Abstract
The Betic Cordillera of southern Spain forms a clear example of a collisional orogen that has undergone large-scale late-orogenic
extension while convergent motion of the bounding plates continued. The orogen provides a unique opportunity to study the tectonics of the
system at different lithospheric levels. At shallow levels in the crust, fault-bounded
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intramontane basins, formed during the early to middle
Miocene, contain coarse continental sediments heavily affected by normal faulting, followed by a less deformed late Miocene marine succession.
Extension was accommodated by coeval shortening in thin-skinned fold-and-thrust belts in the periphery of the system, and much of the region
has now subsided to form a large marine basin, the Alboran basin. The thermal and deformational record of these processes is preserved in
rocks from deeper crustal levels in the internal zone of the Betic Cordillera. These rocks were metamorphosed down to 50 km depth and
are now exposed beneath major low-angle detachment zones that separate them from heavily faulted low-grade rocks above. Cooling ages
of associated mylonites indicate that these detachments were active during the early to middle Miocene. Peridotite massifs in the western
Betics emplaced in the early Miocene provide coherent outcrops of subcontinental upper mantle that allow insight in coeval processes in the
mantle lithosphere. The peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of
exhumation probably occurred during Mesozoic rifting. Cooling at mid-lithospheric depths possibly reflects early crustal thickening, followed
by extension and subsequent heating. A sudden rise of ambient temperatures in the mantle rocks by about 400 °C suggests loss of most of the
underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse.
All of these phenomena can be explained by some form of removal of the lithospheric root beneath a Paleogene collisional orogen, leading to
large-scale extension followed by thermal subsidence of the center of the system. The processes inferred here for the Alboran region are in all
likelihood not unique, as many similarities can be identified with the geology of the Tibetan Plateau, but also with domains in the Variscan and
the Pan-African orogenic belts where extensional processes and associated LP/HT metamorphism and magmatism can be shown to equally have
occurred in a convergent tectonic setting
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