Geology of the San Luis and San Lorenzo Island groups, Gulf of California

Scale of accompanying map on Plate 1: 1:30,000Scale of accompanying map on Plate 2: 1:18,750Title of accompanying map on Plate 1, folded in pocket: Geology, bathymetry and gravity of the San Luis Island group. Size: 106 x 46 cm.Title of accompanying map on Plate 2, folded in pocket: Geology of the northern portion of the San Lorenzo Island group. Size: 106 x 36 cm.Title of accompanying map on Plate 3, folded in pocket: Geology of San Lorenzo Island. Size: 97 x 26 cm.Center coordinates of accompanying map on Plate 1 in decimal degrees [longitude/latitude]: -114.458333?? and 30.041667??Center coordinates of accompanying map on Plate 2 in decimal degrees [longitude/latitude]: -112.891667?? and 28.750000??The base maps for the San Luis Island group were constructed from 1:30,000 aerial photographs taken by the Mexican and United States governments. No source or reference is given as to the date and type of aerial photography. Aerial photography for the ???San Lorenzo Island group was taken with a K-20 reconnaissance camera, using a Cessna 172 airplane flying at 10,000 feet??????. These photographs were used to construct base maps for each island.Bibliography on leaves [69]-71The Gulf of California is believed to be an area\ud of active spreading and movement along transform faults.\ud Astride one of these major transform faults, the Guaymas\ud lineament, lie two island chains, the Isla San Luis\ud group in the north, and Isla San Lorenzo group in the\ud south.\ud The Isla San Luis Island group consists of five\ud islands, all uplifted fault blocks of Miocene calcalkali\ud volcanic rocks. These blocks are upthrown on\ud the west and dip to the northeast. Young acidic volcanic\ud rocks overlie these Miocene rocks on Isla San\ud Luis. Fractures on these islands suggest active faulting\ud related to the regional tectonic framework.\ud The San Lorenzo group also consists of uplifted\ud fault block islands exposing a Cretaceous tonalite\ud basement complex intruded into a metasedimentary\ud sequence. These rocks are overlain by Miocene calcalkali\ud andesitic volcanic rocks. Unconformably over\ud these rocks are fossiliferous lower Pliocene clastic and\ud evaporite sedimentary rocks. Young volcanism is evidenced\ud by Isla Raza which consists of olivine basalt.\ud The chemistry of the volcanic rocks and the\ud geology of the islands are compatible with current\ud theories of the history of the Gulf of California