Tectonostratigraphic Terranes of Magdalena Island, Baja California Sur

Abstract

Magdalena Island, Baja California Sur, consists of two, possibly three, distinctive tectonostratigraphic terranes, the northern, central, and southern(?) Magdalena terranes. The northern Magdalena (ophiolitic) terrane extends from Caho San Lazaro southward to Punta Arena and consists of fault-bound slabs of serpentinized harzburgite, cumulate ultramafic and gabbroic rocks, intrusive gabbro, basalt, and tuff, plus minor chert and limestone all of unknown age. These rocks have a strong penetrative foliation and metamorphic mineral assemblage indicative of the amphibolite to lower greenschist facies.

Near Punta Arena, an east-west-trending high-angle normal fault, down on the south, juxtaposes an unmetamorphosed island-arc sequence, the central Magdalena (arc) terrane, against the northern Magdalena ophiolitic terrane. The central Magdalena terrane sequence consists of andesite and basaltic andesite(?) dikes, sills, pillow lavas, and plagioclase porphyry breccias. Thin-bedded tuffaceous turbidites intercalating and possibly overlying the igneous rocks contain Late Jurassic (Kimmeridgian?) ammonites. Associated with the volcaniclastic sediments are thick mudflow or olistostrome deposits derived from fringing reefs that contain blocks of coralline limestone with an abundant shallow-water fauna of Jurassic age.

At the south end of Magdalena Island, a second prominent east-west-trending high-angle fault juxtaposes basaltic(?) pillow lava, tuff, argillite, and radiolarian chert against the central Magdalena arc terrane. This southernmost fault block, the southern(?) Magdalena terrane, may be a fragment of a terrane of considerable extent present on Santa Margarita Island to the south, or ophiolitic basement that the central Magdalena (arc) terrane was built upon.

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