Pre-Triassic fit and alpine tectonics of continental blocks in the western Mediterranean

Abstract

In the western Mediterranean, during Jurassic to Miocene time, the alpine orogenic belt fragmented into several continental blocks. The relationships between these different blocks, their initial (pre-Triassic) position, and their original fit have not been defined, although various hypotheses have been proposed. To reconstruct the pre-Triassic fit, a new approach is suggested which utilizes the reconstruction of the upper Paleozoic sedimentary basin as the basis for determining relationships between blocks. Examination of the sedimentology and tectonics of the upper Paleozoic of some parts of these scattered blocks, especially those of the Rif (Morocco), Djebel Chenoua (Kabylies, Algeria), and comparison with the upper Paleozoic of the Betic Cordillera (southern Spain) and of the island of Minorca (Balearic Islands, Spain), show the following. (1) Sedimentary zone I, in which Minorca, a part of the Rif, and the Djebel Chenoua have the same sedimentary facies (canyon, deep-sea fan, and slope deposits). These blocks must have been in proximity before the Triassic period. (2) Sedimentary zone II, which joins together southern Spain, the rest of the Rif, and the rest of the Kabylies. (3) These sedimentary zones are parts of a unique, mainly detrital marine basin, including all of these blocks which can then be fitted into a single original Alboran block. (4) A very important Variscan (Late Devonian to Permian) and pre-Variscan fracturing with two main directions (now N120°–140°E and N30°–50°E) existed in the Alboran block. This network has been reactivated during post-Triassic continental motion. A proposed pre-Triassic fit is described, in which the Alboran block is considered to be a sliver of the Iberian block, attached to the latter near the Balearic Islands. In terms of global tectonics, a new alpine evolutionary tectonic pattern is proposed for the Alboran block, with two successive major events: (1) Californian-type tectonics with subduction and Californian-type rifting (Jurassic to Miocene); during this time, the Alboran block slid along the Iberian block, much as southern California and northern Baja California have shifted along the edge of the North American plate. In the Mediterranean, this motion was due to the presence of two oceanic plates which disappeared by subduction in the western Mediterranean. The displacement of the Alboran block was facilitated by the Variscan fracture directions (N30°–50°E), which promoted differential rifting in the block, with horizontal displacements of more than 700 km. (2) Collision tectonics occurred between Africa and Europe (Miocene to Holocene): the differential movements between African and European plates and the Iberian block provoked compressive tectonic episodes in the intermediate region, which constituted the Alboran block. Continental margins of the European and African plates as well as the continental margin of the Alboran block were crushed between the major plates, producing spectacular but superficial thrusts.