Structure of the Northern Altar Pull-Apart Basin Revealed by a 2D Reflection Seismic Survey: Evolution of the Gulf of California Shear Zone in Northwest Mexico

The northern Gulf of California and Salton Trough contain segmented marginal basins abandoned during the oblique rift system's evolution during Late Miocene-Early Pliocene. The Altar basin, in northwestern Sonora, Mexico, contains a > 5 km-thick sedimentary record representing the first marine incursion (Late Miocene) of the Gulf of California seaway followed by the first deltaic deposits of the Colorado River. 2D reflection seismic lines were processed and interpreted to characterize tectonostrati-graphic features of the transtensional Pacific-North America plate boundary in the northern Altar basin (deep structure, faults controlling the subsidence and accumulation of deltaic deposits). The results show the acoustic basement becoming increasingly shallow toward the northeast, new NW-trending faults, and three major seismic reflectors defining the base of three units: A (oldest), B, and C (youngest). Through similarities in sequence stratigraphy and fauna, we correlate Unit A with the Bouse Formation (SW Arizona and SE California), implying its presence in northwest Mexico. The Altar fault strikes similar to N45 degrees W and aligns with the Dunas fault (SE California), suggesting that these faults are the same continuous structure. Seismic horizons above horizon C are less affected by faults. In contrast, horizons A, B and C are cut by faults, have steeper dips, and are laterally discontinuous. We propose the deposition of unfaulted strata occurred after the latest Pliocene abandonment of the Altar basin. Cessation of major transtensional activity in the Altar basin is coincident with a regional westward shift of transtensional plate boundary deformation, preserving a record of the evolving Gulf of California shear zone in northwest Mexico.