Squeezing and lifting – modes of long-term forearc deformation in the Northern Chile subduction zone

Bernd Schurr, GFZ German Research Centre for Geosciences

Subduction zone forearcs deform transiently and permanently due to the frictional coupling with the converging lower plate. Transient stresses are mostly the elastic response to the seismic cycle. Permanent deformation is evidenced by forearc topography, upper plate faulting and earthquakes; its relation to the megathrust seismic cycle is debated. Here we study upper plate seismicity in the northern Chile subduction zone as a proxy for forearc brittle deformation. We find that seismicity is distributed unevenly and a dramatic increase correlates both with a break in the morphology and tectonics of the Coastal Cordillera and the onset of a change in subduction obliqueness. Earthquakes in the South American crust under the sea and under the Coastal Cordillera show a remarkably homogenous north-south, i.e., trench-parallel, compressional stress field. Earthquake fault mechanisms are dominated by trench-perpendicular thrusts. Further inland, where the lower plate becomes uncoupled, the stress field is more varied with direction east-west to southeast-northwest (approx. convergence parallel) dominating. The stress-regime above the plate-coupling-zone, almost perpendicular to plate convergence direction, may be explained by a trench-parallel strain component from a change in subduction obliqueness due to the concave shape of the plate margin, which we demonstrate by investigating inter-plate earthquake slip vectors. From these, we derive a strain rate estimate and compare it to one derived from upper plate earthquakes. In the southern part of our study area, where convergence obliqueness is constant, upper plate seismicity is sparse, but Coastal Cordillera elevation and the uplift rate are higher than in the north. A new relocation of two megathrust earthquake aftershock sequences reveals a flat-ramp-flat structure that we interpret as underplating of a 3 km thick slice of eroded material and/or oceanic crust. This structure is also visible in the residual gravity field and in a tomographic model of the region. Our study demonstrates thus two very different mechanisms of forearc thickening active on two neighboring segments of a subduction zone.

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記

　　　　日　　時：　令和5年1月20日(金)　午後１時３０分～　
　　　　開催方法：　インターネット　WEB会議

1. 13:30-13:45
   演題：Groundwaters and deep-seated fluid circulation around Aso Volcano, Southwest Japan, revealed by multivariate statistical analysis of the geochemical data
   著者：○Hikaru IWAMORI、Hitomi NAKAMURA・Noritoshi MORIKAWA・Masaaki TAKAHASHI・Akihiko INAMURA (AIST)、Satoru HARAGUCHI、Tatsuji NISHIZAWA (Mt. Fuji Res. Inst.)、Shuhei SAKATA
   要旨：Groundwater geochemistry and its statistical analyses revealed how fluids of shallow and deep origins cycle in Aso Volcano and from a subducted slab beneath central Kyushu.
2. 13:45-14:00
   演題：Extraction of P-S converted waves from teleseismic P-wave microseisms
   著者：○加藤翔太・西田 究
   要旨：We developed the source deconvolution method to extract P-S waves converted beneath receivers from teleseismic P-wave microseisms.
3. 14:00-14:15
   演題：Single-station seismic event classification based on a modified deep embedded clustering architecture and its application to Harrison County, Eastern Ohio
   著者：○Jeffrey CHURCH、Dongdong YAO (China University of Geosciences)、Yihe HUANG (University of Michigan)、Zefeng LI (University of Science and Technology of China)
   要旨：We recast the problem of classifying tectonic and anthropogenic waveforms as a clustering problem, thereby avoiding the labor of creating a large labeled dataset.

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〒113－0032　東京都文京区弥生1－1－1　
東京大学地震研究所 共同利用担当
E-mail：k-kyodoriyo(at)eri.u-tokyo.ac.jp

※次回の談話会は令和5年2月17日(金)　午後１時３０分～です。

The Tristan da Cunha mantle plume and its role in the continental breakup and opening of the South Atlantic (insights from geophysical, petrological and geochemical studies along the Walvis Ridge hotspot track)

Wolfram Geissler (Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Germany)

Abstract
Tristan da Cunha is a hotspot in the South Atlantic Ocean, located ~450 km east of the Mid-Atlantic Ridge. The intraplate volcanoes and seamounts that form the Tristan da Cunha archipelago are connected to the Cretaceous (~132 Ma) Etendeka continental flood basalt province in Namibia via the aseismic Walvis Ridge. The ridge is built-up by seamounts chains and submarine volcanic plateaus that show a clear age progression and extend from the Namibian continental margin (northeast) to the volcanic islands of Tristan da Cunha and Gough (southwest). This age-progressive distribution of volcanic rock samples collected from the Walvis Ridge and the Rio Grande Rise west of the Mid-Atlantic Ridge provide evidence for the volcanism at Tristan da Cunha and the formation of Cretaceous flood basalts in Namibia and Brazil to be due to a common hotspot source, with the Walvis Ridge and the Rio Grande Rise documenting the hotspot tracks. The Tristan da Cunha-Walvis Ridge system is one of the few examples of a complete hotspot track, and thus the it is generally assumed to be a surface expression of a long-lasting mantle plume. However, a debate continues about whether the mantle plume beneath Tristan da Cunha is an expression of convection of the whole mantle or of shallower plate-driven convection. The hypothesis of a deep mantle plume origin of the hotspot volcanism at the Tristan da Cunha archipelago is supported by anomalous geochemical data, geochronological constraints, and global seismic tomography. The first proof of the Tristan da Cunha mantle plume activity are the Cretaceous Etendeka-Parana flood basalts, that erupted shortly before the South Atlantic finally opened. Therefore, a debate exists about whether the mantle plume triggered the continental breakup or played at least a strong role in facilitating the breakup. NW Namibia, the Walvis Ridge, Tristan da Cunha, and the South Atlantic Ocean in general were target regions for various international geophysical and geochemical projects over the past two decades, including a recent IODP expedition to the Walvis Ridge. In my talk, I will summarize the recent findings on the Tristan da Cunha mantle plume activity and its role during the breakup of the South Atlantic Ocean.