TALES FROM THE RIFT: OLIGO-MIOCENE TRANSTENSIONAL BASIN DEVELOPMENT AND MAGMATISM IN THE SOLEDAD ROJO BASIN (LOWER COLORADO RIVER CORRIDOR) DURING EARLY PACIFIC-NORTH AMERICAN PLATE INTERACTIONS

Since the mid-Oligocene (ca. 30 Ma), much of SW North America has experienced a complex geologic history related to the development of the Pacific-North American (PNA) plate boundary. Often viewed as an archetypal continental transform plate margin, recent work has suggested this interpretation of the PNA is too simplistic; rather, the boundary is a transtensional continental rift that is progressively “unzipping” and lengthening the East Pacific Rise northward. While a majority of PNA relative motion occurred along the San Andreas Fault, a sizable amount (~25%) is distributed within the ~300 km-wide zone extending eastward into the continental margin that stretches from the Walker Lane east of the Sierra Nevada, south through the Eastern California Shear Zone in the Mojave Desert, into the lower Colorado River corridor (LOCO) east of the Salton Sea, terminating in the Gulf of California Extensional Province. In comparison to the other sections of the rift system, the geologic history of the LOCO is less known, yet may represent an intermediary developmental stage between initial transtensional continental rifting (e.g., Walker Lane) and terminal oblique seafloor spreading (e.g., Gulf of California).

This study presents new mapping and stratigraphic interpretations from the Soledad Rojo basin in the LOCO, NE Imperial Co., SE CA. These late Oligocene-age (~24-25 Ma) synextensional deposits consists of: 1) a basal bimodal volcanic sequence of silicic welded ignimbrites & basaltic lava; 2) volcaniclastic-infilled paleochannels carved into and derived from the basal volcanic unit rocks; and 3) the “Tolbard Fanglomerate”, an alluvial fan/fluvial unit comprised of A) a lower subunit of brick red conglomeratic sandstone, metaplutonic/volcanic subangular (locally rounded) conglomerate and B) an upper subunit of light gray-buff conglomeratic sandstone & subangular metaplutonic/volcanic conglomerate. The Soledad Rojo basin was a synvolcanic half-graben created during regional mid-Tertiary extension associated with the newly developing PNA margin. Subsequently, during the Miocene (pre-Bouse Fm.) the Soledad Rojo basin was offset by several newly identified right-lateral strike-slip and transtensional normal faults associated with the Walker Lane-Gulf of California rift system, with right-lateral displacement of the stratigraphic units up to 2.5 km.