Thermal Imprint Research Articles

Off‐scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc

The Eocene Kayo Formation exposed at the southwestern end of the Shimanto Belt in the Ryukyu Arc records the progressive development of off‐scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint. The formation consists almost wholly of coherent trench turbidites supplied by axial currents. Detailed structural analysis has revealed that two successive deformation stages are recognized in the Formation. The early deformation (stage 1) occurred by horizontal shortening related to the initial stage of off‐scraping accretion. In this stage, sands were water‐rich and nearly unlithified, and the competence contrast between sandstone and shale was low. The late deformation (stage 2) is marked by the formation of seaward verging imbricate fold‐and‐thrust system within the accretionary wedge. Compared with stage 1, sands in stage 2 were more dewatered and lithified, and the competence contrast between sandstone and shale increased. In accordance with this deformation history, structures caused by high pore fluid pressure indicate increased cohesion of sands during off‐scraping accretion. Studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the Formation became buried and suffered thermal imprint; pressure solution cleavage developed as an axial planar to the stage 2 folds. Thermal imprint on the Formation is characterized by high paleotemperatures at approximately 250°C–300°C in a region of the prehnite‐pumpellyite facies metamorphism. This thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate.

Thermal and geodynamic setting of the Buem volcanic rocks near Tiélé, Northwest Bénin, West Africa

The mafic volcanic rocks of Tiélé occur as intercalations within very low-grade metasediments, mainly metapelites and meta-arenites. These Proterozoic supracrustal rocks are considered as part of the Buem Structural Unit (BSU) which is the most external tectonic unit of the Pan-African Dahomeyide fold belt. The geodynamic significance of the BSU is still debated: it could represent either a portion of the West-African passive margin during the rifting and the drifting of the Pan-African ocean (Affaton, 1990), or a continental rift-zone (Jones, 1990) or the Pan-African oceanic crust itself (Burke and Dewey, 1972 and Burke and Dewey, 1973; Shackleton, 1976). The mafic lavas derived from tholeiitic magmas, which originated by partial melting of the lithospheric mantle with a possible asthenospheric contribution. They are quite similar to MORBs and free of any crustal contamination. They are related to a strongly attenuated continental lithosphere, and a passive margin would be the most likely tectonic environment for the coeval volcanic and sedimentary activities. On the other hand, these rocks demonstrate effects of early static recrystallizations related to a very low grade metamorphism of the prehnite-pumpellyite facies developed under temperatures of 200 to 300°C. Such a thermal imprint clearly marks an extensional tectonic stage. Later, the early parageneses were re-equilibrated under slightly higher pressures during the Pan-African collision. Both magmatic features and early metamorphic characters of these metavolcanic rocks are in good agreement with a context of lithospheric thinning characterized by mantle-derived magmatism and high thermal-gradient. These results fit with previous interpretations mainly based on sedimentary and structural features: the BSU represents a portion of a Proterozoic passive margin which suffered strong lithospheric stretching, with deep seated normal faults and mantle derived magmatism, and a subsequent severe subsidence evidenced by the prominent detrital sedimentation. Such a tectonic setting developed during the Pan-African oceanic drifting

Dating the shock wave and thermal imprint of the giant Vredefort impact, South Africa

U-Pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded Vredefort impact structure of South Africa. Results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 ± 3 Ma. Discovery of a synimpact to postimpact dike of norite that intruded at 2019 ± 2 Ma is the first recognition of mafic igneous activity related to impact. The dike is either derived from a Sudbury-type impact melt layer (since eroded) or is the product of decompression melting of Kaapvaal mantle in response to the ablation of >15 km of crust at the center of the crater. The combined heating effects of the shock wave and impact-triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with Kaapvaal craton, and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study.