THE MIOCENE TIHAMA ASIR OPHIOLITE AND ITS BEARING ON THE OPENING OF THE RED SEA

Abstract

In early Miocene time (ca. 2. Ma ago), dike swarms, layered gabbros, granophyres, and basalts of the Tihama Asir ophiolite in S.W. Saudi Arabia were emplaced during the initial stages of Red Sea rifting and separation of the African and Arabian plates. As rifting began, dikes invaded Paleozoic and Mesozoic sedimentary and Precambrian metamorphic rocks of the Arabian plate along a northwesterly trend paralleling the axial trough of the Red Sea. With increased separation along the rift edge, new crust was accreted to the continental margin. This crust consisted of a 4 km wide zone of subparallel dikes having chilled margins against one another and lacking screens of older continental rock. This dike complex is analogous to the sheeted dike swarms described from the Cyprus, Oman, and Newfoundland ophiolites. Layered gabbros and granophyric intrusions within the dike swarm are products of magmatic differentiation developed during the initial stages of rifting. The differentiation trends indicate that the parent magma was tholeiitic and evolved by crystal fractionation in much the same manner as lavas at Thingmuli Volcano, Iceland. Late Miocene tilting of the sedimentary rocks and dike swarm toward the Red Sea axis and later erosion have exposed the continental-oceanic crust boundary along a narrow zone. The continental Baid Formation (19 Ma) rests unconformably on the Tihama Asir ophiolite.