Japan Geoscience Union Meeting 2019

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-CG Complex & General

[S-CG50] Intraslab and intraplate earthquakes

Thu. May 30, 2019 9:00 AM - 10:30 AM A02 (TOKYO BAY MAKUHARI HALL)

convener:Saeko Kita(International Institute of Seismology and Earthquake Engineering, BRI), Tomohiro Ohuchi(Geodynamics Research Center, Ehime University), Marcel Thielmann(Bavarian Geoinstitute, University of Bayreuth), Ryo Okuwaki(Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Chairperson:Saeko Kita(Building Research Insitute), Tomohiro Ohuchi(Geodynamics Research Center, Ehime University), Marcel Thielmann(University of Bayreuth), Ryo Okuwaki(筑波大学), Thomas Ferrand(UC San Diego)

10:15 AM - 10:30 AM

[SCG50-06] Transformation-induced, melt-enhanced faulting in orthoenstatite: implications for global intermediate-depth earthquakes

Feng Shi1, Jianguo Wen2, Tony Yu1, Lupei Zhu3, *Yanbin Wang1 (1.Center for Advanced Radiation Sources, University of Chicago, Chicago, IL, USA, 2.Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL, USA, 3.Department of Earth & Atmospheric Sciences, Saint Louis University, St. Louis, MO, USA)

Intermediate-depth earthquakes, which occur at ~50-300 km depths and account for ~90% of all deep earthquakes between 50 and 700 km, are ubiquitously observed along convergent plate margins and post great hazards in many regions of the world. Earthquake-depth distribution in oceanic subduction zones exhibits a prevalent secondary seismicity peak between 180 and 240 km, where major dehydration reactions are expected to have completed [1, 2]. This secondary seismcity peak is a direct manifestation of earthquake activities in the lower plane of the double seimic zone [3]. One of the main constituents of oceanic slabs is harzburgite, which consists mainly of olivine and orthoenstatite (OEn) [4]. The latter transforms to high-pressure clinoenstatite (HP-CEn) at ~120 – 210 km depths, depending on aluminum content and slab temperature [5, 6]. We conducted deformation experiments on OEn at conditions corresponding to ~ 40 – 250 km depths. Within its stability field, OEn deforms plastically; no brittle failure is detected with acoustic emission (AE) monitoring. In the HP-CEn stability field, metastable OEn generates numerous AEs generated, and fails by macroscopic faulting between ~773 and 1373 K. Outside this temperature range, metastable OEn flows plastically again with no AEs detected. Recovered failed samples contain large conjugated faults, with ultrafine-grained gouge layers containing melts that are more Al-rich. OEn grain boundaries are decorated with fine grained (grainsize < 1 micron) garnet and clinoenstatite (CEn). The latter is interpreted as back-transformed HP-CEn upon pressure release. Within large OEn grains, finer-grained garnet forms thin lamellae preferentially parallel to the OEn (100) twin planes. These results suggest that micro-ruptures start in metastable OEn by syn-deformational transformation, which, aided by exothermic latent heat production, results in exsolution lamellae of garnet and CEn, both along the OEn (100) planes, associated with intragranular ruptures. These micro-ruptures conglomerate along weakened grain boundaries to form intergragular faults consisting of ultrafine grained reaction and shear zones, which, through adiabatic heating and local melting, self-organize into macroscopic faults. We propose that in harzburgite, the observed OEn instability triggers shear localization in olivine, resulting in adiabatic instability, a mechanism hypothesized on theoretical grounds [7, 8] and recently observed in laboratory experiments [9].
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