EAST WEST THRUSTING AND ANOMALOUS MAGNETIC DECLINATIONS IN THE SIERRA-GORDA, BETIC-CORDILLERA, SOUTHERN SPAIN

Structural and palaeomagnetic studies in the Sierra Gorda (Sierra de L oja), located in the External zone of the Betic Cordillera, indicate t hat westward-directed thrusting is not associated with significant rot ations about a vertical axis. Detailed mapping and slip vector analysi s show that the Sierra Gorda is a thrust complex composed of three thr ust sheets. The uppermost thrust places Early Jurassic pelagic carbona tes on top of Jurassic to oligocene sediments that form a large doubly -plunging footwall syncline. The eastern limb of this syncline has bee n overturned and is tectonically thinned as a result of the overthrust ing. Palaeomagnetic results from Mesozoic and Tertiary sediments both within and around the perimeter of the Sierra Gorda indicate that: (1) the average remanence vector of the seven Late Jurassic localities sa mpled within the Sierra Gorda has a direction (D = 328-degrees and I = 38-degrees) that is not significantly different from the expected dec lination for the Upper Jurassic of stable Iberia; and (2) there is no significant difference between the remanences in the two upper thrust sheets indicating that differential rotation did not occur during the initiation and displacement on the thrusts. In contrast, the one Late Jurassic site that was sampled to the west of the Sierra Gorda is rota ted, like the rest of the Subbetics, 60-degrees clockwise of the refer ence direction. The unrotated directions obtained in the Sierra sugges t, either that it has rotated in a clockwise sense concordant with the rest of the Subbetic zone and has then been backrotated, or that it h as never rotated relative to stable Iberia. In the latter, simpler hyp othesis the unrotated declinations may be explained in terms of orthog onal convergence along an irregular continental margin.