Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite

Nature Communications, Aug 2018

Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the mantle to an unusual depth of ~80 km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but the triggering mechanisms remain enigmatic. Here we show that dry granulite rocks, the dominant constituent of the subducted Indian crust, become brittle when deformed under conditions corresponding to the eclogite stability field. Microfractures propagate dynamically, producing acoustic emission, a laboratory analog of earthquakes, leading to macroscopic faults. Failed specimens are characterized by weak reaction bands consisting of nanometric products of the metamorphic reaction. Assisted by brittle intra-granular ruptures, the reaction bands develop into shear bands which self-organize to form macroscopic Riedel-like fault zones. These results provide a viable mechanism for deep seismicity with additional constraints on orogenic processes in Tibet.

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Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite

Abstract Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the mantle to an unusual depth of ~80 km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but the triggering mechanisms remain enigmatic. Here we show that dry granulite rocks, the dominant constituent of the subducted Indian crust, become brittle when deformed under conditions corresponding to the eclogite stability field. Microfractures propagate dynamically, producing acoustic emission, a laboratory analog of earthquakes, leading to macroscopic faults. Failed specimens are characterized by weak reaction bands consisting of nanometric products of the metamorphic reaction. Assisted by brittle intra-granular ruptures, the reaction bands develop into shear bands which self-organize to form macroscopic Riedel-like fault zones. These results provide a viable mechanism for deep seismicity with additional constraints on orogenic processes in Tibet. Introduction Tibetan Plateau, the largest mountain range on Earth, is formed by the collision between Indian and Eurasian continents since ~50 million years ago, resulting in significant deformation in central and eastern Asia with a remarkably thickened crust in southern Tibet (Fig. 1a)1,2,3 and pushing the Mohorovičić seismic discontinuity (Moho) to ~80 km deep1,4,5,6. The underlying geodynamic process has been under intense debate for decades, a central issue being the mechanical nature of subducted Indian crust and the underlying upper mantle3,7,8. Earlier seismic studies report bimodal depth distributions of seismicity, interpreted as an aseismic and weak lower crust sandwiched between the seismogenic upper crust and the upper mantle9. Contrary to this “jelly sandwich” model, the “crème brûlée” model argues for strong upper and lower crust overlying a weak upper mantle10. The two models have drastically different implications in thermal structure, mass transfer, water storage, and melt distribution in the lower crust and upper mantle. While the “jelly sandwich” model requires water and/or partial melt enrichment7,11, suggesting lateral channel flow of materials within the mid-crust to compensate the extra surface load imposed by the Himalaya mountain range12, the “crème brûlée” model suggests compensating flows occurring primarily in the upper mantle13. Fig. 1 Seismicity in southern Tibet and the relation to granulite-eclogite metamorphic reaction. a A map view of the area of interest with earthquake locations from the EHB Bulletin (http://www.isc.ac.uk/ehbbulletin/) (relocated using teleseismic data) and from relocation14 using the Himalaya Napel Tibet Seismic Experiment18 data. b A schematic cross-section view normal to the arc of the Main Central Thrust with earthquake hypocenters shown in a. The blue dashed line marks the approximate boundary between the Tibetan crust (above) and the subducted Indian crust (below). The range of the Moho depths is indicated by the greenish band, whose upper, and lower bounds are from14 and57, respectively. “873 K isotherm” from thermokinematic modeling57 and the “30% eclogitization” region based on received function models32 are indicated. c A schematic phase diagram of granulite-facies rocks in relation with other facies (modified from70). Estimated P-T conditions of xenolith samples are given as the following: eclogite—red rectangle36 and red circle37; granulite—green squares31. Experimental conditions are given as color-coded stars. Red symbols are in eclogite field and green in granulite field Full size image Seismicity is the most direct indicator of mechanical state of Earth’s interior. Most recent studies show that seismicity distribution in southern Tibet is more or less continuous from the surface to as deep as 100 km (Fig. 1b)14,15,16,17. While the debate on crust and mantle strength continues [e.g., see reviews in11,18], one must address another crucial issue, that is, how can earthquakes occur in the lower crust and upper mantle, which are well below the brittle-ductile transition depth19, with plastic yield strength of rocks well below that of Byerlee’s friction20. As far as the lower crust is concerned, three hypotheses have been proposed. The first is a thermal runaway process where a self-amplifying mechanical instability arises from the combination of shear localization and grain-size reduction within a visco-plastic material21. Crucial issue here is the conditions under which shear localizations nucleate and self-amplify, leading to failure22. Such conditions are poorly known and just begun to be investigated in the laboratory23. The second is dehydration embrittlement, during which a metamorphic dehydration reaction has the potential to raise pore pressure, thereby lowering the effective pressure, permitting brittle failure24. This hypothesis requires sufficient structural water in consti (...truncated)


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Feng Shi, Yanbin Wang, Tony Yu, Lupei Zhu, Junfeng Zhang, Jianguo Wen, Julien Gasc, Sarah Incel, Alexandre Schubnel, Ziyu Li, Tao Chen, Wenlong Liu, Vitali Prakapenka, Zhenmin Jin. Lower-crustal earthquakes in southern Tibet are linked to eclogitization of dry metastable granulite, Nature Communications, 2018, Issue: 9, DOI: 10.1038/s41467-018-05964-1