The Morphotectonics

of the Spanish Sierra Nevada

a study using Digital Elevation Modelling.

 Abstract 

The Sierra Nevada of Southern Spain began to emerge as a domed antiform, bordered by clear normal faults, during continent-continent collision of the African and European plates in late Oligocene times.   Isostatic rebound of a thickened crustal root below the mountain is thought to be responsible for over six kilometres of uplift since the Serravallian.   The "trilobite" shape to the mountain block suggests a more complex uplift history, which is reflected in the great diversity of tectonic geomorphology around the mountain front.   The morphotectonics of the NW-SE trending western mountain front suggest more recent uplift than on the E-W trending southern mountain front.

The historical and instrumental record of earthquake activity reflects this geomorphology with a concentration of palaeoseismic activity to the west of the block during the past 500 years.   Fault plane solutions emphasise the dip-slip nature of the NW-SE trending faults and the strike-slip dominated nature of the E-W trending faults.

The Sierra Nevada mountain front is a classic example of segmented faulting and this study has divided the 200 kilometre mountain front into nineteen segments, based on geomorphology, fault strike, drainage and the position of alluvial fans.   The varying patterns of uplift and quiescence on each fault segment have been preserved in the fault controlled mountain slopes above, and in the drainage systems and alluvial fans whose base level is controlled by each fault segment.

Digital Elevation Models (DEMs) of the mountain have been used to quantify the geomorphology of these slopes, the drainage networks and the form of the alluvial fans.   DEM   histograms and profiles along ridges reveal five laterally extensive pediments in the mountain side which are thought to represent five periods of quiescence since the mountain emerged.   Upper Miocene sediments found at 1700 metres above the current mountain front have been used to calculate retreat rates of each major drainage channel.   These erosion  rates have been used to date the embayments in the other pediments to indicate local uplift rates for each fault segment.

From the evidence presented in this thesis it is suggested that the geomorphology of the Spanish Sierra Nevada is caused by uplift of an asymmetrical horst block driven by isostatic rebound of a thickened crustal root, in a context of  varying horizontal 2 compression directions from tectonic convergence of the European and African plates.   Tectonic quiescence and pediment formation is thought to occur during rotation of compression directions and is usually proceeded by renewed uplift on different fault segments, under an altered tectonic regime.

Reconstruction of the rupture histories of different segments has been used to infer the corresponding changes in the convergence direction of the African and European plates.   This research provides new tectonic data on the Sierra Nevada and the recent plate tectonic history of the Gibraltar arc.   It also highlights the application of DEMs to studies of geomorphology and neotectonics.