Abstract
Because earthquakes on large active thrust or reverse faults are not always accompanied with surface rupture, paleoseismological estimation of their associated seismic hazard is a difficult task. To improve the seismic hazard assessments in the Andean foreland of western Argentina (San Juan Province), this paper proposes a novel approach that combines structural geology, geomorphology and exposure age dating. The Eastern Precordillera of San Juan is probably one of the most active zones of thrust tectonics in the world. We concentrated on one major regional active reverse structure, the 145 km long Villic´um-Pedernal thrust, where this methodology allows one to: (1) constrain the Quaternary stress regime by inversion of geologically determined slip vectors on minor or major fault planes; (2) analyse the geometry and the geomorphic characteristics of the Villic´um-Pedernal thrust; and (3) estimate uplift and shortening rates through determination of in situ-produced 1 0 Be cosmic ray exposure (CRE) ages of abandoned and uplifted alluvial terraces. From a structural point of view, the Villic´um-Pedernal thrust can be subdivided into three thrust portions constituting major structural segments separated by oblique N40°E-trending fault branches. Along the three segments, inversion of fault slip data shows that the development of the Eastern Precordillera between 31°S and 32°S latitude is dominated by a pure compressive reverse faulting stress regime characterized by a N110° ′ 10°E-trending compressional stress axis (σ 1 ). A geomorphic study realized along the 18 km long Las Tapias fault segment combined with CRE ages shows that the minimum shortening rate calculated over the previous ∼20 kyr is at least of the order of 1 mm yr - 1 . An earthquake moment tensor sum has also been used to calculate a regional shortening rate caused by seismic deformation. This analysis of the focal solutions available for the last 23 yr shows that the seismic contribution may be three times greater than the shortening rate we determined for the Las Tapias fault (i.e. ∼3 mm yr - 1 ), suggesting that the San Juan region may have experienced a seismic crisis during the 20th century. Moreover, the ramp that controls the development of the Eastern Precordillera appears to be one of the main seismic sources in the San Juan area, particularly the 65 km long Villic´um-Las Tapias segment. A first-order evaluation of the seismic hazard parameters shows that this thrust segment can produce a maximum earthquake characterized by a moment magnitude of ∼7.3 (′0.1) and a recurrence interval of 2.4 (′1.5) kyr. This part of the Villic´um-Pedernal ramp may have ruptured during the Ms =7.4, 1944 San Juan earthquake producing very few surface ruptures and only distributed flexural slip deformation on to the Neogene foreland bedding planes between the Eastern Precordillera and Pie de Palo.
Highlights
Obtaining a paleoseismological estimate of the seismic hazard linked to large reverse or thrust faults is a difficult task
Inversion of fault slip data provides new constraints on the stress regime associated with the growth of the Eastern Precordillera and shows that the development of the Eastern Precordillera is dominated by a pure compressive reverse faulting stress regime characterized by a N110 ± 10◦Etrending compressional stress axis (σ1)
The data and observations reported in this current study lead us to consider the Villicum– Pedernal thrust that borders the Eastern Precordillera between 31◦S and 32◦S latitude as the main ramp emergence of the basement decollement located at depth below the Tullum Valley and Pie de Palo
Summary
Obtaining a paleoseismological estimate of the seismic hazard linked to large reverse or thrust faults is a difficult task. Those faults may generate destructive earthquakes that produce either conspicuous surface ruptures or small and questionable surface displacements, or even no evident surface rupture. The Andean foreland of western Argentina (Fig. 1) is one of the most seismically active zones of thrust tectonics in the world In this region, more than 90 per cent of the total continental seismic moment release in the Andean foreland from Ecuador to Patagonia has occurred since 1960 (Chinn & Isacks 1983). We concentrated on one major regional active reverse structure, the Villicum–Pedernal thrust, where this methodology allows one to: (1) constrain the Quaternary stress regime by inversion of geologically determined slip vectors on minor or major fault planes; (2) analyse the geometry and the geomorphic characteristics of the Villicum–Pedernal thrust; and (3) estimate uplift and shortening rates through determination of in situ-produced 10Be cosmic ray exposure (CRE) ages of abandoned and uplifted alluvial terraces
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