Prehistoric life in the Colorado Desert endured a variety of environmental extremes. Episodic flooding and shifts in the course of the Colorado River resulted in the infilling of the Salton Basin and created a large freshwater lake known as Lake Cahuilla. Settlement along the different segments of the lakeshore is considered variable and may reflect accessibility to nearby viable resources. Remains from archaeological excavations at lakeshore sites show that lacustrine resources and fishing opportunities attracted prehistoric groups to the ancient lake. How prehistoric groups organized themselves and utilized lakeshore and nearby resources offer opportunities to explore the subsistence and mobility strategies of populations living in an oscillating environmental context. Using information generated from past Cultural Resource Management projects, the current study analyzes multiple data sets to address questions of a regional scale to more fully understand the effects of cyclical Lake Cahuilla on desert inhabitants. Analysis of existing collections and their associated documentation from late prehistoric habitation sites adjacent to the northwestern maximum shoreline as well as recessional shoreline sites some 30 miles to the south provide additional information on resource availability in a changing environment. It appears that in some circumstances the northwestern lakeshore inhabitants adapted to a changing environment and maintained occupation spanning multiple lake stands. During high stands, subsistence practices focused on lacustrine resources until no longer viable and habitation sites feature specialized subsistence technology reflecting fish procurement and processing. During lake recession, at least short-term habitation was sustained and corresponded to the exploitation of specific fish and waterfowl species. This study will help us better understand the strategies employed by groups who utilized the resources of this fluctuating lacustrine environment. Examination of resource use and mobility patterns practiced by prehistoric Lake Cahuilla inhabitants allows for interpretations of the adaptations necessary for life within this desert region. Ultimately, this research is applicable to broader anthropological queries on a regional scale. The Salton Basin is positioned within a geographical region that likely experienced influence and change from the surrounding environs. Gaining a deeper understanding of the study area will ultimately aid in future research concerning environmental adaptation, exchange relations, and culture change among the neighboring regions of the Mojave and Great Basin deserts, the agricultural Southwest, adjacent mountains and coast lines, and Baja California (Schaefer and Laylander 2007:381). Additionally, an understanding of how resource availability influenced past populations can contribute to ongoing and future studies concerned with resource management in the Colorado Desert and similar xeric environments.

The Clark fault is one of the primary dextral faults in the San Jacinto fault zone system, southern California. Previous mapping of the Clark fault at its southern termination in the San Felipe Hills reveals it as a broad right lateral shear zone that ends north of the crossing, northeast-striking, left-lateral Extra fault. We investigate the relationship between the dextral Clark fault and the sinistral Extra fault to determine whether the Clark fault continues to the southeast. We present new structural, geophysical and geomorphic data that show that the Extra fault is a ~7 km wide, coordinated fault array comprised of four to six left-lateral fault zones. Active strands of the Clark fault zone persists through the Extra fault array to the Superstition Hills fault in the subsurface and rotate overlying sinistral faults in a clockwise sense. New detailed structural mapping between the San Felipe and Superstition Hills confirms that there is no continuous trace of the Clark fault zone at the surface but the fault zone has uplifted an elongate region ~950 km. sq. of latest Miocene to Pleistocene basin-fill in the field area and far outside of it. Detailed maps and cross sections of relocated microearthquakes show two earthquake swarms, one in 2007 and another in 2008 that project toward the San Felipe Hills, Tarantula Wash and Powerline strands of the dextral Clark fault zone in the San Felipe Hills, or possibly toward the parts of the Coyote Creek fault zone. We interpret two earthquake swarms as activating the San Jacinto fault zone beneath the Extra fault array. These data coupled with deformation patterns in published InSAR data sets suggest the presence of possible dextral faults at seismogenic depths that are not evident on the surface. We present field, geophysical and structural data that demonstrate dominantly left-lateral motion across the Extra fault array with complex motion on secondary strands in damage zones. Slickenlines measured within three fault zones in the Extra fault array reveal primarily strike-slip motion on the principal fault strands. Doubly-plunging anticlines between right-stepping en echelon strands of the Extra fault zone are consistent with contraction between steps of left-lateral faults and are inconsistent with steps in dominantly normal faults. Of the 21 published focal mechanisms for earthquakes in and near the field area, all record strike-slip and only two have a significant component of extension. Although the San Sebastian Marsh area is dominated by northeast-striking leftlateral faults at the surface, the Clark fault is evident at depth beneath the field area, in rotated faults, in microseismic alignments, and deformation in the Sebastian uplift. Based on these data the Clark fault zone appears to be continuous at depth to the Superstition Hills fault, as Fialko (2006) hypothesized with more limited data sets.

Continental rifting ultimately creates a deep accommodation space for sediment. When a major river flows into a late-stage rift, thick deltaic sediment can change the thermal regime and alter the mechanisms of extension and continental breakup. The Salton Trough, the northernmost rift segment of the Gulf of California plate boundary, has experienced the same extension as the rest of the Gulf, but is filled to sea level by sediment from the Colorado River. Unlike the southern Gulf, seafloor spreading has not initiated. Instead, seismicity, high heat flow, and minor volcanoes attest to ongoing rifting of thin, transitional crust. Recently acquired controlled-source seismic refraction and wide-angle reflection data in the Salton Trough provide constraints upon crustal architecture and active rift processes. The crust in the central Salton Trough is only 17-18 km thick, with a strongly layered but relatively one-dimensional structure for ~100 km in the direction of plate motion. The upper crust includes 2-3 km of Colorado River sediment. The basement below the sediment is interpreted to be similar sediment metamorphosed by the high heat flow and geothermal activity. Meta-sedimentary rock extends to at least 7-8 km depth. A 4-5 km thick layer in the middle crust is either additional meta-sedimentary rock or stretched pre-existing continental crust. The lowermost 4-5 km of the crust is rift-related mafic magmatic material underplated from partial melting in the hot upper mantle. North American lithosphere in the Salton Trough has been almost or completely rifted apart. The gap has been filled by ~100 km of new transitional crust created by magmatism from below and sedimentation from above. These processes create strong lithologic, thermal, and rheologic layering. Brittle extension occurs within new meta-sedimentary rock. The lower crust, in comparison, stretches by ductile flow and magmatism is not localized. This seismic interpretation is also supported by 1D thermal and rheological modeling. In this passive rift driven by far-field extensional stresses, rapid sedimentation keeps the crust thick and ductile, which delays final breakup of the crust and the initiation of seafloor spreading. / Ph. D.

The Fish Creek-Vallecito section is the most stratigraphically complete and structurally intact Neogene exposure in the Salton Trough, and thus provides a useful reference section for regional stratigraphie revision and historical interpretation of the early Gulf of California and Colorado Delta. The section comprises a marine sequence (Imperial Formation) bracketed by nonmarine units (Split Mountain and Alverson Formations below, Palm Spring Formation and Canebrake Conglomerate above). Recognition of distinct suites of locally-derived and Colorado River-derived sediment, combined with sedimentological evidence, led to revision of this sequence in terms of informal members and geneticstratigraphic units: (1) pre-rift braided-stream deposits (2) syn-rift fanglomerates and volcanics, with local pre-marine evaporites; (3) pre-deltaic marine units, deposited primarily as small fan deltas; a progradational sequence of the ancestral Colorado delta, consisting of (4) an upward-shoaling marine sequence, and (5) a nonmarine deltaplain sequence; (6) lacustrine units; and (7) locally-derived basinmargin alluvium that interfingers with (4), (5) and (6). Neogene palinspastic base maps for paleogeographic mapping were based on displacement histories for the Pacific-North American plate boundary and its constituent faults. The tectonic-sedimentary history consists of: (1) early to middle Miocene rifting that propagated southward from southern California to the Gulf mouth; (2) northward marine transgression of the rift basin, reaching southern California by the late Miocene; (3) development of the San Andreas-Gulf of California transform boundary by inboard transfer of intraplate slip; (4) earliest Pliocene initiation of the lower Colorado River and Delta by rapid epeirogenic uplift of the Bouse Embayment; and (5) late Pliocene or Pleistocene transpressive uplift in the western Salton Trough caused by outboard transfer of slip from the San Andreas fault. The stratigraphic succession in the western Salton Trough resulted largely from tectonic transport through a series of paleoenvironments anchored to the North American plate by the entry point of the Colorado River.

Using Coachella Valley's Salton Sea ecosystem as a model, several multidisciplinary wetland activities for grades three to six were developed. A resource guide for educators is included.

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