Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Open Quaternary”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 361 records · Page 20Linked to original sources
USGS1993

A response to "Milankovitch theory viewed from Devils Hole" by J. Imbrie, A.C. Mix and D.G. Martinson

The detailed and well-dated 500,000-yr record of oxygen-18 variations found in vein calcite core DH-11 taken from Devils Hole in Nevada provides several challenges to the Milankovitch theory for the occurrence of Quaternary glaciations. A recent discussion paper (Imbrie and others, 1993) has dismissed the relevance of this well-dated core for determining the timing of global climatic fluctuations and, moreover, asserts that the Devils Hole record provides support for the Milankovitch theory. Upon analysis of the arguments found in this discussion, the authors found nothing to dissuade them from the original conclusion that the Devils Hole chronology does present a serious challenge to the Milankovitch theory.

Open-File Report↗
USGS1996

Sediment magnetic and geochemical data from Quaternary lacustine sediment in two cores from Tule Lake, Siskiyou County, California

Sediment magnetic and geochemical results have been obtained from the top 60 meters of lacustrine sediments recovered in two cores from Tule Lake in northern California. The sediment magnetic and geochemical data, presented here in tabular form, complement studies of diatoms and pollen in the cores that are the bases for published paleoclimatic interpretations. This report also documents the methods used to obtain the magnetic properties and geochemical data.

Open-File Report↗
USGS2026

Preliminary geology of the North Meadow Creek Area, Tobacco Root Mountains, southwest Montana: The North Meadow Creek Fault

This report documents a previously unmapped fault, informally referred to herein as the North Meadow Creek fault, on the east flank of the Tobacco Mountains of southwestern Montana. This fault has an apparently long and complex history, including Quaternary offset of an older alluvial terrace, offset of a debris flow of presumed Pleistocene age, offset of rhyolite lava flows of presumed Tertiary age from their presumed source several kilometers east, on the opposite side of the fault, and offset and possible rotation of blocks of Archean gneiss on northeast side of the fault. The western part of the fault is covered by glacial moraines of two apparent ages, both Pleistocene.

Montana↗
USGS1982

Altitude and configuration of the water table in the High Plains Aquifer in Kansas, 1965

The High Plains aquifer in Kansas is a part of a regional system that extends South Dakota to Texas. The aquifer in Kansas underlies an area of 31,000 square miles in the western and south-central part. This aquifer is a hydraulically connected assemblage of unconsolidated water-bearing deposits of Tertiary and Quaternary age. Maps published at a scale of 1:500,000 show the altitude and configuration of the water table in Kansas during 1965. Groundwater moved from higher altitudes in the western part of the High Plains to lower altitudes in the eastern part at a average rate of 10 ft/mi. The upgradient flexure of water table contours along some of the valleys indicates that groundwater was discharged to the streams in those areas. (USGS)

Open-File Report↗
USGS1996

Neogene and Quaternary geology of a stratigraphic test hole on Horn Island, Mississippi Sound

During April and May, 1991, the U.S. Geological Survey (USGS) drilled a 510-ft-deep, continuously cored, stratigraphic test hole on Horn Island, Mississippi Sound, as part of a field study of the Neogene and Quaternary geology of the Mississippi coastal area. The USGS drilled two new holes at the Horn Island site. The first hole was continuously cored to a depth of 510 ft; coring stopped at this depth due to mechanical problems. To facilitate geophysical logging, an unsampled second hole was drilled to a depth of 519 ft at the same location.

Open-File Report↗
USGS1983

A description of aquifer units in western Oregon

Hydrogeologic information for western Oregon was compiled to aid the U.S. Environmental Protection Agency in evaluating proposals for underground injection of waste fluid. Geologic formations were grouped into seven aquifer units according to hydraulic and geologic similarities. The bedrock aquifer units in the Klamath Mountains, Coast Range, and Western Cascade Range all have low permeabilities and yield only small quantities of water to wells for domestic and stock uses. The Columbia River Basalt Group aquifer unit, which crops out along the Columbia River and the northern Willamette Valley, also has overall low permeability; however, the basalt supplies water for public, domestic, and stock, and some irrigation uses in western Oregon. The most important aquifer unit, and generally most permeable is the Tertiary-Quaternary sedimentary deposits that occur in lowlands throughout the area and provide water for irrigation, industry, public supplies and domestic and stock uses. All aquifer units generally contain water with low concentrations of dissolved solids at shallow depths. In the Tertiary marine rocks of the Coast Range, analyses from a limited number of deep wells indicated that water with more than 10,000 milligrams per liter dissolved solids is widespread at depths greater than about 2 ,000 feet. (USGS)

Oregon↗
USGS1986

Water-table decline in the south-central Great Basin during the Quaternary Period; implications for toxic-waste disposal

The distribution of vein calcite, tufa, and other features indicative of paleo-groundwater discharge, indicates that during the early to middle Pleistocene, the water table at Ash Meadows, in the Amargosa Desert, Nevada, and at Furnace Creek Wash, in east-central Death Valley, California, was tens to hundreds of meters above the modern water table, and that groundwater discharge occurred up to 18 km up-the-hydraulic gradient from modern discharge areas. Uranium series dating of the calcitic veins permits calculation of rates of apparent water table decline; rates of 0.02 to 0.08 m/1000 yr are indicated for Ash meadows and 0.2 to 0.6 m/1000 yr for Furnace Creek Wash. The rates for Furnace Creek Wash closely match a published estimate of vertical crustal offset for this area, suggesting that tectonism is a major cause for the displacement observed. In general, displacements of the paleo-water table probably reflect a combination of: (a) tectonic uplift of vein calcite and tufa, unaccompanied by a change in water table altitude; (b) decline in water table altitude in response to tectonic depression of areas adjacent to dated veins and associated tufa; (c) decline in water table altitude in response to increasing aridity caused by major uplift of the Sierra Nevada and Transverse Ranges during the Quaternary; and (d) decline in water altitude in response to erosion triggered by increasing aridity and/or tectonism. A synthesis of geohydrologic, neotectonic, and paleoclimatologic information with the vein-calcite data permits the inference that the water table in the south-central Great Basin progressively lowered throughout the Quaternary. This inference is pertinent to an evaluation of the utility of thick (200-600 m) unsaturated zones of the region for isolating solidified radioactive wastes from the hydrosphere for hundreds of millenia. Wastes buried a few tens to perhaps 100 m above the modern water table--that is above possible water level rises due to future pluvial climates--are unlikely to be inundated by a rising water table in the foreseeable geologic future. (Author 's abstract)

Open-File Report↗
USGS2025

Late Quaternary environmental change in eastern Beringia

Eastern Beringia (Alaska and western Yukon) is an extensive, high-latitude region of North America that remained largely unglaciated throughout the Quaternary. Consequently, its sedimentary deposits preserve long-term environmental records that have intrigued scientists for nearly a century. Recent advances in palaeoecological proxies and dating methods have proved critical in addressing long-standing questions about regional late Quaternary environmental change. At the same time, they have led to new and sometimes controversial hypotheses. This review covers recent discoveries and unresolved questions focused on the period 57,000–10,000 calendar years before C.E. 1950 (cal yr BP). The middle Wisconsin interstadial (57,000–30,000 cal yr BP) was a period of relative warmth in eastern Beringia, compared with the late Wisconsin (30,000–14,000 cal yr BP). Early in the interstadial occasional Picea woodland was present amongst widespread shrub tundra. Palaeoecological, sedimentary and isotopic data indicate that climate was cooler and drier than the Holocene, with high rates of aeolian activity. Megafauna typically associated with the ‘mammoth steppe’ ecosystem (woolly mammoth [ Mammuthus primigenius ], horse [ Equus ] and steppe-bison [ Bison priscus ]) were present in some abundance. The transition towards late Wisconsin cold-stage conditions (35,000–30,000 cal yr BP) coincided with the establishment of the Bering Land Bridge and featured expansion of spatially varied, herbaceous vegetation, sometimes associated with deep active layers. Sedimentary DNA ( seda DNA) and macrofossil evidence show vegetation was not a prairie-like grassland, and the term “steppe-tundra” is a better descriptor. Permafrost pore-ice isotopic (δ 18 O) records suggest a step change in one or more climate drivers ca. 30,000 cal yr BP, by which time steppe-tundra was established across eastern Beringia. It remains uncertain whether Picea survived cold-stage conditions within isolated refugia, or whether it recolonized from south of the Laurentide-Cordilleran ice sheets. Genetic data suggest that Picea probably survived in situ ; however, there is no definitive fossil evidence to support this. The end-Pleistocene transition from steppe-tundra to shrub tundra began ca. 15,000 cal yr BP and took place within decades at local scales. The expansion of woody taxa coincided with rising sea levels, reduced sea-ice extent and an abrupt shift in atmospheric circulation that enhanced precipitation. During this time, Earth's orbital configuration caused high early-summer temperatures and strong seasonality, creating growing conditions very different from today. The vegetation consisted of Salix and Betula shrub tundra with open areas of herbs and graminoids. During the deglacial warming trend, the Younger Dryas oscillation (12,800–11,700 cal yr BP) was variably expressed. It is generally evident in records affected by adjacent oceans but can be absent at sites in continental areas. These past conditions and paleoenvironmental changes have implications for contemporary issues: hypotheses about Pleistocene mammalian extinction; sensitivity of eastern Beringia to major oceanic reorganizations and high-frequency climate variability; the nature of woody plant expansion with climate warming; grazing, hydroclimate and fire as controls over ecosystems; the efficacy of “Pleistocene rewilding” for carbon capture.

Beringia↗
USGS2011

Using luminescence dating of coarse matrix material to estimate the slip rate of the Astaneh fault, Iran

In this paper, we present optically and infrared stimulated luminescence (OSL and IRSL) ages for four samples from alluvial fan surfaces in the Astaneh Valley. This valley is located in the north-east part of the Alborz range in Iran. Our morphologic interpretations recognize at least three generations of fans in the study area, all of which have been displaced along the left-lateral strike-slip Astaneh fault. Because of the dry, loose, and sometimes complex juxtaposition of the target sediments, we collected the samples in total darkness beneath dark plastic layers placed atop the pit openings. Luminescence ages of the fans are ∼55 ka, ∼32 ka and ∼16 ka. These ages are concurrent with periods of loess deposition and wet climatic conditions previously recorded in the Arabia-Iranian region. They allow estimation of a horizontal slip rate of ∼2 mm/yr along the Astaneh fault, which is consistent with additional slip rates determined for the Holocene period along faults further west of the Astaneh fault.

Quaternary Geochronology↗
USGS1975

Preliminary report on the reconnaissance engineering geology of the Yakutat area, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

Yakutat, situated about 225 miles northwest of Juneau, Alaska, near the shores of the Gulf of Alaska, has a setting that calls for superlatives. Within the Yakutat region are some of the tallest mountains, some of the heaviest snowfalls, and the largest glacier in North America. Between the abrupt mountain front and the Gulf of Alaska lies a very gently sloping plain of outwash derived from repeated cycles of advance followed by melt and retreat of glaciers during the Quaternary Period. The latest melting probably took place 500 to 600 years ago. Yakutat is built upon the moderately steep moraine that is the product of melting of one of these glaciers. Near Yakutat, surficial deposits are as much as 700 feet thick and probably overlie siltstone, sandstone, and mudstone. The eight general categories of mapped surficial deposits include artificial fill, organic, eolian, beach, delta-estuarine, alluvial, and outwash deposits, and deposits of the outer Yakutat Bay moraine complex.

Alaska↗
USGS1998

Map and data for Quaternary faults and folds in New Mexico

The "World Map of Major Active Faults" Task Group is compiling a series of digital maps for the United States and other countries in the Western Hemisphere that show the locations, ages, and activity rates of major earthquake-related features such as faults and fault-related folds; the companion database includes published information on these seismogenic features. The Western Hemisphere effort is sponsored by International Lithosphere Program (ILP) Task Group H-2, whereas the effort to compile a new map and database for the United States is funded by the Earthquake Reduction Program (ERP) through the U.S. Geological Survey. The maps and accompanying databases represent a key contribution to the new Global Seismic Hazards Assessment Program (ILP Task Group II-O) for the International Decade for Natural Disaster Reduction. This compilation, which describes evidence for surface faulting and folding in New Mexico, is the third of many similar State and regional compilations that are planned for the U.S. The compilation for West Texas is available as U.S. Geological Survey Open-File Report 96-002 (Collins and others, 1996 #993) and the compilation for Montana will be released as a Montana Bureau of Mines product (Haller and others, in press #1750).

New Mexico↗
USGS1975

Metallogenic provinces of the southeastern Pacific region

Metalliferous mineral deposits of the southeastern Pacific region include: (1) hydrothermal, magmatic, and sedimentary deposits of the Andean region, one of the great mineral belts of the world; 2) Scattered hydrothermal mineral occurrences in the Antarctic Peninsula; and 3) metal-enriched pelagic sediments, ferromanganese nodules, and volcanic rocks(?) in the southeast Pacific basin. Andean metalliferous deposits, the chief concern of this report, are for the most part spatially and genetically related to calc-alkaline plutons, sub-volcanic intrusions, and volcanic rocks emplaced during the Andean orogeny of Late Triassic to Quaternary age. These deposits are components of a single metallogenic province superimposed on two or more pre-Andean metallogenic provinces that are indicated by scattered deposits of Paleozoic and Precambrian(?) age. Occurrences in the Antarctic Peninsula are of age and origin similar to the deposits in the Andes and are considered to belong to the Andean province. The Andean metallogenic province may be divided into several subprovinces, each parallel to the Andes and the continental margin and each having a dominant metal or suite of metals. The central Andes of Peru, northern Chile, and Bolivia, which contain the greatest concentration of exploitable deposits and the greatest variety of ore types, have as many as five linear partly overlapping subprovinces. These sub-provinces, from west to east (Pacific coast to the eastern Andean front), are characterized, respectively, by deposits of : 1) iron; 2) copper, with or without associated gold; 3) polymetallic base metals (zinc, lead, copper), generally containing silver; 4) tin; and 5) gold. Iron deposits chiefly are near the coast in central to northern Chile and in southern Peru. The copper and polymetallic provinces are characterized by abundant deposits in the central Andean region but comparatively few scattered deposits in the north (Ecuador, Colombia, and Venezuela) and in southern Chile and Argentina. Scattered gold-rich veins and placers occur along the western Andean front and coastal region, the general area of the copper province, from central Chile to northern Colombia. Similar deposits occur along a discontinuous belt in the eastern Andes from Bolivia to Ecuador and in the central Andes of Colombia. Tin deposits are almost wholly restricted to the eastern Andes of Bolivia. The position and age distribution of plutonic and volcanic rocks and associated metalliferous deposits of the Andes indicate presence of an active subduction zone in this region since at least Late Triassic time. Magmas of the calc-alkaline igneous rocks are believed to have formed chiefly by partial melting of mantle, oceanic sediments, and oceanic crust along the Benioff zone at depths of 100 to 200 km. Plutonic and volcanic rocks show a general though nonuniform progression of decreasing age from west to east. Rocks of Jurassic and Cretaceous age are most abundant near the coast, whereas those of Tertiary and Quaternary age dominate in the Andes. Locally, intrusive rocks and ore deposits of widely different ages are juxtaposed. Metals of the ore deposits associated with the calc-alkaline rocks were supplied by the source rocks in the Benioff zone, some of which probably had been previously enriched in certain metals at the ancestral East Pacific Rise and were mobilized or assimilated from metal-rich zones in the overlying mantle and continental crust by magmas rising from the Benioff zone.

Open-File Report↗
USGS1989

Geologic studies in Alaska by the U.S. Geological Survey, 1988

This volume continues the annual series of U.S. Geological Survey (USGS) reports on geologic investigations in Alaska. Since 1975, when the first of these collections of short papers appeared under the title "The United States Geological Survey in Alaska: Accomplishments during 1975," the series has been published as USGS circulars. This bulletin departs from the circular style, in part to provide a more flexible format for longer reports with more depth of content, better documentation, and broader scope than is possible for circular articles. The 13 papers in this bulletin represent a sampling of research activities carried out in Alaska by the USGS over the past few years. The topics addressed range from mineral resource studies (including natural gas) and geochemistry, Quaternary geology, basic stratigraphic and structural problems, and the use of computer graphics in geologic map preparation, to the application of geochronology to regional tectonic problems. Geographic areas represented are numbered on figure 1 and include the North Slope (1) and Brooks Range (2, 3) of Arctic Alaska, Seward Peninsula (4), interior Alaska (5-9), and remote locations of the Alaska Peninsula (10, 11) and southeast Alaska (12, 13). Two bibliographies following the reports of investigations list (1) reports about Alaska in USGS publications released in 1988 and (2) reports about Alaska by USGS authors in publications outside the USGS in 1988. A bibliography and index of the short papers in past USGS circulars devoted to Geological Research and Accomplishments in Alaska (1975-1986) is published as USGS Open-File Report 87-420.

Alaska↗
USGS2014

Preliminary bedrock and surficial geologic map of the west half of the Sanders 30' x 60' quadrangle, Navajo and Apache Counties, northern Arizona

The bedrock and surficial geologic map of the west half of the Sanders 30' x 60' quadrangle was completed in a cooperative effort of the U.S. Geological Survey (USGS) and the Navajo Nation to provide regional geologic information for management and planning officials. This report provides baseline geologic information that will be useful in future studies of groundwater and surface water resources, geologic hazards, and the distribution of soils and plants. The west half of the Sanders quadrangle encompasses approximately 2,509 km 2 (980 mi 2 ) within Navajo and Apache Counties of northern Arizona and is bounded by lat 35°30' to 35° N., long 109°30' to 110° W. The majority of the land within the map area lies within the Navajo Nation. South of the Navajo Nation, private and State lands form a checkerboard pattern east and west of Petrified Forest National Park. In the west half of the Sanders quadrangle, Mesozoic bedrock is nearly flat lying except near folds. A shallow Cenozoic erosional basin that developed about 20 Ma in the western part of the map area cut across late Paleozoic and Mesozoic rocks that were subsequently filled with flat-lying Miocene and Pliocene mudstone and argillaceous sandstone and fluvial sediments of the Bidahochi Formation and associated volcanic rocks of the Hopi Buttes volcanic field. The Bidahochi rocks are capped by Pliocene(?) and Pleistocene fluvial sediments and Quaternary eolian and alluvial deposits. Erosion along northeast-southwest-oriented drainages have exposed elongated ridges of Bidahochi Formation and basin-fill deposits that are exposed through shallow eolian cover of similarly oriented longitudinal dunes. Stokes (1964) concluded that the accumulation of longitudinal sand bodies and the development of confined parallel drainages are simultaneous processes resulting in parallel sets of drainages and ridges oriented along the prevailing southwest wind direction on the southern Colorado Plateau.

Arizona↗
USGS2005

Pliocene transpressional modification of depositional basins by convergent thrusting adjacent to the "Big Bend" of the San Andreas fault: An example from Lockwood Valley, southern California

The "Big Bend" of the San Andreas fault in the western Transverse Ranges of southern California is a left stepping flexure in the dextral fault system and has long been recognized as a zone of relatively high transpression compared to adjacent regions. The Lockwood Valley region, just south of the Big Bend, underwent a profound change in early Pliocene time (???5 Ma) from basin deposition to contraction, accompanied by widespread folding and thrusting. This change followed the recently determined initiation of opening of the northern Gulf of California and movement along the southern San Andreas fault at about 6.1 Ma, with the concomitant formation of the Big Bend. Lockwood Valley occupies a 6-km-wide, fault-bounded structural basin in which converging blocks of Paleoproterozoic and Cretaceous crystalline basement and upper Oligocene and lower Miocene sedimentary rocks (Plush Ranch Formation) were thrust over Miocene and Pliocene basin-fill sedimentary rocks (in ascending order, Caliente Formation, Lockwood Clay, and Quatal Formation). All the pre-Quatal sedimentary rocks and most of the Pliocene Quatal Formation were deposited during a mid-Tertiary period of regional transtension in a crustal block that underwent little clockwise vertical-axis rotation as compared to crustal blocks to the south. Ensuing Pliocene and Quaternary transpression in the Big Bend region began during deposition of the poorly dated Quatal Formation and was marked by four converging thrust systems, which decreased the areal extent of the sedimentary basin and formed the present Lockwood Valley structural basin. None of the thrusts appears presently active. Estimated shortening across the center of the basin was about 30 percent. The fortnerly defined eastern Big Pine fault, now interpreted to be two separate, oppositely directed, contractional reverse or thrust faults, marks the northwestern structural boundary of Lockwood Valley. The complex geometry of the Lockwood Valley basin is similar to other Tertiary structural basins in southern California, such those that underlie Cuyama Valley, the Ridge basin, and the east Ventura basin.

Tectonics↗
USGS1985

Altitude and configuration of the water table in the High Plains Aquifer in Kansas, 1970

The High Plains aquifer in Kansas is a part of a regional system that extends from South Dakota to Texas. The aquifer in Kansas underlies an area of 31,000 square miles in the western and south-central part. This aquifer is a hydraulically connected assemblage of unconsolidated water-bearing deposits of Tertiary and Quaternary age. Maps published at a scale of 1:500,000 show the altitude and configuration of the water table in Kansas during 1970. Ground water moved from higher altitudes in the western part of the High Plains to lower altitudes in the eastern part under an average gradient of 10 feet per mile. The upgradient flexure of water-table contours along some of the valleys indicates that ground water was discharged to the streams in those areas. (USGS)

Open-File Report↗
USGS1985

Iowa ground-water-quality monitoring program

A ground-water-quality monitoring network has been designed to collect data to describe the long-term chemical quality of major bedrock and Quaternary aquifer systems in Iowa. The network was designed because of data needs expressed by several State and Federal agencies. Areas where new or additional data are needed are nitrate and bacteria concentrations in alluvial and glacial drift aquifers, fluoride and radionuclides in deep bedrock aquifers, general quality of water in the Missouri River alluvium, changes in water quality of deep bedrock aquifers caused by large withdrawal, effects of land use on shallow aquifers, and the impact of agricultural injection wells on receiving aquifers. Most of the ground-water-quality monitoring in the State has been directed at compliance or ambient monitoring of municipal water supplies. Some site-specific monitoring has occurred. More recently several special studies or research monitoring activities have been completed or begun. The monitoring program utilizes a strategy that includes low frequency and sparse distribution of samples from Cambrian-Ordovician, Silurian-Devonian, Mississippian,Pennsylvanian, and Dakota aquifers. A larger density and greater frequency of samples will be obtained from shallow bedrock wells and from wells penetrating Quaternary aquifers suck as alluvial, buried channel, and other drift aquifers. More than 1,200 wells are available and acceptable for the network. From these and newly completed wells, 200 samples will be collected and analyzed annually. Analyses will be made for common anions and cations, trace metals, nutrients, and radionuclides. One out of ten samples will be analyzed for priority pollutants and pesticides. Data from this program will be published annually in Water Resources Data, Iowa, U.S. Geological Survey Water-Data Report.

Iowa↗
USGS1996

Large-scale right-slip displacement on the East San Francisco Bay region fault system, California: Implications for location of late Miocene to Pliocene Pacific plate boundary

A belt of northwardly younging Neogene and Quaternary volcanic rocks and hydrothermal vein systems, together with a distinctive Cretaceous terrane of the Franciscan Complex (the Permanente terrane), exhibits about 160 to 170 km of cumulative dextral offset across faults of the East San Francisco Bay Region (ESFBR) fault system. The offset hydrothermal veins and volcanic rocks range in age from .01 Ma at the northwest end to about 17.6 Ma at the southeast end. In the fault block between the San Andreas and ESFBR fault systems, where volcanic rocks are scarce, hydrothermal vein system ages clearly indicate that the northward younging thermal overprint affected these rocks beginning about 18 Ma. The age progression of these volcanic rocks and hydrothermal vein systems is consistent with previously proposed models that relate northward propagation of the San Andreas transform to the opening of an asthenospheric window beneath the North American plate margin in the wake of subducting lithosphere. The similarity in the amount of offset of the Permanente terrane across the ESFBR fault system to that derived by restoring continuity in the northward younging age progression of volcanic rocks and hydrothermal veins suggests a model in which 80–110 km of offset are taken up 8 to 6 Ma on a fault aligned with the Bloomfield-Tolay-Franklin-Concord-Sunol-Calaveras faults. An additional 50–70 km of cumulative slip are taken up ≤ 6 Ma by the Rogers Creek-Hayward and Concord-Franklin-Sunol-Calaveras faults. An alternative model in which the Permanente terrane is offset about 80 km by pre-Miocene faults does not adequately restore the distribution of 8–12 Ma volcanic rocks and hydrothermal veins to a single northwardly younging age trend. If 80–110 km of slip was taken up by the ESFBR fault system between 8 and 6 Ma, dextral slip rates were 40–55 mm/yr. Such high rates might occur if the ESFBR fault system rather than the San Andreas fault acted as the transform margin at this time. Major transpression across the boundary between the Pacific and North American plates at about 3 to 5 Ma would have resulted in the transfer of significant slip back to the San Francisco Peninsula segment of the San Andreas fault. Since that time, the ESFBR fault system has continued to slip at rates of 11–14 mm/yr. If this interpretation is valid, the ESFBR fault system was the Pacific-North American plate boundary between 8 and 6 Ma, and this boundary has migrated both eastward and westward with time, in response to changing plate margin geometry and plate motions.

California↗