Riccardo Vassallo

Mongolian summits: An uplifted, flat, old but still preserved erosion surface

In Gobi Altay and Altay, Mongolia, several flat surfaces, worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m, suggests disruption of a single large-scale surface. New thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during Jurassic time. The Gobi Altay and Altay flattopped massifs are tectonically and geomorphologically unique. Their preservation for ~150 m.y. implies that no further tectonic movements occurred before the onset of the last deformation episode, 5 ± 3 m.y. ago. It also suggests that very low erosion rates were maintained by a dry climate over millions of years.

Slope and climate variability control of erosion in the Andes of central Chile

Climate and topography control millennial-scale mountain erosion, but their relative impacts remain matters of debate. Confl icting results may be explained by the infl uence of the erosion threshold and daily variability of runoff on long-term erosion. However, there is a lack of data documenting these erosion factors. Here we report suspended-load measurements, derived decennial erosion rates, and 10 Be-derived millennial erosion rates along an exceptional climatic gradient in the Andes of central Chile. Both erosion rates (decennial and millenial) follow the same latitudinal trend, and peak where the climate is temperate (mean runoff ~500 mm yr ‐1 ). Both decennial and millennial erosion rates increase nonlinearly with slope toward a threshold of ~0.55 m/m. The comparison of these erosion rates shows that the contribution of rare and strong erosive events to millennial erosion increases from 0% in the humid zone to more than 90% in the arid zone. Our data confi rm the primary role of slope as erosion control even under contrasting climates and support the view that the infl uence of runoff variability on millennial erosion rates increases with aridity.

Transpressional tectonics and stream terraces of the Gobi‐Altay, Mongolia

We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi-Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ~600 ka ago, while a more internal sub-parallel fault was still active until ~200-100 ka. Vertical offset of an alluvial fan abandoned ~100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene - Holocene uplift rate. The morphology of the catchment-piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.

Earthquake Geology of the Bulnay Fault (Mongolia)

The Bulnay earthquake of 23 July 1905 (Mw 8.3–8.5), in north‐central Mongolia, is one of the world’s largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in the region during the twentieth century. The 375 km long surface rupture of the left‐lateral, strike‐slip, N095°E‐trending Bulnay fault associated with this earthquake is remarkable for its pronounced expression across the landscape and for the size of features produced by previous earthquakes. Our field observations suggest that in many areas the width and geometry of the rupture zone is the result of repeated earthquakes; however, in those areas where it is possible to determine that the geomorphic features are the result of the 1905 surface rupture alone, the size of the features produced by this single earthquake are singular in comparison to most other historical strike‐slip surface ruptures worldwide. Along the 80 km stretch, between 97.18° E and 98.33° E, the fault zone is characterized by several meters width and the mean left‐lateral 1905 offset is 8.9±0.6  m with two measured cumulative offsets that are twice the 1905 slip. These observations suggest that the displacement produced during the penultimate event was similar to the 1905 slip. Morphotectonic analyses carried out at three sites along the eastern part of the Bulnay fault allow us to estimate a mean horizontal slip rate of 3.1±1.7  mm/yr over the Late Pleistocene–Holocene period. In parallel, paleoseismological investigations show evidence for two earthquakes prior to the 1905 event, with recurrence intervals of ∼2700–4000  yrs.

Spatial and temporal variations in creep rate along the El Pilar fault at the Caribbean‐South American plate boundary (Venezuela), from InSAR

In eastern Venezuela, the Caribbean - South American plate boundary follows the El Pilar fault system. Previous studies based on three GPS campaigns (2003 – 2005 – 2013) demonstrated that the El Pilar fault accommodates the whole relative displacement between the two tectonic plates (20 mm/yr) and proposed that 50 – 60 % of the slip is aseismic [Jouanne et al., 2011; Reinoza et al., 2015]. In order to quantify the possible variations of the aseismic creep in time and space, we conducted an InSAR time-series analysis, using the NSBAS method, on 18 images from the ALOS-1 satellite spanning the 2007 – 2011 period. During this 3.5 year period, InSAR observations show that aseismic slip decreases eastwards along the fault: the creep rate of the western segment reaches 25.3 ± 9.4 mm/yr on average, compared to 13.4 ± 6.9 mm/yr on average for the eastern segment. This is interpreted, through slip distribution models, as being related to coupled and un-coupled areas between the surface and ca. 20 km in depth. InSAR observations also show significant temporal creep rate variations (accelerations) during the considered time span along the western segment. The transient behavior of the creep is not consistent with typical postseismic afterslip following the 1997 Ms 6.8 earthquake. The creep is thus interpreted as persistent aseismic slip during an interseismic period, which have a pulse or transient like behavior.

Pleistocene slip rates on the Boconó fault along the North Andean Block plate boundary, Venezuela

The Bocono fault is a strike-slip fault lying between the North Andean Block and the South American plate which has triggered at least five Mw > 7 historical earthquakes in Venezuela. The North Andean Block is moving toward NNE with respect to a stable South American plate. This relative displacement at ~12 mm · yr−1 in Venezuela (within the Maracaibo Block) was measured by geodesy, but until now the distribution and rates of Quaternary deformation have remained partially unclear. We used two alluvial fans offset by the Bocono fault (Yaracuy valley) to quantify slip rates, by combining 10Be cosmogenic dating with measurements of tectonic displacements on high resolution satellite images (Pleiades). Based upon a fan dated at > 79 ka and offset by 1350–1580 m and a second fan dated at 120–273 ka and offset by 1236–1500 m, we obtained two Pleistocene rates of 5.0 - 11.2 and < 20.0 mm · yr−1, consistent with the regional geodesy. This indicates that the Bocono fault in the Yaracuy valley accommodates 40 to 100% of the deformation between the South American plate and the Maracaibo block. As no aseismic deformation was shown by InSAR analysis, we can assume that the fault is locked since the 1812 event. This implies that there is a slip deficit in the Yaracuy valley since the last earthquake ranging from ~ 1 to 4 m, corresponding to a Mw 7 – 7.6 earthquake. This magnitude is comparable to the 1812 earthquake and to other historical events along the Bocono fault.

Characterizing the Present‐Day Activity of the Tunka and Sayan Faults Within Their Relay Zone (Western Baikal Rift System, Russia)

The Sayan and Tunka faults are located at the boundary between the northernmost mountain belt of Central Asia (the Sayan-Baikal ranges) and the Siberian platform. These prominent crustal structures were involved in the opening of the southern Baikal rift system since the beginning of the Cenozoic and define large-scale sharp morphotectonic features. Despite low instrumental seismic activity, Late Pleistocene-Holocene morphotectonic features along the two faults indicate that the faults are active and have the capacity to produce strong earthquakes. A careful mapping of the most recent trace of activity, within the southeastern parts of the two faults where they merge within a relay zone, demonstrates that they correspond now to left-lateral-reverse faults, suggesting a recent inversion of their vertical component. We also show that the two faults are now structurally connected via a young surface rupture and that no obvious post-Last Glacial Maximum ruptures are observed along the central part of the Sayan Fault beyond its junction zone with the Tunka fault. This suggests that the left-lateral strike-slip deformation is transferred from the eastern Sayan fault to the Tunka fault. A detailed morphotectonic study along the southeastern Sayan fault allows estimating a left-lateral slip rate between 1.3 (min) and 3.9 mm/year (max). Finally, a critical review of Russian paleoseismic data, combined with our paleoseismological investigations, allows us to propose that the mean recurrence time along the two faults is on the order of 4 kyr and that they may have either ruptured together or during seismic clusters.