused by evolution of the Hellenic subduction zone as it increased its curvature during trench rollback and clockwise and counterclockwise rotation of crustal fragments in the west and east, respectively. After formation of the western part of the North Anatolian fault zone, extension within the eastern part of the South Balkan extensional system was related to southward movement of its lithosphere at a slower rate than the extension within the Aegean extensional region. Active extension and basin formation show two provinces of extension that are nearly at right angles to one another and their overlap in the central South Balkan extensional system: east-west extension in central Albania to eastern Macedonia and north-south extension from northwestern Greece and eastern Macedonia to eastern Bulgaria and northwestern Turkey. © 2008 Geological Society of America