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At least 1,261 records · Page 70Linked to original sources
USGS2020

Bedrock geologic map of the Springfield 7.5- x 15-minute quadrangle, Windsor County, Vermont, and Sullivan County, New Hampshire

The bedrock geology of the 7.5- by 15-minute Springfield quadrangle consists of highly deformed and metamorphosed Mesoproterozoic through Devonian metasedimentary and meta-igneous rocks. In the west, Mesoproterozoic gneisses of the Mount Holly Complex are the oldest rocks and form the eastern side of the Chester dome. The Moretown slice structurally overlies the Chester dome along the Keyes Mountain thrust fault which represents the Ordovician Taconic suture (Red Indian Line) between Laurentian and Ganderian crust. The allochthonous Cambrian through Ordovician Moretown slice includes the Moretown and Cram Hill Formations and the North River Igneous Suite. Silurian and Devonian metasedimentary and metavolcanic rocks of the Connecticut Valley trough (CVT) unconformably overlie the Moretown slice. Ordovician to Silurian and Devonian metasedimentary and meta-igneous rocks of the New Hampshire sequence structurally overlie the CVT along the Devonian, Acadian Monroe thrust fault. The oldest part of the New Hampshire sequence consists of Ordovician metamorphosed volcanic, plutonic, and sedimentary rocks of the Bronson Hill arc including the Ammonoosuc Volcanics, the Partridge Formation, and the Oliverian Plutonic Suite. The Ammonoosuc Volcanics are the base of the exposed arc section in the map area. The Bronson Hill arc rocks are exposed in fault-bounded structural belts, including the Monroe thrust sheet, the Claremont belt, the Sugar River and Unity domes, and the footwall of the Brennan Hill thrust fault. Silurian to Devonian metasedimentary rocks of the Clough Quartzite, and Fitch and Littleton Formations unconformably overlie the Bronson Hill arc rocks. Devonian granitic and pegmatitic dikes and sills of the New Hampshire Plutonic Suite intruded previously deformed rocks. Devonian, Acadian F 1 fold nappes have a sheath fold geometry and are truncated by multiple generations of faults. The Bronson Hill arc structurally overlies the CVT along the Acadian Monroe fault with preserved tectonic mélange in the footwall. Upright dome-stage F 2 folds post-date amphibolite facies metamorphism and locally developed into sheath folds in high-strain zones. F 3 folds exhibit sinistral rotation associated with Alleghanian lower-greenschist facies faults. Late Paleozoic Alleghanian to Mesozoic shear zones transpose stratigraphy, early structures, and peak metamorphic isograds. 40 Ar/ 39 Ar white-mica growth ages (300–250 million years before present [Ma]) indicate that retrograde deformation continued into the latest Paleozoic and earliest Mesozoic. Apatite fission track data show that brittle faults were active prior to about 100 Ma and experienced Late Cretaceous and even Paleocene re-activation. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing characterization of the bedrock of Vermont and New Hampshire. This Scientific Investigations Map of the Springfield 7.5- x 15-minute quadrangle consists of sheets 1 and 2 as well as a geographic information system (GIS) database that includes bedrock geologic units, faults, outcrops, and structural geologic information. Sheet 1 of the report includes a bedrock geologic map, a correlation of map units, and a description of map units. Sheet 2 includes a discussion of the geology, references cited, two cross sections from the geologic map on sheet 1, a tectonic map showing major structural features, and a structural domain map showing the orientation of brittle features.

New Hampshire, Vermont↗
USGS2022

Geologic map of the South Boston 30' × 60' quadrangle, Virginia and North Carolina

This 1:100,000-scale geologic map of the South Boston 30 ’ × 60 ’ quadrangle, Virginia and North Carolina, provides geologic information for the Piedmont along the I–85 and U.S. Route 58 corridors and in the Roanoke River watershed, which includes the John H. Kerr Reservoir and Lake Gaston. The Raleigh terrane (located on the eastern side of the map) contains Neoproterozoic to early Paleozoic(?) polydeformed, amphibolite-facies gneisses and schists. The Carolina slate belt of the Carolina terrane (located in the central part of the map) contains Neoproterozoic metavolcanic and metasedimentary rocks at greenschist facies. Although locally complicated, the slate-belt structure mapped across the South Boston map area is generally a broad, complex anticlinorium of the Hyco Formation (here called the Chase City anticlinorium) and is flanked to the west and east by synclinoria, which are cored by the overlying Aaron and Virgilina Formations. The western flank of the Carolina terrane (located in the western-central part of the map) contains similar rocks at higher metamorphic grade. This terrane includes epidote-amphibolite-facies to amphibolite-facies gneisses of the Neoproterozoic Country Line complex, which extends north-northeastward across the map. The Milton terrane (located on the western side of the map) contains Ordovician amphibolite-facies metavolcanic and metasedimentary gneisses of the Cunningham complex. Crosscutting relations and fabrics in mafic to felsic plutonic rocks constrain the timing of Neoproterozoic to late Paleozoic deformations across the Piedmont. In the eastern part of the map, a 5- to 9-kilometer-wide band of tectonic elements that contains two late Paleozoic mylonite zones (Nutbush Creek and Lake Gordon) and syntectonic granite (Buggs Island pluton) separates the Raleigh and Carolina terranes. Amphibolite-facies, infrastructural metaigneous and metasedimentary rocks east of the Lake Gordon mylonite zone are generally assigned to the Raleigh terrane. In the western part of the map area, a 5- to 8-kilometer-wide band of late Paleozoic tectonic elements includes the Hyco and Clover shear zones, syntectonic granitic sheets, and amphibolite-facies gneisses along the western margin of the Carolina terrane at its boundary with the Milton terrane. This band of tectonic elements is also the locus for early Mesozoic extensional faults associated with the early Mesozoic Scottsburg, Randolph, and Roanoke Creek rift basins. The map shows fluvial terrace deposits of sand and gravel on hills and slopes near the Roanoke and Dan Rivers. The terrace deposits that are highest in altitude are the oldest. Saprolite regolith is spatially associated with geologic source units and is not shown separately on the map. Mineral resources in the area include gneiss and granite quarried for crushed stone, tungsten-bearing vein deposits of the Hamme district, and copper and gold deposits of the Virgilina district. Surface-water resources are abundant and include rivers, tributaries, the John H. Kerr Reservoir, and Lake Gaston. Groundwater flow is concentrated in saprolite regolith, along fractures in the crystalline bedrock, and along fractures and bedding-plane partings in the Mesozoic rift basins.

North Carolina, Virginia↗
USGS2011

Control of Precambrian basement deformation zones on emplacement of the Laramide Boulder batholith and Butte mining district, Montana, United States

What are the roles of deep Precambrian basement deformation zones in the localization of subsequent shallow-crustal deformation zones and magmas? The Paleoproterozoic Great Falls tectonic zone and its included Boulder batholith (Montana, United States) provide an opportunity to examine the importance of inherited deformation fabrics in batholith emplacement and the localization of magmatic-hydrothermal mineral deposits. Northeast-trending deformation fabrics predominate in the Great Falls tectonic zone, which formed during the suturing of Paleoproterozoic and Archean cratonic masses approximately 1,800 mega-annum (Ma). Subsequent Mesoproterozoic to Neoproterozoic deformation fabrics trend northwest. Following Paleozoic through Early Cretaceous sedimentation, a Late Cretaceous fold-and-thrust belt with associated strike-slip faulting developed across the region, wherein some Proterozoic faults localized thrust faulting, while others were reactivated as strike-slip faults. The 81- to 76-Ma Boulder batholith was emplaced along the reactivated central Paleoproterozoic suture in the Great Falls tectonic zone. Early-stage Boulder batholith plutons were emplaced concurrent with east-directed thrust faulting and localized primarily by northwest-trending strike-slip and related faults. The late-stage Butte Quartz Monzonite pluton was localized in a northeast-trending pull-apart structure that formed behind the active thrust front and is axially symmetric across the underlying northeast-striking Paleoproterozoic fault zone, interpreted as a crustal suture. The modeling of potential-field geophysical data indicates that pull-apart?stage magmas fed into the structure through two funnel-shaped zones beneath the batholith. Renewed magmatic activity in the southern feeder from 66 to 64 Ma led to the formation of two small porphyry-style copper-molybdenum deposits and ensuing world-class polymetallic copper- and silver-bearing veins in the Butte mining district. Vein orientations parallel joints in the Butte Quartz Monzonite that, in turn, mimic Precambrian deformation fabrics found outside the district. The faults controlling the Butte veins are interpreted to have formed through activation under shear of preexisting northeast-striking joints as master faults from which splay faults formed along generally east-west and northwest joint plane orientations.

Scientific Investigations Report↗
USGS2017

Emplacement of Holocene silicic lava flows and domes at Newberry, South Sister, and Medicine Lake volcanoes, California and Oregon

This field guide for the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Scientific Assembly 2017 focuses on Holocene glassy silicic lava flows and domes on three volcanoes in the Cascade Range in Oregon and California: Newberry, South Sister, and Medicine Lake volcanoes. Although obsidian-rich lava flows have been of interest to geologists, archaeologists, pumice miners, and rock hounds for more than a century, many of their emplacement characteristics had not been scientifically observed until two very recent eruptions in Chile. Even with the new observations, several eruptive processes discussed in this field trip guide can only be inferred from their final products. This makes for lively debates at outcrops, just as there have been in the literature for the past 30 years. Of the three volcanoes discussed in this field guide, one (South Sister) lies along the main axis defined by major peaks of the Cascade Range, whereas the other two lie in extensional tectonic settings east of the axis. These two tectonic environments influence volcano morphology and the magmatic and volcanic processes that form silicic lava flows and domes. The geomorphic and textural features of glass-rich extrusions provide many clues about their emplacement and the magma bodies that fed them. The scope of this field guide does not include a full geologic history or comprehensive explanation of hazards associated with a particular volcano or volcanic field. The geochemistry, petrology, tectonics, and eruption history of Newberry, South Sister, and Medicine Lake volcanic centers have been extensively studied and are discussed on other field excursions. Instead, we seek to explore the structural, textural, and geochemical evolution of well-preserved individual lava flows—the goal is to understand the geologic processes, rather than the development, of a specific volcano.

California, Oregon↗
USGS2022

Field-trip guide to continental arc to rift volcanism of the southern Rocky Mountains—Southern Rocky Mountain, Taos Plateau, and Jemez Mountains volcanic fields of southern Colorado and northern New Mexico

The southern Rocky Mountains of northern New Mexico and southern Colorado preserve the Oligocene to Pleistocene record of North American continental arc to rift volcanism. The 35–23 million year old (Ma) southern Rocky Mountain volcanic field (SRMVF), spectacularly preserved in the San Juan Mountains of southern Colorado, records the evolution of large andesitic stratovolcanoes to complex caldera clusters, from which at least 22 major ignimbrite sheets (each 150–5,000 cubic kilometers) were erupted. Outflow deposits of the SRMVF preserved along the broadly uplifted northwest flank of the northern Rio Grande rift basin (the San Luis Valley) provide critical structural and temporal constraints on the inception of crustal extension. Coincident with waning stages of SRMVF caldera-forming volcanism (~25.4 Ma), extensional tectonism was accompanied by a transition from bimodal early Miocene to intermediate-composition late Miocene and dominantly basaltic Pliocene rift volcanism of the Taos Plateau in the southern San Luis Basin. Concomitant rift volcanism in the Española Basin and bordering Jemez Mountains of northern New Mexico records a similar Miocene eruptive history dominated by intermediate-composition volcanism that transitioned locally to Pliocene rift-related basaltic volcanism of the Cerros del Rio volcanic field and culminated in eruptions of the iconic rhyolitic Pleistocene Bandelier Tuff and formation of the Valles Caldera along the northwestern rift-basin margin. This 6-day, 7-night field trip will focus, in broadly equal proportions, on rift-related extensional volcanism of the Jemez Mountains and Taos Plateau regions during the first half of the trip, and on caldera-forming volcanism of the southern Rocky Mountain volcanic field during the second half of the trip. The 35-million-year volcanic history of the region highlighted by new geologic mapping, high-resolution geochronology, petrologic, geochemical, and geophysical data facilitates discussion of (1) the magmatic response to the tectonic transition from subducted-slab arc to continental-rift volcanism; (2) the nature and temporal evolution of rift magmas; (3) fault controls on the spatial evolution of rift magmatism; (4) the diversity of continental-arc ignimbrite volcanism and associated lavas; (5) ignimbrite caldera structure and associated intrusions in three-dimension; (6) the role of recycled crystal mush and magmatic cumulates during growth of Cordilleran batholiths; and (7) high-precision geochronologic contributions to interpretation of relations between regional tectonic and volcanic processes. Most stops will be along roads, but there will be moderate hikes on trails of less than 1-hour duration covering 1–2 kilometers (0.6–1.2 miles) with modest elevation gain of <150 meters (<492 feet). The route will progress in reverse stratigraphic order, starting in the Jemez Mountains of New Mexico and proceed northward to San Luis Basin and San Luis Hills before turning west to the southeast and central San Juan Mountains. Our last full day takes us to the little-visited and only recently mapped, Bonanza caldera of the northeastern San Juan Mountains and on the final day, we leave the San Luis Valley to briefly explore the Tertiary subvolcanic plutons of the Collegiate Range along the west side of the Arkansas Valley rift valley, en route to Denver. The authors of all daily contributions acknowledge the helpful reviews by Amy Gilmer and Joe Colgan and thank Christine Chan and Jeremy Havens for assistance with figures, tables, and guidebook text.

Colorado, New Mexico↗
USGS1974

Erosion and sediment yields in mountain watersheds of the Transverse Ranges Ventura and Los Angeles Counties, California; analysis of rates and processes

Major-storm and long-term erosion rates in mountain watersheds of the western Transverse Ranges of Ventura County are estimated to range from low values that will not require the construction of catchments or channel-stabilization structures to values as high as those recorded anywhere for comparable bedrock erodibilities. A major reason for this extreme variability is the high degree of tectonic activity in the area--watersheds are locally being uplifted by at least as much as 25 feet per 1,000 years, yet the maximum extrapolated rate of denudation measured over the longest available period of record is 7.5 feet per 1,000 years adjusted to a drainage area of 0.5 square mile. Evidence of large amounts of uplift continuing into historic time includes structurally overturned strata of Pleistocene age, active thrust faulting, demonstrable stream antecedence, uplifted and deformed terraces, and other results of base-level change seen in stream channels. Such evidence is widespread in the Transverse Ranges, and aspects of the landscape, such as drainage-net characteristics and hillslope morphology, are locally more a function of tectonic activity than of denudational process. Many of the 72 study watersheds are located on frontal escarpments of mountain blocks cut by recently active thrust faults, along which the upper part of the drainage basin has overthrusted either the lower part of the basin or the adjacent valley area. To define erosion rates in 35 small watersheds in the western Transverse Ranges, a group of 37 similar watersheds with measured sediment yields in debris basins was selected from the eastern Transverse Ranges in Los Angeles County. Sediment yields from this group of watersheds during the record-breaking 1969 storms ranged from relatively low rates to values equivalent to reduction of the entire land surface of a watershed by more than 2 inches. Correlation of erosion rates from the watersheds with measured rates to the group with unknown rates required definition of the chief factors that control the erosion rates. Numerous types and combinations of variables measuring physiography, soil erodibility, slope stability, hydrologic factors, wildfire effects, vegetation, and land use were analyzed by regression. A slope-stability variable retained in regressions at significant levels was the proportion of watershed drainage area underlain by slope failures, a logical measure of increased erodibility caused by uplift. The importance in the area of debris flows, mudflows, and mass movements--forms of sediment transport not involving normal aqueous entrainment--is also a reflection of the active tectonic setting of the Transverse Ranges. Implicit in the detailed study of selected physiographic and slope-failure variables was the logical assumption that correlation with the probability of transport by these exotic but quantitatively important sedimentation processes would be achieved. So prominent and widespread was evidence of debris flows in the small study watersheds after the 1969 storms, that it was possible to formulate a model for the dispersal of sediment in such watersheds: Lateral supply of sediment to stream channels is a relatively continuous process, accomplished in significant part during the dry season by dry-sliding, in addition to wet-season contributions from overland flow and mass movements. During periods without major storms, stream channels undergo more-or-less time-continuous fill. Then, during a storm of high recurrence interval, channel-bed material is mobilized and dispersed in large part by debris flows--coarse granular slurries, some of which are induced by mass movements triggered by the storm. Channels undergo substantial net scour, accomplished by removal of bed material in debris flows and by scour during recession flow. Valley-side slopes are undercut by bank erosion, and a new cycle of channel infilling by hillslope processes is initiated.

California↗
USGS2023

Diving deeper into seep distribution along the Cascadia Convergent Margin, USA

Previous margin-wide studies of methane seep distribution along the Cascadia Subduction Zone indicate peaks in seep density within the landward limit of the of gas hydrate stability zone (GHSZ; ≤500 m depth), suggesting a link between current ocean warming, acceleration of hydrate dissociated, and methane emissions. This inferred connection, however, may not account for regional geologic and/or structural complexities driving methane seepage. Expanding upon an existing seep database by adding new seeps data, we conducted statistical and spatial analyses to determine margin-wide distribution trends and offer a tectonic framework for understanding the tendency toward non-normality and spatial clustering. We then highlight the role of local-scale drivers of seep formation in addition to the first-order tectonic framework, using systematic geologic/geomorphic characterization of seep emission sites in southern Cascadia and case studies using meta-attribute analysis of seismic reflection data.. Seep distribution along the margin is non-random, but instead of clustering along the 500-m isobath, regions of high seep density occur in canyons and topographic highs. New findings from this study conclude that co-location of the outer arc high (OAH) and the landward limit of the GHSZ may explain high concentrations of seeps where deformation is the greatest and hydrates are unstable. Detailed analysis of the spatial relationships between seep sites and geologic-geomorphic features in southern Cascadia reveal a link between seeps and anticlines, with 52% of the seeps found in association with anticlines, 36% found at faults, 16% associated with canyons, and 11% at seafloor failure scarps. Given that a majority of anticlines are located along or seaward of the OAH in the actively deforming outer wedge, we suggest that the location of the OAH is a primary structural control on seep distribution. This scenario is supported by neural network analysis of multichannel seismic data revealing zones of probable fluid migration along vertical pipes, faults, and chimneys in the vicinity of active seep sites on anticlines. Determining linkages between seeps and submarine tectonic geomorphology is a crucial first step for understanding and forecasting the distribution of methane seepage, but also a necessity for evaluating causal relationships between ocean warming and gas-hydrate stability.

California, Oregon, Washington↗
USGS2022

MIS 5e sea-level history along the Pacific coast of North America

The primary last interglacial, marine isotope substage (MIS) 5e records on the Pacific coast of North America, from Washington (USA) to Baja California Sur (Mexico), are found in the deposits of erosional marine terraces. Warmer coasts along the southern Golfo de California host both erosional marine terraces and constructional coral reef terraces. Because the northern part of the region is tectonically active, MIS 5e terrace elevations vary considerably, from a few meters above sea level to as much as 70 m above sea level. The primary paleo-sea-level indicator is the shoreline angle, the junction of the wave-cut platform with the former sea cliff, which forms very close to mean sea level. Most areas on the Pacific coast of North America have experienced uplift since MIS 5e time, but the rate of uplift varies substantially as a function of tectonic setting. Chronology in most places is based on uranium-series ages of the solitary coral Balanophyllia elegans (erosional terraces) or the colonial corals Porites and Pocillopora (constructional reefs). In areas lacking corals, correlation to MIS 5e often can be accomplished using amino acid ratios of fossil mollusks, compared to similar ratios in mollusks that also host dated corals. Uranium-series (U-series) analyses of corals that have experienced largely closed-system histories range from ∼124 to ∼118 ka, in good agreement with ages from MIS 5e reef terraces elsewhere in the world. There is no geomorphic, stratigraphic, or geochronological evidence for more than one high-sea stand during MIS 5e on the Pacific coast of North America. However, in areas of low uplift rate, the outer parts of MIS 5e terraces apparently were re-occupied by the high-sea stand at ∼100 ka (MIS 5c), evident from mixes of coral ages and mixes of molluscan faunas with differing thermal aspects. This sequence of events took place because glacial isostatic adjustment processes acting on North America resulted in regional high-sea stands at ∼100 and ∼80 ka that were higher than is the case in far-field regions, distant from large continental ice sheets. During MIS 5e time, sea surface temperatures (SSTs) off the Pacific coast of North America were higher than is the case at present, evident from extralimital southern species of mollusks found in dated deposits. Apparently, no wholesale shifts in faunal provinces took place, but in MIS 5e time, some species of bivalves and gastropods lived hundreds of kilometers north of their present northern limits, in good agreement with SST estimates derived from foraminiferal records and alkenone-based reconstructions in deep-sea cores. Because many areas of the Pacific coast of North America have been active tectonically for much or all of the Quaternary, many earlier interglacial periods are recorded as uplifted, higher-elevation terraces. In addition, from southern Oregon to northern Baja California, there are U-series-dated corals from marine terraces that formed at ∼80 ka, during MIS 5a. In contrast to MIS 5e, these terrace deposits host molluscan faunas that contain extralimital northern species, indicating cooler SST at the end of MIS 5. Here I present a review and standardized database of MIS 5e sea-level indicators along the Pacific coast of North America and the corresponding dated samples. The database is available in Muhs et al. (2021b; https://doi.org/10.5281/zenodo.5903285 ).

Earth System Science Data↗
USGS2005

Sediment-hosted lead-zinc deposits: A global perspective

Sediment-hosted Pb-Zn deposits contain the world’s greatest lead and zinc resources and dominate worldproduction of these metals. They are a diverse group of ore deposits hosted by a wide variety of carbonate andsiliciclastic rocks that have no obvious genetic association with igneous activity. A range of ore-forming processes in a variety of geologic and tectonic environments created these deposits over at least two billion years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic to low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host rocks that includes stratiform, strata-bound, and discordant ores. These ores are divided into two broad subtypes: Mississippi Valley-type (MVT) and sedimentary exhalative (SEDEX). Despite the “exhalative” component inherent in the term “SEDEX,” in this manuscript, direct evidence of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX. The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores. The distinction between some SEDEX and MVT deposits can be quite subjective because some SEDEX ores replaced carbonate, whereas some MVT deposits formed in an early diagenetic environment and display laminated ore textures. Geologic and resource information are presented for 248 deposits that provide a framework to describe and compare these deposits. Nine of the 10 largest sediment-hosted Pb-Zn deposits are SEDEX. Of the deposits that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits and 15 MVT (excluding Irish-type) deposits. Despite the skewed distribution of the deposit size, the two deposits types have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn), with a fairly consistent ratio of about 10/1, regardless of the size of the deposit or district. Zinc grades are approximately the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits. The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX deposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT deposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits hosted in carbonate rocks are virtually indistinguishable. The distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon. The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in the Phanerozoic. MVT deposits dominantly formed in platform carbonate sequences typically located within extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intracontinental or failed rifts, and rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their host rocks; however, a few are close (<~5 m.y.) to the age of their host rocks. In the absence of direct dates for SEDEX deposits, their age of formation is generally constrained by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation. Fluid inclusion, isotopic studies, and deposit modeling suggest that MVT and SEDEX deposits formed from basin brines with similar temperatures of mainly 90° to 200°C and 10 to 30 wt percent NaCl equiv. Lead isotope compositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal sources. Lead isotope compositions do not provide criteria that distinguish MVT from SEDEX subtypes. However, sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVT sulfur isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive isotopic compositions from 0 to 20 per mil. Isotopic values for MVT ores extend over a wider range and include more data with negative isotopic values. Given that there are relatively small differences between the metal character of MVT and SEDEX deposits and the fluids that deposited them, perhaps the most significant difference between these deposits is their de-positional environment, which is determined by their respective tectonic settings. The contrasting tectonic setting also dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology, deposit morphology, and ore textures. Brief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a subset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct ore type (Irish type). There are no significant differences in grade tonnage values between MVT deposits and the subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical MVT ores. The dominance of bacteriogenic sulfur in the Irish ores commonly ascribed as uniquely Irish type is in fact no different from several MVT deposits or districts. A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain significantly higher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits are almost three times more enriched in Ag and one and a half times more enriched in Pb compared to other SEDEX deposits. Metamorphism is a characteristic feature but not a prerequisite for inclusion in the Broken Hill-type category, and known Broken Hill-type examples appear to occur in Paleo- to Mesoprotero-zoic terranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to support their inclusion as a separate category of SEDEX deposits.

Book chapter↗
USGS2009

The Portland Basin: A (big) river runs through it

Metropolitan Portland, Oregon, USA, lies within a small Neogene to Holocene basin in the forearc of the Cascadia subduction system. Although the basin owes its existence and structural development to its convergent-margin tectonic setting, the stratigraphic architecture of basin-fill deposits chiefly reflects its physiographic position along the lower reaches of the continental-scale Columbia River system. As a result of this globally unique setting, the basin preserves a complex record of aggradation and incision in response to distant as well as local tectonic, volcanic, and climatic events. Voluminous flood basalts, continental and locally derived sediment and volcanic debris, and catastrophic flood deposits all accumulated in an area influenced by contemporaneous tectonic deformation and variations in regional and local base level.

Oregon↗
USGS1989

Appalachian Piedmont landscapes from the Permian to the Holocene

Between the Potomac and Susquehanna Rivers and from the Blue Ridge to the Fall Zone, landscapes of the Piedmont are illustrated for times in the Holocene, Late Wisconsin, Early Miocene, Early Cretaceous, Late Triassic, and Permian. Landscape evolution took place in tectonic settings marked by major plate collisions (Permian), arching and rifting (Late Triassic) and development of the Atlantic passive margin by sea floor spreading (Early Cretaceous). Erosion proceeded concurrently with tectonic uplift and continued after cessation of major tectonic activity. Atlantic Outer Continental Shelf sediments record three major erosional periods: (1) Late Triassic-Early Jurassic; (2) Late Jurassic-Early Cretaceous; and (3) Middle Miocene-Holocene. The Middle Miocene-Holocene pulse is related to neotectonic activity and major climatic fluctuations. In the Piedmont upland the Holocene landscape is interpreted as an upland surface of low relief undergoing dissection. Major rivers and streams are incised into a landscape on which the landforms show a delicate adjustment to rock lithologies. The Fall Zone has apparently evolved from a combination of warping, faulting, and differential erosion since Late Miocene. The periglacial environment of the Late Wisconsin (and earlier glacial epochs) resulted in increased physical erosion and reduced chemical weathering. Even with lowered saprolitization rates, geochemical modeling suggests that 80 m or more of saprolite may have formed since Late Miocene. This volume of saprolite suggests major erosion of upland surfaces and seemingly contradicts available field evidence. Greatly subdued relief characterized the Early Miocene time, near the end of a prolonged interval of tropical morphogenesis. The ancestral Susquehanna and Potomac Rivers occupied approximately their present locations. In Early Cretaceous time local relief may have been as much as 900 m, and a major axial river draining both the Piedmont and Appalachians flowed southeast past Baltimore. The Late Triassic landscape was influenced by rift basin development. Streams drained into a hydrologically closed basin: no through-flowing rivers seem to have been present. A limestone escarpment along the Blue Ridge may have existed as a consequence of a semi-arid climate. The Permian may have been a time of Himalayan-like mountains and mountain glaciers. Streams (and glaciers) generally flowed southwest and west. ?? 1989.

Geomorphology↗
USGS1990

Multidisciplinary hydrologic investigations at Yucca Mountain, Nevada

Future climatic conditions and tectonic processes have the potential to cause significant changes of the hydrologic system in the southern Great Basin, where a nuclear-waste repository is proposed for construction above the water table at Yucca Mountain, Nevada. Geothermal anomalies in the vicinity of Yucca Mountain probably result from the local and regional transport of heat by ground-water flow. Regionally and locally irregular patterns of hydraulic potential, local marsh and pond deposits, and calcite veins in faults and fractures probably are related principally to climatically imposed hydrologic conditions within the geologic and topographic framework. However, tectonic effects on the hydrologic system have also been proposed as the causes of these features, and existing data limitations preclude a full evaluation of these competing hypotheses. A broad program that integrates many disciplines of earth science is required in order to understand the relation of hydrology to past, present and future climates and tectonism.

Conference Paper↗
USGS1995

Comparison of flank margin cave development on San Salvador island, Bahamas, and Isla de Mona, Puerto Rico

San Salvador Island, Bahamas is a 161 Km2 tectonically stable late Quaternary carbonate island located 600 km east-southeast of Miami FL. San Salvador contains numerous flank margin caves (phreatic karst features) that developed primarily in late Pleistocene eolianites. These caves developed during a short time in versy small fresh-water lenses. Cave elevations and Uranium-series ages from stalagmites indicate that all currently subaerial flank margin caves developed during the last interglacial seal-level highstand that was 6 m above current mean seal level 125,000 years ago (oxygen isotope substage 5e), which lasted no more than 14,000 years. The caves were formed by dissolution in the mixing zone at the margin of a freshwater lens that was elevated by the substage 5e highstand, and which resided within the small emergent portions of eolianite ridges. The flank margin caves have chambers with volumes greater than 1000 m3 on San Salvador; on other Bahamian islands, chambers as large as 14,000 m3 are known. Isla de Mona is 55km2, uplifted Late Miiocene-Pleistocene carbonate platform located in the Mona Passage 60 km west of Puerto Rico. Most of the island has 40 to 80 m high vertical cliffs to the sea which contain numerous flank margin caves at various elevations. A few cave chambers have volumes in excess of 100,000 m3. Paleomagnetic reversals observed in cave deposits and speleothems indicate the caves are at lease 780,000 years old as pre-Pleistocene. This age indicates that the caves may have developed in a stable, freshwater lens prior to Pleistocene glacio-eustasy, and have been subsequently tectonically elevated to their current position. The great size of the flank margin caves indicate mixing-zone dissolution in a large freshwater lens over an extended period of time. Flank margin caves in Isla de Mona, and some in the Bahamas contain evidence of two cycles of dissolution, in that speleothems and the cave wall rock to which they are attached show evidence on contemporaneous phreatic dissolution. In the Bahamas, the cause is most likely a minor change in sea level during the last interglacial highstand which caused a brief period of emergence, during which the subaerial speleothems grew, followed by re-submergence and dissolution. On Isla de Monta either tectonic motion, or the onset of glacio-eustasy, created the conditions that caused renewed phreatic dissolution after a period of subaerial exposure and speleothem precipitation. Despite differences in rock age and geologic setting, both San Salvador and Isla do Mona show evidence of re-invasion of the flank margin caves by dissolutionally aggressive water following a vadose interval. The flank margin caves have very similar morphologies and characteristics, and the only major difference is attrutable to the larger lens size and the longer duration of stable lens position on Isla de Mona. The data indicate that dissolution occurs rapidly in these environments, and despite the development of large voids, the same geochemical environment can be re-established after an emergence episode.

Conference Paper↗
USGS2016

Some contrasting biostratigraphic links between the Baker and Olds Ferry Terranes, eastern Oregon

New stratigraphic and paleontologic data indicate that ophiolitic melange windows in the Olds Ferry terrane of eastern Oregon contain limestone blocks and chert that are somewhat different in age than those present in the adjacent Baker terrane melange. The melange windows in the Olds Ferry terrane occur as inliers in the flyschoid Early and Middle Jurassic age Weatherby Formation, which depositionally overlies the contact between the melange-rich Devonian to Upper Triassic rocks of the Baker terrane on the north, and Upper Triassic and Early Jurassic volcanic arc rocks of the Huntington Formation on the south. The Baker terrane and Huntington Formation represent fragments of a subduction complex and related volcanic island arc, whereas the Weatherby Formation consists of forearc basin sedimentary deposits. The tectonic blocks in the melange windows of the Weatherby Formation (in the Olds Ferry terrane) are dated by scarce biostratigraphic evidence as Upper Pennsylvanian to Lower Permian and Upper Triassic. In contrast, tectonic blocks of limestone in theBaker terrane yield mostly fusulinids and small foraminifers of Middle Pennsylvanian Moscovian age at one locality.Middle Permian (Guadalupian) Tethyan fusulinids and smaller foraminifers (neoschwagerinids and other Middle Permian genera) are present at a few other localities. Late Triassic conodonts and bryozoans are also present in a few of the Baker terrane tectonic blocks. These limestone blocks are generally embedded in Permian and Triassic radiolarian bearing chert or argillite. Based on conodont, radiolarian and fusulinid data, the age limits of the meange blocks in the Weatherby Formation range from Pennsylvanian to Late Triassic.

Oregon↗
USGS1990

Evolution of the Tertiary La Honda basin, central California

Tertiary strata of the La Honda basin are exposed in the Santa Cruz Mountains along the central California coast south of San Francisco. The basin fill has a composite thickness of more than 14,500 m and consists of sedimentary and volcanic rocks that in places rest on granitic basement rocks of the Salinia terrane. Paleogene strata are mainly turbidite sandstone and hemipelagic mudstone that accumulated in deep-sea fan and basin plain environments at lower bathyal to abyssal depths. Neogene rocks are mainly shallow-marine shelf sandstone and upper to middle bathyal siliceous mudstone. Both Paleogene and Neogene strata exhibit rapid lateral variations in thickness and facies, several local and regional unconformities, numerous folds, and ubiquitous faults. The complicated geology and geologic history of the La Honda basin reflect the fact that, throughout its history, the basin has been located at or near the tectonically active plate boundary between the North American continent and various oceanic plates of the Pacific basin. The La Honda basin originated during the Paleocene, perhaps during an episode of wrench tectonism associated with oblique subduction and arrival of the Salinia terrane. Major restructuring of the basin during the Oligocene-including uplift and erosion of the basin margins, movement along the Zayante-Vergeles fault, and deposition of two sand-rich deep-sea fans–apparently resulted from the approach of the Farallon-Pacific spreading ridge and its collision with the California continental margin. During the late Oligocene and early Miocene, widespread volcanism and marine transgression accompanied an episode of regional transtension along the San Andreas fault system. Deposition of shallow-marine sandstones and deeper-water siliceous mudstones occurred during much of the Miocene and Pliocene but was interrupted at least three times by brief episodes of uplift and erosion associated with transpressional wrench tectonism along the San Andreas fault. Marine deposition ended and uplift of the modern Santa Cruz Mountains began during the late Pliocene in response to the most-recent episode of regional transpression. Five small oil fields in the La Honda basin have produced a total of 1.7 million barrels of oil and 300 million cubic feet of gas, mostly from reservoirs in Eocene turbidite sandstone and Miocene limestone.

Conference Paper↗
USGS1978

Hypothesis: Many earthquakes in the central and southeastern United States are causally related to mafic intrusive bodies

Assessment of earthquake hazards in the United States is based largely upon knowledge acquired in the seismically active parts of the western United States. Earthquakes in the central and southeastern United States are seismologically and geologically very anomalous, however, compared with those in the western United States. For example, shallow earthquakes of magnitude 7-f in the western United States in general have tectonic surface faulting associated with them. No tectonic surface faulting, however, has been found to be associated with the New Madrid, Mo., earthquakes of 1811 and 1812, several of which had estimated magnitudes of 7-f. The lack of surface faulting is, in part, responsible for the fact that the New Madrid earthquakes have not been related with certainty to any particular geologic structural element, which makes assessment of the seismic hazards associated with possible similar future earthquakes very difficult. Geologic and seismologic data from the central and southeastern United States, however, suggest a spatial correlation between mafic intrusive rocks and seismic activity. An additional supportive correlation is that the trend of many dikes coincides with the trend of nodal planes in many fault-plane solutions. These correlations differ greatly from the direct correlation of earthquakes with faults in the western United States. Consideration of different concepts of earthquake sources in the central and southeastern. United States seems necessary- The preferred concept is that local high-stress concentrations, which may result in earthquakes, occur at or close to the contact of mafic intrusives and their felsic host rocks. Ancient rift zones may have been the primary control of the location of the mafic intrusives. Implications of a causal relation between mafic intrusives and seismicity are that more useful seismo-tectonic maps may be possible and that the source dimensions of the earthquakes would be small. Because of low attenuation, however, effects of earthquakes in the central and southeastern United States, even if relatively small because of smaller source dimensions, must be considered hazardous.

Journal of Research of the U.S. Geological Survey↗
USGS1978

Porphyry copper exploration model for northern Sonora, Mexico

The regional tectonic pattern of the porphyry copper province of northern Sonora, Mexico, is similar to, but more complex than, the Colorado mineral belt. Four northeast-trending shear zones, spaced at 30- to 50-kilometer intervals from Hermosillo, Mexico, north to Nogales, Mexico, are interpreted from analysis of lineament data from Landsat-1 images. From comparison with other areas, these zones are believed to have Precambrian ancestry. North-northwest-trending lineaments, which commonly occur along range fronts, are interpreted to be the principal basin and range faults. North-south- and west-northwest-trending lineaments are believed to be related to lesser faults and fractures. Areas of hydrothermal alteration have been mapped on a regional scale by use of digital image-processing techniques. These altered areas occur on the flanks of the northeast-trending shear zones, generally near intersections with north-northwest-trending lineaments. This tectonic framework with associated alteration is similar to patterns in the Colorado Plateau, the porphyry copper province of Arizona, and central Mexico and is supported by relationships found in limited local mapping. The pattern of northeast-trending shear zones and north-northwest-trending faults in northern Sonora closely resembles the tectonic framework of the Colorado mineral belt. The younger sedimentary and volcanic cover, however, has not been eroded in Sonora as it has been in the Colorado mineral belt. This comparison with the Colorado mineral belt suggests that mineral deposits are most likely to be found in altered areas where a northeast-trending shear zone is intersected by north-northwest-trending faults. Seven areas with these characteristics have been defined in Sonora and are being evaluated by means of regional geophysical and geochemical data.

Sonora↗
USGS2017

The story of a Yakima fold and how it informs Late Neogene and Quaternary backarc deformation in the Cascadia subduction zone, Manastash anticline, Washington, USA

The Yakima folds of central Washington, USA, are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line- and area-balanced cross sections, the crust has horizontally shortened by 11% (0.8–0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west-northwest striking frontal thrust faults and shorter north to northeast striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault-cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west trending Yakima folds and west striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low strain rate environment in the backarc of the Cascadia Subduction Zone.

Washington↗