The geologic record of coastal California includes evidence of numerous volcanic centers younger than 30 Ma that do not appear to have erupted in an arc setting. By correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the Pacific plate, we add new constraints to tectonic reconstructions since 30 Ma. Our correlations, such as erupting the Morro Rock–Islay Hill complex south of the Pioneer fracture zone and the Iversen Basalt south of the Mendocino fracture zone, require larger displacements within western North America than advocated by most previous authors. Specifically, we infer at least 315 km of motion between the Sierra Nevada and rigid North America at an azimuth of about N60°W and at least 515 km between Baja California and rigid North America in a similar direction. A consequence of inferring a large displacement of Baja California is that the Pacific–North American plate boundary must have developed most of its current form prior to 10 Ma. We interpret a slab window developing between Cocos and Monterey plates after 19 Ma that reconstructs under nearly all of the southern California volcanic centers dated at 18–14 Ma. Most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism, followed by slow subsidence of variable but often extended duration, consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere.

No abstract

Cataloging-in-Publication data is on fi le with the Library of Congress FRONT COVER Photograph of pump unit atop an oil well in Los Gatos, Calif. The pump unit and well are in a fenced enclosure in the parking lot of a commercial offi ce building on Los Gatos Boulevard near Garden Lane. According to the current owner, the well has not produced oil for many years, but unconfi rmed reports suggest that the well yielded about 8 barrels of oil per day in 1953. Geologic and organic-geochemical information from this and other historical oil wells in the Los Gatos area provide important insights into the subsurface geology and geologic history of the southwestern Santa Clara Valley ("Silicon Valley"); see text for further discussion.

The Neogene Peters Hills basin is a small terrestrial basin that formed along the south flank of the Alaska Range during a time in which there was regional shortening. The formation of the Peters Hills basin is consistent with it being a wedge-top basin that formed on top of the active southeast-vergent Broad Pass thrust fault. Movement along this thrust raised a ridge of Jurassic and Cretaceous metasedimentary rocks, which then trapped behind it Miocene and Pliocene sediments that were derived from the growing Alaska Range. The presence of this thrust fault is consistent with regional structural, stratigraphic, seismicity, gravity, and aeromagnetic data. The Peters Hills basin is no longer a depocenter, but if the Broad Pass thrust remains active, it would help explain the westward decrease in Quaternary slip rate along the Denali fault system, and it may constitute a seismic hazard that could produce earthquakes in the M w 7.6-7.8 range. PETERS HILLS BASIN Sedimentary basins of south-central Alaska are easily identified on an isostatic residual gravity map (Fig. 2). The largest of these is the Cook Inlet basin, which is almost 8 km deep and has active fault-cored folds (Haeussler et al.

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Suggested citation: Lewis, K.A., Potter, C.J., Shah, A.K., Stanley, R.G., Haeussler, P.J., and Saltus, R.W., 2015, Preliminary interpretation of industry two-dimensional seismic data from Susitna Basin, south-central Alaska: U.S. Geological Survey Open-File Report 2015-1138, 51 p., http://dx.doi.org/10.3133/ofr20151138. ISSN 2331ISSN -1258 Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. AbstractLocated approximately 80 kilometers northwest of Anchorage, Alaska, the Susitna Basin is a complex sedimentary basin whose tectonic history has been poorly understood. Recent interpretation of two-dimensional seismic reflection data integrated with well, aeromagnetic, and gravity data provides new insights into the structural and stratigraphic nature of the basin.This report presents an interpretation of 41 two-dimensional seismic reflection lines, acquired by industry from the 1960s to the 1980s. Our interpretation of the seismic data focused mainly on picking two Eocene stratigraphic units and a presumed base of Tertiary horizon. Based on our interpretation of the seismic data, the structural features in the basin appear to be generally contractional, as evidenced by the presence of many reverse faults, thrust faults, and folds, with the contraction mainly oriented east-west. This result is contrary to prior inferences of most previous geologic studies that showed normal faults. Several regional reverse faults have been identified in the seismic data and appear to divide the basin into three regions or "sides": east, west, and south.The eastern seismic lines show evidence of numerous short-wavelength antiforms that appear to correspond to a series of northeast-trending lineations observed in aeromagnetic data, which have been interpreted as being due to folding of Paleogene volcanic strata. The eastern side of the basin is also cut by a number of reverse faults and thrust faults, the majority of which strike north-south. The western side of the Susitna Basin is cut by a series of regional reverse faults and is characterized by synformal structures in two fault blocks between the Kahiltna River and Skwentna faults. These synforms are progressively deeper to the west in the footwalls of the east-vergent Skwentna and northeast-vergent Beluga Mountain reverse faults. Although the seismic data are limited to the south, we in...