Large peridotite massifs are scattered along the 1500 km length of the Yarlung-Zangbo Suture Zone (southern Tibet, China), the major suture between Asia and Greater India. Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low fO(2) (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO2 II, coesite, possible stishovite). New physical and isotopic (C, N) studies of the diamonds indicate that they are natural, crystallized in a disequilibrium, high-T environment, and spent only a short time at mantle temperatures before exhumation and cooling. These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks. Possible evidence for metamorphism in or near the upper part of the Transition Zone includes the following: (1) chromite (in disseminated, nodular and massive chromitites) containing exsolved pyroxenes and coesite, suggesting inversion from a high-P polymorph of chromite; (2) microstructural studies suggesting that the chromitites recrystallized from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite; (3) a new cubic Mg-silicate, with the space group of ringwoodite but an inverse-spinel structure (all Si in octahedral coordination); (4) harzburgites with coarsely vermicular symplectites of opx + Cr-Al spinel +/- cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to > 13 GPa. Evidence for a shallow pre-metamorphic origin for the chromitites and peridotites includes the following: (1) trace-element data showing that the chromitites are typical of suprasubduction-zone (SSZ) chromitites formed by magma mixing or mingling, consistent with Hf-isotope data from magmatic (375 Ma) zircons in the chromitites; (2) the composition of the new cubic Mg-silicate, which suggests a low-P origin as antigorite, subsequently dehydrated; (3) the peridotites themselves, which carry the trace element signature of metasomatism in an SSZ environment, a signature that must have been imposed before the incorporation of the UHP and low-fO(2) phases. A proposed P-T-t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c. 375 Ma, residence in the upper Transition Zone for > 200 Myr, and rapid exhumation at c. 170-150 Ma or 130-120 Ma. Os-isotope data suggest that the subducted mantle consisted of previously depleted subcontinental lithosphere, dragged down by a subducting oceanic slab. Thermomechanical modeling shows that roll-back of a (much later) subducting slab would produce a high-velocity channelized upwelling that could exhume the buoyant harzburgites (and their chromitites) from the Transition Zone in < 10 Myr. This rapid upwelling, which may explain some characteristics of the diamonds, appears to have brought some massifs to the surface in forearc or back-arc basins, where they provided a basement for oceanic crust. This model can reconcile many apparently contradictory petrological and geological datasets. It also defines an important, previously unrecognized geodynamic process that may have operated along other large suture zones such as the Urals.

/pdf/mantle-recycling-transition-zone-metamorphism-of-tibetan-4xm79elj4r.pdf

Mantle Recycling: Transition Zone Metamorphism of Tibetan Ophiolitic Peridotites and its Tectonic Implications

SUMMARY

We developed an integrated technique for estimating crustal anisotropy with a horizontal axis using radial (R) and transverse (T) receiver functions. The technique includes computing three individual and one joint objective function (JOF) and a reliability analysis of the estimated anisotropy. The individual objective functions (IOFs) are designed to: (1) maximize the peak energy of the stacked R receiver function after a cosine moveout correction in the Ps arrival time; (2) to maximize the correlation of the radial receiver functions after a full correction of anisotropy or (3) to minimize the total energy of transverse receiver functions stacked after a removal of crustal anisotropy. The JOF was computed by a weighted average of the three IOFs, and the reliability analysis uses the principle that stacking coherent signals can lead to an increase of signal-to-noise ratio. We applied the technique to synthetic receiver functions generated with 30–60 per cent white noise from a variety of anisotropic and heterogeneous models. The synthetic tests indicate that the proposed technique has good capability to recover the input models. Despite the presence of random and other coherent noises, such as those caused by inhomogeneous structures, in the data, the technique can always provide accurate estimates of crustal anisotropy. We applied the technique to two permanent seismic stations in western China and found significant crustal anisotropy beneath one station located at the northern edge of the Tibetan plateau. The observed fast polarization direction at this station follows the direction of the maximum horizontal tensile stress, suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust. The station situated in the Sichuan basin, on the other hand, shows little to no seismic anisotropy, which may suggest that the crust beneath the basin is nearly rigid with very little deformation. The developed technique can be applied to any broadband seismic stations that have a good backazimuthal coverage of teleseismic events.

/pdf/estimating-crustal-seismic-anisotropy-with-a-joint-analysis-2d7ilxytan.pdf

Estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data

Slab rupture and delamination under the Betics and Rif constrained from receiver functions

The recent tectonic evolution of the western Mediterranean region is enigmatic. The causes for the closure of the Moroccan marine gateway prior to the Messinian salinity crisis, for the ongoing shortening of the Moroccan Rif and for the origin of the seismogenic Trans-Alboran shear zone and eastern Betics extension are unclear. These puzzling tectonic features cannot be fully explained by subduction of the east-dipping Gibraltar slab in the context of the regional relative plate motion frame. Here we use a combination of geological and geodetic data, as well as three-dimensional numerical modelling of subduction, to show that these unusual tectonic features could be the consequence of slab dragging—the north to north-eastward dragging of the Gibraltar slab by the absolute motion of the African Plate. Comparison of our model results to patterns of deformation in the western Mediterranean constrained by geological and geodetic data confirm that slab dragging provides a plausible mechanism for the observed deformation. Our results imply that the impact of absolute plate motion on subduction is identifiable from crustal observations. Identifying such signatures elsewhere may improve the mantle reference frame and provide insights on subduction evolution and associated crustal deformation.

Puzzling features of western Mediterranean tectonics explained by slab dragging

Spatial and temporal radiogenic isotopic trends of magmatism in Cordilleran orogens

Seismic images of the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region show that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins, driving surface tectonics and pre-conditioning the margins for further deformation. Alan Levander and co-authors present seismic images that show larger than expected volumes of high-seismic-velocity mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region. The anomalous regions are under and aligned with the continental margins at depths greater than 200 km, with the lithospheric mantle under the continental margins significantly thinner than expected. The authors conclude that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins, driving surface tectonics and preconditioning the margins for further deformation by creating topography along the lithosphere–asthenosphere boundary. Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood1. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts2,3,4,5. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region6,7; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis8,9 finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere–asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc8,10, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc11. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones12,13,14,15,16,17.

/pdf/subduction-driven-recycling-of-continental-margin-p1y909sfg4.pdf

Subduction-driven recycling of continental margin lithosphere

https://www.gseis.rice.edu/Reprints/R067_MasyEL11EPSL.pdf

J. Masy

Papers

Subduction-driven recycling of continental margin lithosphere

Mantle flow beneath northwestern Venezuela: Seismic evidence for a deep origin of the Mérida Andes

Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela

Seismic anisotropy and mantle flow beneath western Venezuela

Seismic imaging of the Upper Mantle structure and dynamics beneath the Southern Caribbean plate boundary and Venezuela