Terrane

About: Terrane is a research topic. Over the lifetime, 11025 publications have been published within this topic receiving 442596 citations. The topic is also known as: tectonostratigraphic terrane.

Zircon Inheritance Reveals Exceptionally Fast Crustal Magma Generation Processes in Central Iberia during the Cambro-Ordovician

Abstract: The Variscan basement of the Central Iberian Zone contains abundant Cambro-Ordovician calc-alkaline to peraluminous metagranites and metavolcanic rocks with two notable features: first, they were apparently produced with no connection to any major tectonic or metamorphic event; second, they have an unusually high zircon inheritance. U^Pb dating combined with cathodoluminescence imaging reveals that about 70^80%, in some samples nearer 100%, of the zircon grains contain inherited pre-magmatic cores, despite the temperature reached by the magmas (about 9008C, calculated using the Ti-in-zircon thermometer) being high enough to dissolve all the available zircon (from the rock’s zircon saturation temperature, 770^8608C). The fact that the dissolution of zircon was so incomplete can only be attributed to the kinetics of heat transfer to and from the magmas. Three-dimensional modeling of zircon dissolution behavior in melts with a composition similar to the Iberian Cambro-Ordovician magmas indicates that the survival of zircons from the suggested late Pan-African protolith would be possible only if melt production was rapid, specifically less than 10 4 years, and probably about 2 � 10 3 years, from the beginning of melting (7008C) to the thermal peak (9008C). Melt production was followed by fast magma transfer to upper crustal levels resulting either in surface eruption or in the emplacement of small (5400 m thick) sills or laccoliths. We suggest that these elevated rates of crustal melting could only have been caused by intrusion of mantlederived mafic magmas, most probably at the base of the crust. This scenario is consistent with a rifting regime in which crust and mantle were mechanically decoupled; this would explain the scarcity of contemporaneous crustal deformation. Furthermore, fast melting rates in the lower crust followed by fast melt transportation to the upper crust could also explain the lack of contemporaneous metamorphism. The speed of the partial melting process resulted in the production of felsic magmas that inherited the geochemical characteristics of their granitoid crustal protolith. This explains the apparent contradiction between the calc-alkaline to peraluminous geochemical characteristics of the magmas and the inferred extensional (i.e. rift-related) tectonic setting. Our model is compatible with the hypothesis of fragmentation and dispersal of terranes from the northern margin of Gondwana that led to the opening of the Rheic and Galicia^South Brittany oceans and, ultimately, caused the detachment of the Iberian microplate from Armorica and Gondwana during the early Paleozoic.

INDEPTH III seismic data: From surface observations to deep crustal processes in Tibet

Abstract: [1] During the summer of 1998, active-source seismic data were collected along a transect running 400 km NNW-SSE across the central Tibetan Plateau as the third phase of project INDEPTH (International Deep Profiling of Tibet and the Himalaya). The transect extends northward from the central Lhasa block, across the Jurassic Bangong-Nujiang Suture (BNS) at 89.5°E, to the central Qiangtang block. A seismic velocity model for the transect to ∼25 km depth produced by inversion of P wave first arrivals on ∼3000 traces shows (1) a ∼50-km-wide region of low velocity (at least 5% less than surrounding velocities) extending to the base of the model at the BNS; (2) sedimentary cover for the southern Qiangtang block that is ∼3.5 km thick; (3) a distinct interface between sedimentary cover and Qiangtang basement or underplated Jurassic melange in the central Qiangtang block; and (4) evidence that the Bangoin granite extends to a depth of at least 15 km. The BNS has little geophysical signature, and appears unrelated to the ∼5 km northward shallowing of the Moho which is associated with the BNS in central Tibet. Geophysical data along the main INDEPTH III transect show little evidence for widespread crustal fluids, in contrast to the seismic “bright spots” found in southern Tibet and to magnetotelluric evidence of fluid accumulations in eastern Tibet. A comparison between the global average and Tibetan velocity-depth functions offers constraints for models of plateau uplift and crustal thickening. Taken together with the weak geophysical signature of the BNS, these velocity-depth functions suggest that convergence has been accommodated largely through pure-shear thickening accompanied by removal of lower crustal material by lateral escape, likely via ductile flow. Although we cannot resolve the details, we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of Tibet as a series of accreted terranes.

Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc

Abstract: Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Shelburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.

The blueschist-bearing Qiangtang metamorphic belt (northern Tibet, China) as an in situ suture zone: Evidence from geochemical comparison with the Jinsa suture

Abstract: Metasiliciclastic rocks and metabasalts from the blueschist-bearing Qiangtang metamorphic belt and the Jinsa suture zone were analyzed for major and trace elements in an attempt to evaluate the affinities of these two tectonic entities. Tholeiitic mid-oceanic-ridge basalts (MORBs) from the Jinsa suture can be distinguished from metabasalts of Qiangtang, which have alkalic compositions and exhibit a range of characteristics typical of many within-plate oceanic islands. The Qiangtang metasiliciclastic rocks were derived from a passive continental margin source, whereas those from the Jinsa suture zone were sourced from a continental island arc or an active continental margin source. The geochemical distinction of metasiliciclastic rocks and metabasalts of the Qiangtang metamorphic belt from their counterparts within the Jinsa suture indicates that there is no affinity between these two tectonic entities and that the Qiangtang metamorphic belt could not have been underthrust from the Jinsa suture. It most likely represents an exhumed accretionary complex composed of sediments derived from a passive continental margin and fragments of seamount sequences, and marks an in situ suture zone that separates northern and southern Qiangtang terranes.