Late Quaternary faulting history of the Carrizal and related faults, La Paz region, Baja California Sur, Mexico

Abstract

The southwest margin of the Gulf of California has an array of active normal faults despite this being an oblique-divergent plate boundary with spreading centers that localized deformation along the plate boundary 2-3 million years ago. The Carrizal and Centenario faults form the western border fault of the Gulf of California marginal fault system within and south of La Paz Bay, and ~20-30 km west of the capital city of La Paz, Baja California Sur, Mexico. Geologic and geomorphic mapping, optically stimulated luminescence (OSL) geochronology, and paleoseismic investigations onshore, compressed high-intensity radar pulse (CHIRP) profi ling offshore, and analysis of uplifted marine terraces in the footwall of the offshore Carrizal fault provide some of the fi rst numerical and geometrical constraints on late Pleistocene-Holocene faulting along the Carrizal fault. The onshore Carrizal fault has ruptured with up to ~1-2 m of vertical displacement per event, likely producing ~M 6.3-6.9 earthquakes, and at least two to three surface rupturing earthquakes have occurred since 22 ka. Onshore paleoseismic excavations and uplifted marine terraces on the western side of La Paz Bay both suggest offset rates of 0.1-0.2 mm/yr, with a footwall uplift rate of 0.13 mm/yr since 128 ka, and an approximately constant rate since marine oxygen-isotope stage (MIS) 11 terraces (420 ka). A CHIRP survey identifi ed underwater fault scarps with heights ranging from 21 to 86 m on the Carrizal fault in La Paz Bay and from 3 to 5 m along the Centenario fault. The offshore Carrizal fault lies 8-10 km east of the western edge of La Paz Bay, forming a right step from the onshore Carrizal fault. The offshore Carrizal fault is the oldest fault of the fault system, and the fault likely grew in the latest Miocene to Pliocene in a complex way to the south toward the onshore Centenario and Carrizal faults. When the Alarcon spreading center started its modern rates at 2.4 Ma, the Carrizal fault likely slowed to the 0.1-0.2 mm/yr rates of the late Quaternary determined in this study.

Strain partitioning in highly oblique rift settings: Inferences from the southwestern margin of the Gulf of California (Baja California Sur, México)

“…However, both field observations and thermochronologic data cast doubt on the Neogene activity of the La Paz fault (Fletcher et al, ). The most recent structures in this region consist of an extensive array of normal faults, which are generally steeply dipping, ~N to NNW‐striking and ~E to ENE‐dipping (Fletcher & Munguía, ; Fletcher et al, ; Busch et al, ; Coyan et al, ; Umhoefer et al, ; Figure ). These faults cut through the Los Cabos block, and bound two ~N‐S–oriented extensional basins (i.e., the San José del Cabo and the San Juan de Los Planes basins) that are filled with Middle Miocene to Quaternary sediments.…”

“…On the basis of geomorphological observations and seismic data, many of these faults are Quaternary‐active. In particular, recent normal faulting in the southernmost Baja California Peninsula is evidenced by the offset of Quaternary alluvial deposits and geomorphologic features along the main normal faults (Busch et al, ; Coyan et al, ; Umhoefer et al, ). Cerralvo Island is bounded to the east by a normal fault scarp that may have generated some extensional earthquakes (Fletcher & Munguía, ; Figure ).…”

“…Simplified geological map of southern Baja California Peninsula (location in Figure ; adapted from Hausback, , Drake, , and Bot et al, ) and offshore faults (mainly from Umhoefer et al, , Duque‐Trujillo et al, , and Ferrari et al, ). Lower left inset is schematic geological cross section A–A′ (San Evaristo); see location on northern part of main map.…”

“…The timing and spatial distribution of extensional deformation at the Pacific‐North America plate‐boundary that led to rifting are thus uncertain and debated (e.g., Fletcher et al, ; Oskin & Stock, ; Umhoefer, ). In this regard, an intriguing point regards the observation that, in spite of the highly transtensional plate motion, the margins of the main rift zone are often characterized by dominant dip‐slip normal faulting (e.g., Busch et al, ; Umhoefer et al, ; Umhoefer et al, ; Umhoefer & Dorsey, ). In addition, focal mechanism solutions of earthquakes document the presence of NNW‐trending active normal fault systems along the south‐easternmost part of the Baja California Peninsula (Fletcher & Munguía, ).…”

Miocene detachment faulting predating EPR propagation: Southern Baja California

“…This GOC normal extension turned finally to NW‐SE [ Angelier et al ., , Colletta and Angelier ; Geoffroy and Pronost , ; Umhoefer et al ., ]. The development of some newly formed N‐S to NE‐SW high‐angle normal faults [ Umhoefer et al ., ], including possibly the San José del Cabo fault, is consistent with this latest evolution. However this late NW‐SE extension is weak and probably transient since the present‐day stress field indicated by earthquakes along the margin is more consistent with NE‐SW extension [ Munguía et al ., ].…”