Geology topics

Michelle A. Roberts

Publications and source records attributed to Michelle A. Roberts.

4 recordsLinked to original sources

USGS2026

Three-dimensional geologic map of the San Andreas Fault Zone between Gold Hill and Pinnacles National Park, California (ver. 1.1, October 2026)

This three-dimensional (3D) geologic map of the San Andreas Fault Zone spans approximately 100 kilometers from Gold Hill in the southeast to Pinnacles National Park in the northwest. It was created by assembling eight geologic cross sections derived from geologic mapping, potential-field geophysical models, and petroleum well log descriptions. The resulting 3D geologic map depicts both surface and subsurface interpretations of geologic units, faults, and folds in a model that can be viewed from any perspective and sliced in any direction. Subsurface interpretations in this map are based on geologic mapping completed between 2005 and 2014, existing two-dimensional geologic maps, and existing gravitational anomaly and magnetic anomaly maps. After completion of the 3D geologic map, kinematic models were created using forward and reverse deformation algorithms in a structural geology modelling software. Kinematic modeling was applied to parts of the map to help better understand the effects of transpressive deformation, and how resulting structures might affect regional seismic hazards. The results of this kinematic modeling yielded insights into the geologic history of the region, including a possible mechanism for the creation of secondary faults, and quantifying the amount of contraction caused by regional compressional stress.

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USGS2023

Geology along the Yuba Pass and Highway 70 corridors: A complex history of tectonics and magmatism in the northern Sierra Nevada

This field trip traverses a cross section of northern Sierra Nevada geology and landscape along two major corridors, Highway 49 (Yuba Pass) and Highway 70. These highways, and adjacent roadways, offer roadcuts, outcrops, and overviews through diverse pre-Cenozoic metamorphic rocks along the Laurentian margin, Mesozoic batholithic rocks, and Miocene volcanic rocks. Observing this array of rocks on a single trip provides an opportunity to examine the progression of tectonic forces in this region since the Paleozoic Era. Inspiration for this trip is a 1:100,000-scale geologic map and geophysical maps of the Portola 30′ × 60′ quadrangle that integrate decades of published and unpublished mapping with new geophysical data. The quadrangle map will seamlessly depict a geologically complex region along the boundary between the Sierra Nevada and Basin and Range provinces, dominated by transtensional tectonics of the Walker Lane. This field trip highlights many of the major units of the geologic map and will also feature new geochronological data on plutonic rocks.

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USGS2015

Offshore geology and geomorphology from Point Piedras Blancas to Pismo Beach, San Luis Obispo County, California

Marine geology and geomorphology were mapped along the continental shelf and upper slope between Point Piedras Blancas and Pismo Beach, California. The map area is divided into the following three (smaller) map areas, listed from north to south: San Simeon, Morro Bay, and Point San Luis. Each smaller map area consists of a geologic map and the corresponding geophysical data that support the geologic mapping. Each geophysical data sheet includes shaded-relief multibeam bathymetry, seismic-reflection-survey tracklines, and residual magnetic anomalies, as well as a smaller version of the geologic map for reference. Offshore geologic units were delineated on the basis of integrated analysis of adjacent onshore geology, seafloor-sediment and rock samples, multibeam bathymetry and backscatter imagery, magnetic data, and high-resolution seismic-reflection profiles. Although the geologic maps are presented here at 1:35,000 scale, map interpretation was conducted at scales of between 1:6,000 and 1:12,000. Sea level was approximately 120 to 130 m lower during the Last Glacial Maximum (about 21 ka). This approximate depth corresponds to the modern shelf break, a lateral change from the gently dipping (0.8° to 1.0°) outer shelf to the slightly more steeply dipping (about 1.5° to 2.5°) upper slope in the central and northern parts of the map area. South of Point San Luis in San Luis Bay, deltaic deposits offshore of the mouth of the Santa Maria River (11 km south of the map area) have prograded across the shelf break and now form a continuous low-angle (about 0.8°) ramp that extends to water depths of more than 160 m. The shelf break defines the landward boundary of slope deposits. North of Estero Bay, the shelf break is characterized by a distinctly sharp slope break that is mapped as a landslide headscarp above landslide deposits. Multibeam imagery and seismic-reflection profiles across this part of the shelf break show evidence of slope failure, such as slumping, sliding, and soft-sediment deformation, along the entire length of the scarp. Notably, this shelf-break scarp corresponds to a west splay of the Hosgri Fault that dies out just north of the scarp, suggesting that faulting is controlling the location (and instability) of the shelf break in this area.

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USGS2000

Landslides in Alameda County, California: A digital database extracted from preliminary photointerpretation maps of surficial deposits by T.H. Nilsen in USGS Open-File Report 75-277

All or part of 25 7.5-minute quadrangles identifying 8465 landslides - largely slow-moving slides and earth flows - in Alameda County, California, have been converted to a digital-map database, compiled at 1:24,000 scale and plotted at 1:62,500 scale, that can be acquired from the U.S. Geological Survey over the Internet or on magnetic tape.

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