Assessing seismic hazards in slowly deforming orogens. – SLOWDEF

Rather than starting from short-term modern interseismic deformation rates as classically done, we propose to investigate active deformation first from a long-term perspective - ie over a time scale sufficient to cumulate observable and measurable deformation – and to progressively downscale to intermediate and then short-term active deformation. The proposed strategy to solve for active tectonics and associated seismic hazards in slowly deforming orogens therefore relies on the tight synthesis of geological data retrieved over various - but complementary - spatial and temporal scales. These data will be acquired using state-of-the-art tools and techniques of the Earth Sciences (structural geology, geomorphology, geodesy, mineralogy...). The originality and contribution of this project is their specific combination and integration for the particular purpose of assessing active tectonics in slowly deforming regions.
Our approach is tested from our work on the field example of the Western Kunlun mountain range (Xinjiang China).

-> Kinematics of deformation of the largest active thrust sheet: the Mazar Tagh thrust sheet (Guilbaud et al 2017, in prep a+b) within the Tarim Basin.
-> Could the Mazar Tagh thrust sheet rupture during mega-earthquakes (M>8) ? we unraveled the presence of uplifted incised surfaces over the Mazar Tagh ramp that could be related to paleo-earthquakes (Guilbaud, 2019).If these surfaces were of co-seismic origin, the associated coseismic uplift, as inferred from their relative incision, indicates the possibiliy of mega-earthquakes (M>8).
-> Is the large Mazar Tagh thrust sheet coupled during the interseismic ? Published GPS data were re-analyzed in a combined displacement field relative to a Tarim reference as defined from our structural results (Tissandier, 2019). We evidenced a gradient of horizontal velocities of ~2-3 mm/yr across the range. Also, our results indicate that the thrust sheet could be coupled along its eastern part. Geodesy interpreted in an adequate structural frame could be envisaged here even though deformation rates are slow.
-> Mechanical asperities and seismicity along evaporitic decollements: the nase of the Digne nappe (France). The possibility of earthquales on such decollements goes against the paradigm that these lithologies continuously creep. However, seismicity is often observed in such contexts. To further explore this , we analyzed the mineralogy of an exotic basement block trapped within the Digne Nappe (Hakkinen et al, in prep). White mica rims, contemporaneous of the alpine nappe, reveal significant overpressures, most probably related to a local mechanical asperity that could have possibly originated seismicity.

Our investigations aim at emphasizing the value of the geological and morphological record of active deformation in slowly deforming orogens, where sole classical geodetic approaches fail at resolving modern interseismic loading of active faults. Fast convergent settings such as the Himalayas have inspired – and will keep inspiring – the way we view the seismic cycle of active faults, but the line of attack to tackle the issue of seismic hazards in slowly convergent mountain ranges is forcedly different and needs to be adjusted. Our work will provide a comprehensive methodology to better assess seismic hazards in slowly convergent orogens, by illustrating the idea that active deformation should be quantified over various times scales, from long-term deformation down in time to short-term interseismic processes (Figure 1). As such the impact is expected to be both scientific and societal, and our idea is to be applied to other slow convergent settings, such as the European Alps or the Andes. In general, we expect our approach to be overall applicable not only to convergent settings building mountain ranges, but also to other (extensional, strike-slip) contexts.
Apart from this main methodological outcome, we will provide new and unique data on active tectonics and associated hazards for the particular case example of the southern Tarim Basin and Western Kunlun mountain range, chosen here to illustrate our approach. In particular, we will investigate the possibility of continental major earthquakes rupturing the uniquely and exceptionally wide frontal thrust sheet of the Mazar Tagh. This region has not faced a major earthquake in the historic past and is therefore not prepared for such eventuality.

1. Hakkinen M. , Angiboust A., Simoes M., Squeezed under a thrust sheet: white mica records high tectonic stresses within an evaporitic decollement., in prep for Terra Nova.
2. Guilbaud C., Simoes M., Barrier L., van der Woerd J., Li H., Baby G., Pan J., Investigating kinematic and structural variations along the Western Kunlun mountain front (Xinjiang, China): structural and morphological analysis of the Hotan anticline and Tiekelike fault., in prep (a) for Tectonics.
3. Guilbaud C., Simoes M., van der Woerd J., Barrier L., Pan J., Li H., Tapponnier P., Geology, structure and kinematics of shortening of the presently largest active thrust sheet: the case of the Mazar Tagh (Xinjiang, China), in prep (b) for Tectonics.
4. Guilbaud C., Kinematics of active deformation of the Western Kunlun range (Xinjiang, China): implications for potential seismic hazards., Thèse de doctorat, Université de Paris, Institut de physique du globe de Paris, France, 325 pages.

Convergent plate boundaries are the loci of destructive earthquakes, such as along subduction zones or within the Himalayas where fast convergence rates and frequent large earthquakes have attracted numerous studies in the past. The seismic behavior of active faults is to the first-order reasonably well understood in these fast settings. Unfortunately, such is not the case of active faults within orogens where convergence rates are slow (< 5 mm/yr), so that assessing seismic hazards in these particular contexts remains a scientific and societal challenge. Taking up this challenge requires developing a new approach and strategy to unravel active deformation and elastic strain storage prior to earthquakes, as an alternative to interseismic deformation directly retrieved from classical geodetic techniques. Rather than starting from short-term modern interseismic deformation rates as classically done, we propose to investigate active deformation first from a long-term perspective - ie over a time scale sufficient to cumulate observable and measurable deformation - and to progressively downscale to intermediate and then short-term active deformation. The proposed strategy to solve for active tectonics and associated seismic hazards in slowly deforming orogens therefore relies on the tight synthesis of geological data retrieved over various - but complementary - spatial and temporal scales. These data will be acquired using state-of-the-art tools and techniques of the Earth Sciences (structural geology, geomorphology, geodesy, mineralogy...). The originality and contribution of this project is their specific combination and integration for the particular purpose of assessing active tectonics in slowly deforming regions. The four main young researchers involved in the project are experts in each one of these various domains of the Earth Sciences, and their synergy will be ensured in a voluntarily designed limited research group coordinated by the PI.