Abstract
In this work we present a study of an alluvial fan system, which is affected by the Quaternary activity of the leftlateral, reverse Alhama de Murcia Fault (Betic Cordillera). Paleoseismic studies in this area yield data that can be compared and correlated with the morphologic and tectono-sedimentary evolution of the alluvial fan. The spatial arrangement of the sedimentary alluvial fan units near the fault zone, shown in trenches, is controlled by the recurrent reverse, left-lateral coseismic events. We analysed the morphology of the drainage network using a 1:5000 scale orthoimage to identify and measure horizontal deflections along the fault. The channel pattern analysis allowed us to estimate the average horizontal slip rate of the SAMF for the last 130 ka. This value is 0.21 mm/a, which is slightly higher than the range of values obtained by trenching analysis for the last 30 ka, (0.06 to 0.15 mm/yr). The interpretation of the stratigraphic sequence exposed along the trench walls constrained the occurrence of at least two surface faulting earthquakes during the last 30000 years. The most recent event happened after the El Saltador Creek dissected the alluvial fan. The penultimate event occurred while the alluvial fan was still active.
Highlights
We present a study of the El Saltador alluvial fan, which is situated in the Southeastern Betic Cordillera, and is associated with an active mountain front created by the Lorca-Totana section of the Alhama de Murcia Fault
The mountain front was created by the activity of the NAMF, but the strand of the Alhama de Murcia Fault (SAMF) has absorbed most of the shortening in this area since the Messinian, and affects the upper layers of the larger Pleistocene alluvial fans (Martínez-Díaz, 2002)
Sedimentology and morphology of a sequence of Quaternary alluvial fans in relationship with the paleoseismic activity of the left-lateral, reverse Alhama de Murcia Fault, with the aim of interpreting the tectonic control on the fan dynamics and the fault evolution
Summary
The Southeastern Betic Cordillera is located in the southeast of the Iberian Peninsula close to the area where the African and the Eurasian plates converge. Recent paleoseismic studies in this area using trenching data (Hernández-Enrile et al., 2000; Martínez-Díaz et al, 2001) yield data that can be compared and correlated with the data obtained from our study of the morphologic and tectono-sedimentary evolution of the alluvial fan. To achieve this correlation we first carried out an analysis of the tectono-sedimentary features of the fan using field data from natural outcrops and from two trenches dug on the alluvial fan surface across the fault trace. The main subject of this paper is to understand how the tectonic activity influenced the development of geomorphic features and how to use it to correctly interpreting the punctual information coming from trenches, and to do a preliminary estimate of the horizontal displacement of an left-lateral, oblique-slip fault with paleoseismic activity
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