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. 2021 Mar 16;12(1):1685.
doi: 10.1038/s41467-021-21760-w.

The Hindu Kush slab break-off as revealed by deep structure and crustal deformation

Sofia-Katerina Kufner  1   2   3 , Najibullah Kakar  4   5 , Maximiliano Bezada  6 , Wasja Bloch  7 , Sabrina Metzger  7 , Xiaohui Yuan  7 , James Mechie  7 , Lothar Ratschbacher  8 , Shokhruhk Murodkulov  9 , Zhiguo Deng  7 , Bernd Schurr  7
Affiliations

The Hindu Kush slab break-off as revealed by deep structure and crustal deformation

Sofia-Katerina Kufner et al. Nat Commun. .

Abstract

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Geographic setting and data input for seismic tomography.
a Tectonic setting, local earthquakes used as inversion input, colour-coded by depth (circles), and seismic stations (triangles) with the campaign stations of the recent Afghanistan network (active 2017–2019) in light red. Faults (in black with white background) simplified after refs. ,, and references therein. Political boundaries in dark red. NE/NW/SE/SW-HK abbreviate north-east/north-west/south-east/south-west Hindu Kush. Starting points of profiles shown in Fig. 2 are highlighted in red. b Teleseismic earthquakes used as inversion input. c Study region in the context of the Indian-Asian collision zone where India converges at ~34 mm/year, northward towards Asia. The Cenozoic Indus-Yarlung suture (southern brown line) separates Indian from Asian rocks. The northern brown line represents the late Paleozoic-Triassic suture separating cratonic Asia in the north from the accreted Gondwana terranes in the south.
Fig. 2
Fig. 2. P-wave velocity tomographic model.
a–e Cross-sections with the topography on top; velocity anomaly is colour-coded by the percentage of variation relative to the initial 1D velocity model used in the inversion (Supplementary Fig. 1). The 1.0/1.5/2.0% velocity-anomaly contours are highlighted. The white line marks the resolution limit (see ‘Methods’). The dark red line represents the crust-mantle boundary (Moho) constrained from receiver functions. Local earthquakes within 15 km of the profile (circles) are colour-coded with depth as in Fig. 1a. Crustal events include those obtained from manual picking of the events recorded by the recent network in Afghanistan and those used in the inversion (see ‘Methods’). Yellow stars represent magnitude 6.5+ earthquakes from USGS within the last 30 years projected from ±35 km swaths to account for the larger location uncertainties compared to the local catalogue. LVZ Hindu Kush low-velocity zone, HVZ Hindu Kush high-velocity zone, ATD Afghan-Tajik depression. Other abbreviations as in Fig. 1a. f–k Depth sections; section-depth and absolute P-wave velocity at this depth are given in the top right of each plot. Political boundaries in grey, faults in black with a white background as in Fig. 1a. Local earthquakes are plotted from ±5 km depth swaths in the crust and ±10 km depth swaths in the mantle. All other features as in panels (a)–(e).
Fig. 3
Fig. 3. Data subset inversion and end member synthetic test for mantle anomaly geometry.
a Real data subset inversion for local only, teleseismic only and joint dataset. b Synthetic data subset inversion as in (a). The blue/red contours represent the ±3% velocity anomaly outline of the synthetic input model. c The synthetic input model used in (b), featuring a gradual increase of the velocity anomaly from 0 to 5%. d Synthetic models based on the combined dataset, testing different end-member scenarios for the geometry of the mantle anomaly. Local earthquakes, resolution limit and topography are plotted as in Fig. 2. All cross-sections are along with the profile of Fig. 2c.
Fig. 4
Fig. 4. End member synthetic test for mantle anomaly resolution and depth extent.
a, b Map view sections. c Profile along Fig. 2e. i Real data inversion results as in Figs. 2 and 3a for comparison. ii North-dipping slab scenario as in Fig. 3b. iii A model with similar geometry to (ii) but no mantle high-velocity zone exists between 100 and 180 km depth and deeper than 360 km depth. Local earthquakes, resolution limit and topography are plotted as in Fig. 2.
Fig. 5
Fig. 5. Crustal event catalogue, GNSS rates and tectonic interpretation.
a Histograms of deep crustal seismicity (25–40 km depth, 2012–2019; this study), intermediate-depth seismicity related to break-off (160–300 km depth, 2012–2019; this study and ref. ), and tomography results for P-wave velocity (vP) at 30 km and P-wave velocity anomaly (dvP) at 400 km depth along the longitudinal transect highlighted in (b). Positive dvP values east of 69.5°E indicate the presence of the stretched and tearing slab at depth. This longitudinal range is characterized by middle/lower crustal low-velocities. b Crustal event catalogue shallower than 40 km depth, scaled by event magnitude (see Supplementary Fig. 4 for details). Intermediate-depth seismicity (this study and ref. ) is plotted for orientation. The depth from 40 to ~60 km is mostly aseismic. Compression(P)-axes and focal mechanisms of crustal earthquakes from single event solutions (small beach-balls) and strain inversion (large beach balls; see Supplementary Fig. 5 for details). GNSS rates with 95% confidence ellipses relative to Asia, re-evaluated based on refs. ,. GNSS1-2 highlight the locations of dense GNSS station profiles across the Hindu Kush. NE/NW/SE/SW-HK, north-east/north-west/south-east/south-west Hindu Kush. c Interpretation sketch illustrating the process of slab break-off and the crustal response. The slab experiences stretching and steepening during advancing break-off resulting in a greater penetration depth. Parts of the Indian crust are pulled to depth together with the slab and are buoyantly exhuming (white arrows) providing a heat input to the upper plate crust from below (wavy red arrows). Seismicity related to break-off is highlighted in pink; shallower intermediate-depth seismicity, possibly triggered by phase-transition reactions in the subducted crust, is in purple. The degree of crust-mantle coupling in the upper plate, the Hindu Kush orogen, decreases alongside the advance of slab break-off at depth. This is expressed in a change in crustal deformation style.
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