Break-off of part of the down-going plate during continental collision occurs due to tensile stresses built-up between the deep and shallow slab, for which buoyancy is increased because of continental-crust subduction. Break-off governs the subsequent orogenic evolution but real-time observations are rare as it happens over geologically short times. Here we present a finite-frequency tomography, based on jointly inverted local and remote earthquakes, for the Hindu Kush in Afghanistan, where slab break-off is ongoing. We interpret our results as crustal subduction on top of a northwards-subducting Indian lithospheric slab, whose penetration depth increases along-strike while thinning and steepening. This implies that break-off is propagating laterally and that the highest lithospheric stretching rates occur during the final pinching-off. In the Hindu Kush crust, earthquakes and geodetic data show a transition from focused to distributed deformation, which we relate to a variable degree of crust-mantle coupling presumably associated with break-off at depth.
Bibliographical noteFunding Information:
We thank the Department of Geology of Kabul University, the Afghanistan Geological Survey, the Norwegian Afghanistan Committee and the Tajik Academy of Sciences for installing and maintaining the Afghan and Tajik campaign sites, respectively. The Geophysical Instrument Pool Potsdam (GIPP) provided seismic instruments for the temporary networks; these (FDSN codes: 7B 2008–2010; 6C 2009–2010 and 2013–2014; 5C 2012–2014; 4C 2017–2019) are archived at the GEOFON Data Centre and can be obtained via the GEOFON website (https://geofon.gfz-potsdam.de/waveform/archive/ index.php?type=t). Permanent station waveform data can be obtained through the IRIS DMC (https://ds.iris.edu/SeismiQuery/). This study was supported by GFZ expedition funds, and the TIPTIMON and CATENA projects, funded by the German Ministry of Science and Education (support codes 03G0809A/B and 3G0878A/B). NK was supported by an alumni scholarship from the ‘UNESCO International Training Course for Seismology’ at GFZ Potsdam. SK was supported by a postdoc fellowship of the ‘German Academic Exchange Service (DAAD)’ at the University of Minnesota. Tomographic inversion used the HPC units of the Minnesota Supercomputing Institute. Figures and calculations used the Generic Mapping Tools (gmt.soest.hawaii.edu/), matplotlib (mat-plotlib.org), obspy (obspy.org) and Matlab (mathworks.com/products/matlab.html). M. Dziggel (GFZ) helped in preparing Fig. 5c and A. Gaete (GFZ) helped with first onset picking of teleseismic waveforms.
© 2021, The Author(s).
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