[1] The shortening between the African and the Iberian plates is absorbed by a number of faults distributed over a very wide zone with very low slip rates and long periods of seismic loading. Thus a seismotectonic map based only on faults associated with seismicity or with expressive geomorphic features is incomplete. It is possible to characterize seismogenic faults using paleoseismology. First, paleoseismological results based on trenching analysis in the eastern Betics (Lorca-Totana segment of the Alhama de Murcia fault) are presented. The main paleoseismic parameters of this fault segment are (1) a minimum of two to three M w 6.5-7 earthquakes in the last 27 kyr (shortly before 1650 A.D., between 830 and 2130 B.C. and shortly before 16.7 ka, respectively), with a mean recurrence period of 14 kyr, and a very short elapsed time, and (2) a net slip rate of 0.07-0.6 mm/yr during the last 30 kyr. These results were extrapolated to the rest of the known active faults in the eastern Betics and were added to the slip rates of the active faults at the African margin. The total slip rate of the transect, which crosses de Alhama de Murcia fault in Spain and reaches the Cheliff basin (Algeria), would represent 21-82% of the total shortening between Africa and Eurasia estimated from plate motion models and seismic moment summation. A number of factors could account for this discrepancy: (1) hidden seismogenic faults in the emerged areas, (2) absence of correlation between current and late Pleistocene slip rates, (3) extensive small faults that are undetected and that absorb a significant amount of the deformation, and (4) possible overestimation of the convergence rates.
Most previous studies related to the recent tectonic development of southeastern Betic Cordillera, and in particular the south of Almería, agree with the kinematic variety that can be observed in the postMiocene structures at different scales. This variety is difficult to explain if we consider only the regional stress field related to the convergence of the Eurasian and African plates that remained relatively steady over the last 9 Ma. This work proposes a block tectonics with horizontal and vertical relative movements driven by strikeslip movements of major faults to explain the variety of kinematics observed in the area from Middle Miocene to present. The combined activity of the Alpujarras fault zone (dextral strike-slip) and the Carboneras fault zone (sinistral strike-slip) induces, within the major block bounded by them, the squeezing of minor pre-existing fault bounded crustal blocks. This process generates traction areas inside the escaping block, which are linked with the existence of gradients of westward movement rate. This tectonics can explain some of the structures related with the extensional tectonics predominant in the south of Almería province and the development of the minor and major morphostructures driven by faults with different kinematics. The tectonics of blocks proposed is useful to give a coherent interpretation of apparently incoherent local structural data into a regional and homogeneous compressional stress field.
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.
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