Abstract

The Ronda massif is an outcrop of generally unaltered peridotite covering approximately 300 km2 on the southern coast of Spain west of Malaga. An emplacement age between Triassic and Miocene is indicated from geological evidence.

One margin of the intrusion was undisturbed by later tectonic movements, and the structural development along a traverse outward from this steep contact of the massif was examined in detail. The progressive increase in deformation toward the contact to form phyllite and two generations of gneiss, the spatial attitude of deformation structures and their tectonic interpretation, and the interaction of the deformation with contact metamorphic minerals all support the hypothesis that the structures observed are related to the emplacement of the massif. The structural information is combined with that from a petrologic study and gravity work to suggest that the massif penetrated vertically as a hot solid ridge.

The peridotite ridge intruded the crust vertically, dragging up pelitic rock on the contacts from the lower crust, and displaced roof rock laterally away from the area above the massif as it penetrated the crust. The heat flow from the massif was modeled in an attempt to fit the observed gradient as deduced from the metamorphic aureole; this gradient can be matched only by taking into account the mass movement of material near the contact to bring up geothermal heat. An intrusion temperature of 1,000° to 1,200° C is considered to be most probable based on the metamorphic contact temperature and the petrogenesis of basic layers within the massif. Higher temperatures in the mantle are indicated.

The massif was probably a diapiric intrusion with a rigid top pushed by a root extending through the lithosphere to the warmer astheno-sphere. Mechanical considerations suggest that the massif could have been driven part way through the crust by buoyant pressure in the mantle, as implied by the presence of crustal rock on the margins with a high-pressure history. A model of intrusion which would reduce the potential energy requirements of crustal thickening by the displaced crust allows the separation of adjacent crustal (lithospheric) blocks to open the volume of intrusion; plastic necking of the crust would occur near the hot massif.

The ultramafic massif is considered to be part of two narrow ridges of high-density material along the margins of the Alboran Sea (continental graben) which were identified in a regional gravity survey. It is proposed that the ridges represent asthenospheric rock that welled up through extensional zones rapidly developed by rifting of lithospheric blocks. The geometry suggests a system in which the extensional zones radiate from a point near Gibraltar. These zones may be related to a counterclockwise rotation of Spain with respect to Africa about this point. Such a system would be expected if Spain acted as a roller block between Europe and Africa and is analagous to the opening of the Bay of Biscay.

Continued extension and upwelling (as in other Mediterranean orogenic areas such as the Alps and Appenines) would have resulted in higher temperature diapirs, expulsion of the basic layers as basaltic magma, and a thinner crust to produce the ophiolite suite. In the extreme, a mid-ocean ridge system would be produced.

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