Rates of thrust faulting at the front of the Precordillera in western Argentina: Implications for the seismic hazard in Mendoza city from surface expose dating and paleoseismology
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During the last ~250 years several destructive earthquakes occurred along the eastern mountain front of the Andean Precordillera. To evaluate the seismic hazard generated by two active thrust faults near Mendoza city − the Peñas and Cal faults − this study quantifies their coseismic displacements and Holocene slip rates. Both faults cut through well preserved alluvial fan surfaces and inset terraces. Fault scarps on the lowest terrace level T1 reveal vertical offsets of 0.8−1.0 m on both faults, which are interpreted as coseismic displacements during the last earthquake. Together with the fault dip, these offsets indicate that both faults are capable of producing magnitude Mw 6.9 earthquakes, which is corroborated by a magnitude Ms 7.0 event on the Cal fault that destroyed Mendoza in 1861. Older terraces show stepwise increasing cumulative offsets, indicating that elastic strain energy was repeatedly released during strong earthquakes. Terrace T2 at the Cal fault has a vertical displacement of 2.5 m, which was generated by three earthquakes as revealed by paleoseismologic investigations. Based on an optically stimulated luminescence age, these seismic events occurred in the past 770 ± 76 years. The youngest earthquake corresponds to the 1861 event, whereas the penultimate deformation may be related to an Ms ~7 earthquake in 1782. Terrace T3 at the Cal fault yielded a calibrated 14C age of 3.90 ± 0.14 ka obtained from a plant remnant. Together with the vertical displacement of 3.6 m and the fault dip of ~32°, this implies a shortening rate of 1.5 ± 0.3 mm/a. In contrast, the age and offset of the ~770 years old terrace T2 result in a significantly higher shortening rate of 5.3 ± 1.5 mm/a, implying an increased slip rate on the Cal fault during the past millennial. North of Mendoza, the Peñas thrust fault offsets two terraces (T2 and T3) by ~2 and ~11 m. The terrace ages were quantified by 10Be exposure dating of sand samples from depth profiles and amalgamated sandstone pebbles. Both methods yield similar ages of 3−5 ka for terrace T2 and 11−13 ka for T3. The age of terrace T3 has been confirmed by a 14C age of 12.61 ± 0.20 ka cal. BP. In contrast, 10Be dating of sandstone boulders yielded variable exposure ages that overestimate the depositional age due to a high and variable predepositional 10Be component. The offset and age of terrace T3 in combination with the fault dip of ~25° define a millennial shortening rate of ~2.0 mm/a on the Peñas fault. The present-day shortening rate of 4.5 ± 1.7 mm/a at the eastern margin of the Precordillera was determined by modeling regional GPS data. Thus, the millennial shortening rates determined for the Peñas and Cal faults indicate that both faults have accumulated a significant amount of the present-day shortening rate. The millennial slip rates determined in this study and their comparison to long-term shortening rates, which integrate over several million years, imply that the shortening rate at the Andean mountain front has remained roughly constant through time. This suggests that the high number of historic earthquakes in the Precordillera is not exceptional. The seismic activity will most likely continue at a similar level in the future and especially the Cal thrust fault poses a serious threat to the one million inhabitants of Mendoza.
Projektbezogene Publikationen (Auswahl)
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(2010). Late Quaternary slip rates of two thrust faults at the mountain front of the Andean Precordillera, Mendoza, Argentina. AGU Fall Meeting, San Francisco, 13.- 17. December 2010, Abstract T33A-2204
Hetzel, R., Schmidt, S., Ramos, V.A. & Mingorance, F.
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(2011). 10Be, 14C and OSL dating of alluvial sediments from the eastern Andes. Geophysical Research Abstracts, Vol. 13, EGU 2011-6793, Vienna, 3.-8. April 2011
Schmidt, S., Hetzel, R., Tsukamoto, S. & Frechen, M.
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(2011). A note of caution on the use of boulders for exposure dating of depositional surfaces. Earth Planet. Sci. Lett. 302, 60-70
Schmidt, S., Hetzel, R., Kuhlmann, J., Mingorance, F. & Ramos, V.A.
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(2011). Coseismic displacements and Holocene slip rates for two active thrust faults at the mountain front of the Andean Precordillera (~33°S). Tectonics 30, TC5011
Schmidt, S., Hetzel, R., Mingorance, F. & Ramos, V.A.
