MAPPING AND MODELING OF AN 18-KM-DEEP, SUBDUCTION-RELATED FLEXURAL BASIN WEST OF THE STRAIT OF GIBRALTAR

Abstract:

The existence of eastward subduction is beneath the Gibraltar between southern Spain and Morocco is a long-lived controversy because of: 1) the absence of a recently active volcanic arc; 2) the lack of instrumentally-observed, eastward-dipping thrust earthquakes in the inferred subduction area west of the Strait of Gibraltar; and 3) an eastward-dipping subducting slab that is largely aseismic and poorly expressed on tomographic data. If the area of the Atlantic Ocean is subducting into such a laterally-confined, 150-km-wide subduction zone and is what is inferred by some to be a vertically-dipping slab, we can infer that the oceanic plate will be strongly flexed – especially because the plate is known from magnetic anomalies to be of Jurassic age. Cold, dense lithosphere will be prone to readily flex and sink into the mantle even if the westward rate of subduction is known from GPS studies to be as slow as 5-6 mm/yr. In this presentation we describe on the 14-km-deep basin on Jurassic oceanic crust west of the Gibraltar arc that is largely filled by an accretionary wedge. The basin forms a roughly circular depression with a diameter of 120 km and whose steeper, western edge is inferred to the lithospheric contact between the eastward subducting Atlantic plate and the westward-facing Gibraltar arc. We present industry seismic reflection data that images the outer hinge of the flexural basin along which oceanic crust is flexed downward towards the basin along a zone of recent deformation. We present three flexural profiles that extend from the Gibraltar 500 km to the west that are constrained by the properties of Jurassic oceanic lithosphere and predict the observed basin geometry.