Abstract

New primary data from northwest Africa show that the lower Mesozoic rocks of Morocco rest with profound unconformity on Hercynian metamorphic and/or Autunian sedimentary rocks and occur in three distinct, partially synchronous volcanic-sedimentologic provinces: the Oran Meseta, the High Atlas, and the Moroccan Meseta. The Oran Meseta of northwestern Morocco contains a Middle to Late Triassic andesite and carbonate-evaporite facies related genetically to the Tethys basin. The High Atlas province of southwestern Morocco consists of Late Triassic to Early Jurassic red beds and evaporites interbedded with tholeiite lavas. These tholeiites are underlain by the Minutosaccus-Patinasporites Concurrent Range Zone of middle Carnian age and yield an average isotopic age of about 196 m.y. They are time- and rock-stratigraphic correlatives of the First and Second Watchung-York Haven suite and the Quarryville basalts of Pennsylvania and New Jersey. The Moroccan Meseta of western and central Morocco consists of Lower Jurassic (Liassic) evaporites intercalated with low-alkali quartz tholeiites, yielding an isotopic mean age of 186 ± 8 m.y. This tholeiite is a rock- and time-stratigraphic correlative of the Rossville basalt of Pennsylvania. The tectonic model best explaining the chemical and stratigraphic distribution of lower Mesozoic rock now on the margins of the North Atlantic includes the following sequence: (1) Permian to Late Triassic uplift and crustal thinning along the axis of the future Atlantic Ocean; (2) Middle to Late Triassic strike-slip faulting and andesitic volcanism along east-trending fracture zones, followed by a westward advance of the Tethys Sea across northern Morocco; (3) Late Triassic rifting along the axis of the proto-Atlantic Ocean and shearing along east-west fracture zones, which had the combined effect of decoupling segments of the African and North American plates and providing a pathway for the marine transgression of the Tethys Sea across northern Morocco and south along the axis of rifting; and (4) Late Triassic to Early Jurassic crustal extension and extrusion of olivine and quartz tholeiites, followed by extrusion of subalkalic quartz tholeiites and collapse of the continental margins with the concomitant deposition of marine carbonates.

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