Gravity anomalies, ultramafic intrusions, and the tectonics of the region around the Strait of Gibraltar

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A new compilation of gravity data in the region surrounding the Strait of Gibraltar between 2° and 9°W and 34° and 37°30′N is presented in the form of a Bouguer anomaly map. Published data and new data obtained by Princeton University in southern Spain are included. There are four major features of the anomaly map: (1) An arcuate zone of negative anomalies, more than 200 km wide, parallels the trend of the Betic and Rif orogens. The gravity low crosses the Strait of Gibraltar and includes the Atlantic approach to the Strait and the western Alboran Sea. Values as low as −130 mgal in Spain and −150 mgal in Morocco were reduced. (2) Two coastal gravity high zones along the Moroccan and Spanish margins of the western Alboran Sea show steep gradients and closures as high as 110 mgal superimposed on the arcuate negative zone. The gravity highs in part coincide with a belt of outcrop of ultramafic and associated metamorphic rocks. (3) A central high in the Alboran Sea begins about 90km east of Gibraltar and extends eastward into the axial high of the western Mediterranean. (4) Positive anomalies trend WSW from southern Spain and westward from northern Morocco into the Atlantic approaches to the Strait of Gibraltar. All four major gravity features show a rough symmetry about a plane striking E-W through the Strait of Gibraltar. The most important conclusions that can be drawn from the data are the following: (1) The steep gravity gradients and magnitude of the coastal highs require steeply dipping density discontinuities probably reaching the mantle. The data support recent petrological and structural evidence from Spain of diapiric intrusion of ultramafic rock from the mantle and conflict with previous hypotheses that propose a thin thrust sheet structure of these masses. (2) Continental crust extends across the Strait of Gibraltar in a belt more than 200 km wide, including the western Alboran Sea. The continuity and symmetry of the anomaly pattern across the Strait agree with similar geological observations and correlations of rocks as old as Paleozoic and imply that the Betic and Rif have been part of a single tectonic system including the Alboran Sea probably since Precambrian time. Neither gravity nor geological evidence supports proposals of a major zone of plate discontinuity (transform faulting) between Spain and Morocco as a continuation of the Azores-‘Gibraltar’ ridge from the Atlantic. (3) The central Alboran Sea high is consistent with either a crustal break or a crustal thinning model. Significant zones of thin crust and possible crustal extension are found also in the Atlantic approaches to the Strait of Gibraltar.