The Bahia Tortugas fault basement mélange, Baja California Sur, Mexico: a record of forearc extension and orogenic mixing?
Department of Geological Sciences
San Diego State University
Advisor Dr. David Kimbrough
Friday, April 29th, 2011
South of the California Coast Ranges, the Vizcaino Peninsula, Baja California Sur provides the best view of Mesozoic forearc basement along the outboard edge of the southwest Cordillera. Relatively intact ophiolite complexes of Late Triassic and Middle Jurassic age are well-known from the Vizcaino region and related to forearc extension in the Mesozoic. A lesser known serpentinite matrix basement mélange is well-exposed within an approximately 11 km-long belt in the northern Vizcaino Peninsula where it is bounded by the northwest-trending Bahia Tortugas Fault. The mélange is interpreted as a serpentinite protrusion related to transpression along the Cenozoic fault. The talc-serpentinite matrix of the mélange has a weak subvertical foliation that wraps around the mélange blocks. Mélange blocks range in size up to approximately 100 x 1000m, average 10 m2 and occur in poorly defined zones where specific block types may dominate. Mélange block types include serpentinized peridotite, amphibolite, heterogeneous gabbro, trondhjemite/plagiogranite, metabasalt and ophicalcite. Although, there is no internal stratigraphy, the basic elements of a classic ophiolite are present and local semi-coherent zones exist. The semi-coherent zones occur dominantly in the southeast third of the mélange, where plagiogranite intrudes heterogeneous gabbro. Zircon analysis from plagiogranite blocks, at two localities, yields U-Pb crystallization ages of approximately 188.7 ± 1.2 and 194.6 ± 3.2 Ma. The ages are tentatively interpreted as the crustal formation age of the ophiolite sequence. These zircon ages confirm the presence of early Jurassic ophiolite basement in the region. The presence of antigorite indicates that shearing and serpentinization of the ophiolite occurred at depth. Whole rock major and trace element geochemistry of metabasalts reveal fractionated compositions with arc geochemical affinities. Geochemistry of amphibolite blocks indicate they have a boninitic character characteristic of a supra-subduction zone setting. The presence of exotic high grade amphibolites blocks indicate a potential underthrust component similar to blue-schist grade rocks exposed regionally. The ophiolite and underthrust high grade exotic blocks have been tectonically "mixed" within the BTF transpressive boundary serpentinite matrix mélange. Possible time equivalent and geochemical correlatives to the Bahia Tortugas Fault basement mélange occur in the western Sierran foothills and Klamath Mountains of California-Oregon.