STRUCTURAL SURVEY IN PLUTONS FROM THE PACIFIC COAST TO THE SIERRA SAN PEDRO MARTIR, CENTRAL BAJA CALIFORNIA

Based on a satellite image structural interpretation of 10,000 km2 between the Pacific coast and the Gulf Escarpment (GE), Cretaceous individual and nested plutonic bodies were identified from curvilinear lineaments. At the latitude of San Pedro Mártir (SPM) and San Quintín (SQ), the central part of the area is characterized by series of up to 25 km-long lineaments oriented 340° to 320°, respectively. Such lineaments parallel the well-known magnetite-ilmenite boundary that divides the western Fe-rich form the eastern Ti-rich plutons. It is observed that the eastern plutons are larger and intrude metamorphic rocks (mostly Paleozoic) while the western intrusives are associated to the volcanic arc sequence of the Cretaceous Alisitos Group. Dominant orientation of lineaments in the SPM sector is NW-SE, being N-S in the SQ sector near the Gulf Escarpment. It is inferred that the well-defined 040° lineaments of the entire area resulted from the capture of present Baja California peninsula by the Pacific Plate. We measured primary and fault structures in the same sites of a paleomagnetic survey crossing 8 plutons: San Telmo (T), Aguaje del Burro (B), La Zarza (ZA), San José (SJ), Potrero (PO), San Pedro Mártir (SPM), El Milagro (MI) and El Socorro (SOC). Faults are presumed to be contemporaneous to a time span between the end of subduction (ca. 15 Ma) and the beginning of the San Andreas-Gulf of California transform system (ca. 6 Ma). Faults with cinematic indicators were only observed in 10 sites of B, 15 of SJ and 21 of SPM. We applied the direct inversion method to calculate the principal stress directions of each site and obtained the following resultant orientations for B, SJ and SPM, respectively: s1=141/76, 21/48 y 76/58; s2=286/12, 308/11 y 297/17; and s3=17/8, 216/18 y 205/37. The results indicate extensional vectors averaging 026°, almost perpendicular to the orientation of the GE normal faults of the SPM sector, where deformation is more intense. Finally, by contrast with seismic alignments connecting NW-SE active faults in the northern Baja California, it is interpreted that NE-SW oriented lineaments result from simple shear deformation associated to the movement of the peninsula towards NW.