ABSTRACT The Carrizo Plain, the only closed basin in California’s Southern Coast Ranges, preserves landforms and deposits that record both climate change and tectonic activity. An extensive system of clay dunes documents the elevations of late Pleistocene and Holocene pans. Clay dune elevations, drowned shorelines, eroded anticlinal ridges, and zones of perturbed soil chemistry provide evidence of two lake levels higher than today’s (currently 581 m above sea level [masl]), one at ~591 masl at ca. 20 ka and another at ~585 masl that existed at ca. 10 ka, based on optically stimulated luminescence (OSL) dates on clay dune sediment. Two cores from the abandoned floor of the lake provide additional evidence of a long-lived lake in the Carrizo Plain during the late Pleistocene. The longer of the two cores (~42 m) was sampled for palynology, environmental magnetism, and scanning electron microscope–petrography. The magnetic susceptibility signal contains two notable features corresponding to sedimentary materials consistent with reducing conditions. The higher of these features occurs near the surface, and the lower occurs at ~18 m depth. A 14 C date on charcoal from the upper reduced zone places the top of this zone at no older than 22.6–20.9 cal ka. This date is consistent with the OSL date on geomorphic features associated with a highstand above ~591 masl. Assuming that reducing conditions correspond to at least a few meters’ depth of relatively fresh water, the new 14 C date suggests that the upper reduced zone represents a marine isotope stage (MIS) 2 pluvial maximum lake in the Carrizo Plain. Pollen and ostracodes from the reduced sediments indicate a wetter and cooler climate than today. These conditions would have been capable of sustaining a lake with water much less saline than that of the modern lake. The timing of the oldest documented highstand (no later than 20 ka) is consistent with a modified jet stream migration model and is not consistent with a tropical incursion model. Northeast-to-southwest asymmetry across the lake floor may be consistent with southwestward tilting driven by Coast Range shortening normal to the San Andreas fault, as is seen throughout the region.

The Hubbell Spring fault system lies near the eastern margin of the Albuquerque–Belen Basin in the central Rio Grande rift, and it is one of the most active normal faults in the region. Recent mapping and geophysical studies indicate that fault geometry is more complex and longer than previously thought, with several significant, subparallel, anastomosing, west-dipping splays that form a broad zone as wide as 18 km and ~74 km long. We conducted a paleoseismic investigation of the previously untrenched central Hubbell Spring fault splay (splay L) at the Carrizo Spring site. Our study included mapping, trenching, drilling, and luminescence analyses. We found structural, stratigraphic, and pedologic evidence for the occurrence of at least four, possibly five, large earthquakes that occurred since deposition of piedmont deposits on the Llano de Manzano surface ca. 83.6 ± 6.0 ka. All of these events included warping across a broad deformation zone, whereas the three largest events also included discrete slip across four fault zones. Behavior appears noncharacteristic (i.e., highly variable slip per event), with preferred vertical displacements ranging from 0.4 to 3.7 m. The total down-to-the-west throw of piedmont deposits is 7.3 ± 1.0 m. Luminescence ages indicate that the timing of the four largest surface-deforming events on fault splay L overlaps with the timing of the four youngest faulting events from previous studies of the western Hubbell Spring fault splay (or splay J), suggesting simultaneous rupture of fault splays J and L. Displacement data and correlation of buried soils on event horizons between sites also support simultaneous rupture; however, timing constraints are on the order of thousands of years, and so triggering of events between splays cannot be precluded. The smallest warping event on fault splay L, event Y(?), does not appear to correlate to any events on splay J, suggesting that independent rupture of fault splay L also occasionally occurs. Assuming simultaneous rupture of splay J and L, the average recurrence interval over the past three complete seismic cycles is 19 (+5/−4) k.y., consistent with recurrence intervals estimated for individual cycles, which are 17 k.y., 27 k.y., and 14 k.y. We estimate an average vertical slip rate for the past four complete seismic cycles on splays J and L of ~0.2 mm/yr. In comparison, recent average late Quaternary slip rate estimates for the entire Hubbell Spring fault system range between 0.2 and 1.0 mm/yr, based on topographic profiles transecting all the splays. Slip rates for individual complete seismic cycles for splays L and J vary through time by an order of magnitude, ranging from 0.044 mm/yr to 0.46 mm/yr. This is not due to temporal clustering of earthquakes but instead is primarily due to large variations in slip per event, a finding that may have significant implications for seismic hazards elsewhere in the Rio Grande rift. Additional investigations are needed to determine the paleoseismic behavior of the many other splays of the Hubbell Spring fault system and better characterize this complex fault system for seismic hazard evaluations in the Albuquerque region.

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