Abstract

Coastal Sonora between Puerto Lobos and Bahia Kino can be subdivided into four structural-petrographic subprovinces: an inland subprovince in which unmetamorphosed upper Precambrian and Cambrian strata rest on older Precambrian gneiss and three other subprovinces in which Cenozoic volcanic strata rest on metamorphosed strata of post-Precambrian age intruded by granitic rocks of Mesozoic age. One of these latter subprovinces displays basin-and-range fault blocks; a second has northwest-trending strike-slip(?) faults; and the third, Isla Tiburón, shows structure related to the Neogene dilation of the Gulf of California depression.

The pregranitic rocks include upper Precambrian and Cambrian carbonate rocks, a chert-graywacke-volcaniclastic sequence of probable Carboniferous age, and volcanic-volcaniclastic rocks of Jurassic age. The granitic rocks range from gabbro to granite and have K-Ar cooling ages of from 91 to 30 m.y. Dikes of basaltic to dacitic composition and quartz porphyritic bodies of late Mesozoic or early Cenozoic age cut the granitic rocks.

The lowermost Cenozoic volcanic strata (pre–22 m.y. B.P.) are predominantly composed of rhyolite and basalt. These are followed by a sequence of predominant andesite (∼20 to 18 m.y. old), a sequence of partially marine conglomerate and pyroclastic deposits, and a widespread, predominantly rhyolitic sequence (∼14 to 10 m.y.). All strata 10 m.y. old or older are involved in basin-and-range–type tilting. Volcanic strata less than 8 m.y. old are nearly flat-lying.

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