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USGS1985

40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England

40 Ar/ 39 Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mountain massifs, as well as 40 Ar/ 39 Ar and K-Ar mineral and whole-rock ages from Paleozoic metamorphic rocks, suggest that the thermal peak for the dominant metamorphic recrystallization in western New England occurred 465 ± 5 m.y. ago (Taconian). Although textural data indicate a complex metamorphic-tectonic history for Paleozoic rocks, no evidence in rocks at least as high as kyanite grade dictates an Acadian age for the Barrovian metamorphism. Available 40 Ar/ 39 Ar and K-Ar data suggest that the low-grade metamorphism and cleavage formation in Taconic allochthons and the higher-grade metamorphism and emplacement of the Berkshire massif allochthon are Taconian. 40 Ar/ 39 Ar age data from a poorly defined terrane beginning near the east margin of the Green Mountain massif and extending along the eastern one-third of the Berkshire massif as far south as Otis, Massachusetts, suggest that the area has been retrograded during a metamorphism that peaked at least 376 ± 5 m.y. ago (Acadian). Available age and petrologic data from western New England indicate the presence of at least three separate metamorphic-structural domains of Taconian age: (1) a small area of relict high-pressure and low-temperature metamorphism in northern Vermont (T-1 domain), (2) a broad area in Vermont and eastern New York of normal Barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient (T-2 domain), and (3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks extending from near the Cortlandt Complex northeastward through Dutchess County, New York, to the Berkshire massif in western Massachusetts (T-3 domain). Areas of maximum metamorphic intensity within the T-3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting (Berkshire massif) or from recumbent folding (Manhattan Prong) of remobilized North American continental crust in the later stages of the Taconic orogeny.

Geological Society of America Bulletin↗

USGS1985

Carboniferous U-Pb age of the Sebago batholith, southwestern Maine: Metamorphic and tectonic implications

Two phases (pink and white granite) of the Sebago batholith of southwestern Maine have been dated by the U-Pb zircon method. Identical upper concordia intercepts of both rocks indicate an intrusive age of 325 ± 3 m.y. for the batholith. The lower intercept of the pink-phase sample, 114 ± 13 m.y., is inferred to represent episodic lead loss due to the intrusion of the nearby Cretaceous Pleasant Mountain stock. The lower intercept of the white-phase sample, 18 ± 21 m.y., suggests only modern dilatancy lead loss. Monazites have ages of 272 m.y. (pink) and 282 m.y. (white) which are thought to be cooling ages. Rb-Sr whole-rock data have low initial 87 Sr/ 86 Sr ratios of 0.7031 (pink) and 0.7053 (white). These data, in conjunction with published 40 Ar/ 39 Ar, Rb-Sr, K-Ar, and fission-track ages, suggest that little or no uplift occurred in this part of New England until the Permian and that the uplift rate from 275 m.y. to 225 m.y. was ∼3 times as rapid as was the rate for 225 m.y. to the present. The Carboniferous age of the Sebago batholith suggests that currently accepted metamorphic and tectonic interpretations for southwestern Maine and for east-central New Hampshire require revision.

Maine↗

USGS1988

The relative contribution of accretion, shear, and extension to Cenozoic tectonic rotation in the Pacific Northwest

Large Cenozoic clockwise rotations defined by paleomagnetic data are an established fact in the Pacific Northwest, and many tectonic models have been proposed to explain them, including (1) rotation of accreted oceanic microplates during docking, (2) dextral shear between North America and northward-moving oceanic plates to the west, and (3) microplate rotation in front of an expanding Basin and Range province. Stratigraphic onlap relations and local structure indicate that microplate rotation during docking was not a major contributor to the observed rotations. Coast Range structures, Basin and Range extension, and paleomagnetic data from middle Miocene (15 Ma) Coast Range rocks indicate that dextral shear is responsible for at least 40% of the post-15 Ma rotation of the Coast Range and that Basin and Range extension is responsible for the remainder. Reconstructions based on extrapolation of this ratio back to 37 and 50 Ma are consistent with reconstructions based on paleomagnetic and stratigraphic relations in older rocks and suggest that dextral shear has, been a significant contributor to rotation during most of Tertiary time. Changes in the dextral-shear rotation rate over the past 50 m.y. correlate directly with changes in the velocity of the Farallon plate parallel to the coast and provide a strong argument for oblique subduction as the driving mechanism. Continental reconstructions incorporating shear may provide constraints on the rate of extension in the northernmost Basin and Range region and suggest 17% extension since 15 Ma, 39% since 37 Ma, and 72% since 50 Ma near latitude 42°N.

GSA Bulletin↗

USGS1995

Tectonic implications of post-30 Ma Pacific and North American relative plate motions

The Pacific plate moved northwest relative to North America since 42 Ma. The rapid half rate of Pacific-Farallon spreading allowed the ridge to approach the continent at about 29 Ma. Extinct spreading ridges that occur offshore along 65% of the margin (Lonsdale, 1991) document that fragments of the subducted Farallon slab became captured by the Pacific plate and assumed its motion prior to the actual subduction of the spreading ridge. This plate-capture process can be used to explain much of the post–29 Ma Cordilleran North America extension, strike slip, and the inland jump of oceanic spreading in the Gulf of California. The Pacific and North American contact zone lengthened with each successive plate capture event, underpinning the parts of western North America directly inland with a strong plate undergoing Pacific relative motion. We suggest that much of the post–29 Ma continental tectonism is the result of the strong traction imposed on the deep part of the continental crust by the gently inclined slab of subducted oceanic lithosphere as it moved to the northwest relative to the overlying continent. The plate-capture hypothesis is distinctly different from theories involving shallow slab gaps. Kinematic problems associated with shallow slab-gap models cause us to question them. This conclusion is consistent with seismic refraction interpretations that suggest there is an inclined layer with high velocities like that of basalt or gabbro at the base of the continental crust beneath much of the Californian margin and the documented reduction of slab-pull forces and density associated with young subducting slabs. Thermal and rheologic modeling suggests that coastal California was a strong zone at all depths allowing it to be firmly linked to Pacific motion. Our model shows that deformed regions such as the basin and range and borderland provinces developed in predicted weak parts of the crustal section, but they have been incompletely linked to the deep plate across the ductile middle and lower crustal layer.

Geological Society of America Bulletin↗

USGS1995

Tectonic setting of the Portland-Vancouver area, Oregon and Washington: Constraints from low-altitude aeromagnetic data

Seismic activity in the Portland-Vancouver metropolitan area may be associated with various mapped faults that locally offset volcanic basement of Eocene age and younger. This volcanic basement is concealed in most places by young deposits, vegetation, and urban development. The U.S. Geological Survey conducted an aeromagnetic survey in September 1992 to investigate the extent of these mapped faults and possibly to help identify other seismic and volcanic hazards in the area. The survey was flown approximately 240 m above terrain, along flight lines spaced 460 m apart, and over an area about 50 × 50 km. These magnetic data indicate a pronounced northwest-striking magnetic lineation east of the Willamette River in downtown Portland associated with a fault concealed beneath Quaternary sedimentary deposits and previously inferred from shallow well data. The magnetic lineation confirms the existence of the fault and suggests that it has had a prolonged history: (1) Although well data indicate <200 m of vertical offset of underlying volcanic basement, models based on the aeromagnetic data from downtown Portland suggest reverse faulting with up to 1 km of offset deeper in the section. (2) The magnetic lineation associated with this fault extends southeast to the Clackamas River drainage, a distance of 50 km and considerably beyond the mapped extent of the fault. A northwest-striking magnetic anomaly located southwest of the Tualatin Mountains corresponds closely with another mapped fault and with mixed reverse and strike-slip faulting during a seismic swarm (M ≤ 3) in 1991. We believe these and other anomalies in the aeromagnetic data reflect the Portland Hills fault zone, believed to be the southwestern boundary of a structural basin now occupied by Portland and Vancouver. The postulated northeastern boundary of the basin, the Frontal fault zone, is also evident, although less well represented in the aeromagnetic data. Aeromagnetic anomalies, geologic mapping, and earthquake focal-plane solutions demonstrate a complex deformational history in the Portland-Vancouver area since middle Miocene time that includes elements of compression, extension, and dextral slip. These complexities reflect Portland-Vancouver's unique position within a north-south transition in tectonic styles along the Cascadia margin, from compressional in the north to extensional in the south.

Oregon, Washington↗

USGS1998

Upper crustal structure and Neogene tectonic development of the California continental borderland

Multichannel seismic-reflection data, sonobuoy seismic-refraction data, and regional geology are used to define the upper crustal structure of the southern California continental borderland and to delineate the characteristics of the main lithotectonic belts of the region. The Catalina Schist belt is separated on its west side from the gently deformed Nicolas forearc belt by faults that have steep west dips and pronounced normal separations. On its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the Peninsular Ranges belt at the coastline near Oceanside. The Catalina Schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early Miocene time in conjunction with about 10° of clockwise rotation of the western Transverse Ranges belt. Part of the uplift of the Catalina Schist could have occurred on the detachment fault, but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt. A large amount of uplift is required, and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western Transverse Ranges and Nicolas forearc belts. Extension, accompanied by probable large amounts of right slip, continued in the borderland region during and after middle Miocene time. The later stage of extension was accompanied by rapid clockwise rotation of the western Transverse Ranges of at least 90°. Most of the borderland, including the belt of schist that was uplifted in early Miocene time, was further deformed into numerous basins and ridges during this stage of oblique extension. The primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the Pacific plate after it had become coupled to the Farallon plate system, which had previously been subducted beneath the borderland.

Geological Society of America Bulletin↗

USGS1983

Tectonic uplift of a middle Wisconsin marine platform near the Mendocino triple junction California

An uplifted wave-cut marine platform eroded across bedrock of the Franciscan Complex at Point Delgada, northern California, is overlain by 0.5 to 5 m of wave-worked pea gravel, which is in turn directly overlain by fluvial gravel and silt deposited as alluvial fans. Woody plant debris at the base of the fluvial deposits includes cones of Brewer spruce ( Picea breweriana ), which today are found only at higher elevations and latitudes. Fossil wood debris from this horizon yields a 14 C date of 44,800 ± 1,300 yr. The 44,800 yr radiocarbon age for the base of the fluvial deposits establishes an approximate age for the immediately underlying marine gravels and wave-cut platform. We tentatively correlate this terrace with the middle Wisconsin high sea-level stand at −37 m, dated at about 45,000 yr B.P. If this age is correct, the tectonic uplift since middle Wisconsin time has been 44 m, and the average rate of uplift has been at least 1.0 m/1,000 yr. This relatively high rate is probably related to interaction among the Pacific, Gorda, and North American plates at the Mendocino triple junction.

Geology↗

USGS1985

Evidence for Quaternary tectonism in the northern Bighorn basin, Wyoming and Montana

Irregularities in the reconstructed gradients of latest Pliocene and Quaternary terraces in the northern Bighorn Basin, northwest Wyoming and south-central Montana, suggest that Quaternary movements have occurred on the Frannie anticline, the Nye-Bowler–Sage Creek fault zone, and the North Pryor fault. This area has been considered tectonically inactive. The timing of the interpreted movements is derived from age estimates of fluvial deposits determined from numerical ages and altitudes of associated deposits of volcanic ash. South of the Pryor Mountains, the 1.4 Ma Polecat Bench terrace of the Shoshone River rises 6–7 m over the axis of the Frannie anticline; this displacement may be related to folding or faulting since 1.4 Ma. Near the Nye-Bowler–Sage Creek and North Pryor faults in the Pryor Mountains, bedrock is 25 and 55 m shallower than depths predicted from the projected 1.4 Ma gradient of the Shoshone River through Pryor Gap. The 2.0 Ma terrace of Rock Creek may have been offset up to 7 m by a splay of the Nye-Bowler fault zone between 2.0 and 0.6 Ma. The downstream divergence of terraces along Rock Creek suggests ongoing uplift of the northern Bighorn Basin, perhaps caused by crustal rebound due to post-Oligocene erosion.

Wyoming, Montana↗

USGS1986

Eureka Quartzite in Mexico? - Tectonic implications

At Cerro Cobachi, 90 km east of Hermosillo, Sonora, an Ordovician to Permian miogeoclinal assemblage and an Ordovician to Permian siliceous deep-water assemblage were juxtaposed by thrust faulting between mid-Permian and latest Cretaceous time. Both assemblages resemble counterparts in the Great Basin. One formation, an ultramature quartzite unit in the miogeoclinal assemblage, closely resembles the Middle Ordovician Eureka Quartzite. In the southern Great Basin, isopach lines of the Eureka trend south-southwestward. From a maximum thickness of 134 m near Owens Lake, California, the Eureka thins and splays northward in the southern Inyo Mountains and thins southeastward in the Nopah Range. But south-southwestward, parallel with the isopach lines, it apparently ends abruptly as if faulted. Because the Paleozoic stratigraphy of the western Great Basin and that of west Texas have elements in common, it is quite possible that the southwest-trending facies belts of the Great Basin originally wrapped around the southern border of the continent through northern Mexico and joined corresponding belts in Texas. Two hypotheses are suggested: (1) the Cerro Cobachi terrane, of which the quartzite is a part, is indigenous to northern Mexico, and (2) the Cerro Cobachi terrane is indigenous to California and was displaced tectonically to northern Mexico. The second hypothesis is favored by the apparently abrupt termination of the Eureka Quartzite near Owens Lake, the nearly identical thickness of the two quartzites, and their nearly identical lithic composition and texture.

Geology↗

USGS2001

Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California

A seismic refraction and low-fold reflection survey, known as the Los Angeles Region Seismic Experiment (LARSE), was conducted along a transect (line 1) extending from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. The chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern California that have been debated for decades and have important implications for earthquake hazard assessment. The results (or constraints) are as follows. (1) The maximum depth of the Los Angeles basin along line 1 is 8–9 km. (2) The deep structure of the Sierra Madre fault zone in the northern San Gabriel Valley is as follows. The Duarte branch of the Sierra Madre fault zone forms a buried, 2.5-km-high, moderately north dipping buttress between the sedimentary and volcanic rocks of the San Gabriel Valley and the igneous and metamorphic rocks of the San Gabriel Mountains. (For deeper structure, see following.) (3) There are active crustal décollements in southern California. At middle-crustal depths, the Sierra Madre fault zone appears to sole into a master décollement that terminates northward at the San Andreas fault and projects southward beneath the San Gabriel Valley to the Puente Hills blind thrust fault. (4) The dip and depth extent of the San Andreas fault along line 1 dips steeply (∼83°) northward and extends to at least the Moho. (5) The subsurface lateral extent of the Pelona Schist in southern California is as follows. Along line 1, the Pelona Schist underlies much, if not all of the San Gabriel Mountains south of the San Andreas fault to middle-crustal depths. North of the San Andreas fault, it is apparently not present along the transect.

California↗

USGS2009

The Boring Volcanic Field of the Portland-Vancouver area, Oregon and Washington: Tectonically anomalous forearc volcanism in an urban setting

More than 80 small volcanoes are scattered throughout the Portland-Vancouver metropolitan area of northwestern Oregon and southwestern Washington. These volcanoes constitute the Boring Volcanic Field, which is centered in the Neogene Portland Basin and merges to the east with coeval volcanic centers of the High Cascade volcanic arc. Although the character of volcanic activity is typical of many monogenetic volcanic fields, its tectonic setting is not, being located in the forearc of the Cascadia subduction system well trenchward of the volcanic-arc axis. The history and petrology of this anomalous volcanic field have been elucidated by a comprehensive program of geologic mapping, geochemistry, 40Ar/39Ar geochronology, and paleomag-netic studies. Volcanism began at 2.6 Ma with eruption of low-K tholeiite and related lavas in the southern part of the Portland Basin. At 1.6 Ma, following a hiatus of ~0.8 m.y., similar lavas erupted a few kilometers to the north, after which volcanism became widely dispersed, compositionally variable, and more or less continuous, with an average recurrence interval of 15,000 yr. The youngest centers, 50–130 ka, are found in the northern part of the field. Boring centers are generally monogenetic and mafic but a few larger edifices, ranging from basalt to low-SiO2 andesite, were also constructed. Low-K to high-K calc-alkaline compositions similar to those of the nearby volcanic arc dominate the field, but many centers erupted magmas that exhibit little influence of fluids derived from the subducting slab. The timing and compositional characteristics of Boring volcanism suggest a genetic relationship with late Neogene intra-arc rifting.

Portland, Vancouver↗

USGS2010

Integration of tectonic, sedimentary, and geohydrologic processes leading to a small-scale extension model for the Mormon Mountains area north of Lake Mead, Lincoln County, Nevada

Scattered remnants of highly diverse stratigraphic sections of Tertiary lacustrine limestone, andesite flows, and 23.8–18.2 Ma regional ash-flow tuffs on the north flank of the Mormon Mountains record previously unrecognized deformation, which we interpret as pre–17 Ma uplift and possibly weak extension on the north flank of a growing dome. Directly to the north of the Mormon dome, 17–14 Ma ash-flow tuffs and rhyolite are interstratified with landslides, debris avalanches, debris flows, and alluvial-fan deposits that accumulated to a thickness of more than 2 km in an extension-parallel basin. The source for the landslides and debris avalanche deposits is unknown, but it was probably an adjacent scarp along a transverse fault bounding an early part of the Mormon dome. An average 45° of easterly tilt of the entire Tertiary basin-fill succession represents the major post–14 Ma deformation event in the region. We question the basis for the published estimate of 22 km of westerly displacement on the Mormon Peak detachment fault and, on the basis of landslides in the upper plate having a probable source in the adjacent Mormon dome, constrain the heave to ~4 km. We interpret the dome and basin as coupled strains similar to others in the region and suggest that these strains reflect a waveform pattern of extension-normal lateral midcrustal ductile flow. Previously, doming was interpreted as an isostatic response to tectonic unloading by large-displacement detachment faults or as pseudo-structural highs stranded by removal of middle crust from adjacent areas. Moreover, we argue that the strong thinning of upper-plate rock successions throughout the Mormon Mountains and Tule Springs Hills resulted from a loss of rock volume by protracted fluid flow, dissolution, and collapse, seriously limiting the usefulness of upper-plate strain in evaluating extension magnitude. We present a geohydrologic model that couples uplift driven by ductile inflow with dissolution driven by fluid infiltration, possibly augmented by mantle-derived CO 2 -rich fluids. Karsting in the uplands led to carbonate sedimentation in adjacent lowlands. Whether or not our downward revision of extension in the Mormon Mountains is valid, extension at that latitude is isolated from extension in the Lake Mead area by a low-strain corridor between the two areas. Recognition of the isolated and potentially diminished strain impacts estimates of maximum finite elongation of the Basin and Range Province because one of three vector paths used in those estimates passes through the Mormon Mountains.

Nevada↗

USGS2014

Thermochronology of Cretaceous batholithic rocks in the northern Peninsular Ranges batholith, southern California: Implications for the Late Cretaceous tectonic evolution of southern California

The thermochronology for several suites of Mesozoic metamorphic and plutonic rocks collected throughout the northern Peninsular Ranges batholith (PRB) was studied as part of a collaborative isotopic study to further our understanding of the magmatic and tectonic history of southern California. These sample suites include: a traverse through the plutonic rocks across the northern PRB ( N = 29), a traverse across a central structural and metamorphic transition zone of mainly metasedimentary rocks at Searl ridge ( N = 20), plutonic samples from several drill cores ( N = 7) and surface samples ( N = 2) from the Los Angeles Basin, a traverse across the Eastern Peninsular Ranges mylonite zone ( N = 6), and a suite of plutonic samples collected across the northern PRB ( N = 13) from which only biotite 40 Ar/ 39 Ar ages were obtained. These geochronologic data help to characterize five major petrologic, geochemical, and isotopic zonations of the PRB (western zone, WZ; western transition zone, WTZ; eastern transition zone, ETZ; eastern zone, EZ; and upper-plate zone, UPZ). Apparent cooling rates were calculated using U-Pb zircon (zr) and titanite (sphene) ages; 40 Ar/ 39 Ar ages from hornblende (hbl), biotite (bi), and K-feldspar (Kf); and apatite fission-track (AFT) ages from the same samples. The apparent cooling rates across the northern PRB vary from relatively rapid in the west (zr-hbl ~210 °C/m.y.; zr-bio ~160 °C/m.y.; zr-Kf ~80 °C/m.y.) to less rapid in the central (zr-hb ~280 °C/m.y.; zr-bio ~90 °C/m.y.; zr-Kf ~60 °C/m.y.) and eastern (zr-hbl ~185 °C/m.y.; zr-bio ~180 °C/m.y.; zr-Kf ~60 °C/m.y.) zones. An exception in the eastern zone, the massive San Jacinto pluton, appears to have cooled very rapidly (zr-bio ~385 °C/m.y.). Apparent cooling rates for the UPZ samples are consistently slower in comparison (~25–45 °C/m.y.), regardless of which geochronometers are used. Notable characteristics of the various ages from different dating methods include: (1) Zircon ages indicate a progressive younging of magmatic activity from west to east between ca. 125 and 90 Ma. (2) Various geochronometers were apparently affected by emplacement of the voluminous (ETZ and EZ) La Posta–type plutons emplaced between 99 and 91 Ma. Those minerals affected include K-feldspar in the western zone rocks, biotite and K-feldspar in the WTZ rocks, and white mica and K-feldspar in rocks from Searl ridge. (3) The AFT ages record the time the rocks cooled through the AFT closure temperature (~100 °C in these rocks), likely due to exhumation. Throughout most of the northern traverse, the apatite data indicate the rocks cooled relatively quickly through the apatite partial annealing zone (PAZ; from ~110 °C to 60 °C) and remained at temperatures less than 60 °C as continued exhumation cooled them to present-day surface temperatures. The ages indicate that the western “arc” terrane of the WZ was being uplifted and cooled at ca. 91 Ma, during or shortly after intrusion of the 99–91 Ma La Posta–type plutons to the east. Uplift and cooling occurred later, between ca. 70 Ma and ca. 55 Ma, in the central WTZ, ETZ, and EZ rocks, possibly as upwarping in response to events in the UPZ. The UPZ experienced differential exhumation at ca. 50–35 Ma: Cooling on the western edge was taking place at about the same time or shortly after cooling in the younger samples in the ETZ and EZ, whereas on the east side of the UPZ, the rocks cooled later (ca. 35 Ma) and spent a prolonged time in the apatite PAZ compared to most northern traverse samples. Apparent cooling rates from Los Angeles Basin drill core samples of plutonic rocks show that four are similar to the WTZ thermal histories, and two are similar to the WTZ histories, indicating that the eastern part of the Los Angeles Basin area is underlain by mainly western zone PRB rocks. Thermal histories revealed by samples from Searl ridge indicate that the WTZ magmatism intruded the metasedimentary rocks prior to their deformation and metamorphism at ca. 97 Ma. Both low-grade schists and metasandstones of the western side of the ridge and high-grade gneisses of the eastern side of the ridge have thermal histories consistent with eastern zone rocks—suggesting a temporal/thermal relationship between the western transition zone and the eastern zones. Limited ages from six samples across the Eastern Peninsular Ranges mylonite zone (EPRMZ) indicate that this zone underwent cooling after emplacement of the youngest UPZ rocks at 85 Ma, suggesting that thrusting along the EPRMZ was either coeval with emplacement of the UPZ plutonic rocks or occurred shortly afterwards (~10–15 m.y.). Alternatively, the EPRMZ thrusting may have occurred at temperatures under ~180 °C at yet a later date. The geochronology presented here differs slightly from previous studies for similar rocks exposed across the middle and southern portions of the PRB, in that our data define a relatively smooth progression of magmatism from west to east, and the transition from western, oceanic-arc plutonism to eastern, continental arc plutonism is interpreted to have occurred at ca. 99–97 Ma and not at ca. 105 Ma.

Book chapter↗

USGS2019

Mesozoic to Cenozoic sedimentation, tectonics, and metallogeny of Sonora, Mexico

We will embark on a five-day journey through northern, western, and central Sonora, in which we will see excellent examples of mostly Mesozoic to Cenozoic tectonics, sedimentation, and metallogeny. On Day 1, we will visit the porphyry copper deposit at Ajo, Arizona, and several Pleistocene cinder cones and maar craters in the Pinacate Biosphere Reserve. On Day 2, we will see L- and L-S tectonites at the type locality of the Mojave-Sonora megashear in Sierra Los Tanques, Noche Buena orogenic gold deposit, Ediacaran Gamuza beds in Caborca, and have an overview of the Carnero detachment fault on the south side of Sierra La Gloria. Day 3 will explore faults and related sedimentary and volcanic rocks associated with the late Miocene oblique opening of the Gulf of California rift and visit outcrops that record late Miocene timing constraints for flooding of the Gulf of California seaway, including several localities on southern Isla Tiburón accessible only by boat. Day 4 will visit exposures of Permian sedimentary to Paleogene igneous rocks in Hermosillo (Cerro La Campana); Puerto del Sol detachment fault zone; Aconchi batholith and a hot spring localized on a Basin and Range normal fault; Santa Elena low-sulfidation epithermal gold mine; and the Upper Jurassic Cucurpe Formation. On Day 5, we will visit several exposures of different crustal levels of the Magdalena-Madera metamorphic core complex, including the spectacular stretched pebble conglomerates in Arroyo Amolares.

Sonora↗

USGS2021

Paleozoic and Mesozoic tectonic events west of the Waterbury Dome: Results of new mapping in the western Connecticut Highlands

This field trip highlights the results of recent U.S. Geological Survey (USGS) bedrock geologic mapping in four 7.5 min quadrangles in the western Connecticut highlands near Southbury, Connecticut, USA. The rocks are broadly within what Rodgers (1985) called the Hartland and Gneiss Dome belts of the Connecticut Valley Synclinorium ( Rodgers, 1985 ; Fig. 1 ), the latter of which is now known as the Connecticut Valley–Gaspe Trough (Hibbard et al., 2006). The mapping occurred over two intervals: 2003–2005 and 2016–present. In the first, the goal was a detailed map of the early Mesozoic Pomperaug basin, which overlaps the four quadrangles. Portions of the basin had been separately mapped during the statewide 7.5 min quadrangle mapping campaign spanning the 1950s–1970s, resulting in an inaccurate depiction of the basin on the 1985 state geologic map ( Rodgers, 1985 ). A new map of the basin was proposed in part to benefit an ongoing project by the USGS Connecticut Water Science Center to determine the contributions of natural and artificial contaminants to a public water-supply well in Woodbury, Connecticut, under the National Water-Quality Assessment (NAWQA) Program ( Starn and Brown, 2007) . The NAWQA project was partially funded by the Pomperaug River Watershed Coalition, which also provided logistical support to the geologic mapping project. Mapping of the basin was also a high priority for the Connecticut State Geologist at the time (Ralph Lewis, 2002, pers. comm.). The mapping resulted in an NEIGC (New England Intercollegiate Geologic Conference) field guide ( Burton et al., 2005) and a 1:12,000-scale USGS open-file map ( Burton, 2006) and was funded by the USGS National Cooperative Geologic Mapping Program (NCGMP). The second phase of the mapping began in 2016 after the discovery of elevated levels of uranium and arsenic in domestic water wells in the igneous and metamorphic rocks that surround the sedimentary and volcanic rocks of the Pomperaug basin ( Flanagan and Brown, 2017) . Structural measurements in the surrounding crystalline rocks were made during Pomperaug basin mapping to better understand the tectonic setting, but a revision of the crystalline map units on Rodgers’ 1985 geologic map was not attempted. Nonetheless, discrepancies were noted between the new mapping and the 1985 map, particularly within the two northern quadrangles of Woodbury and Roxbury, which were originally mapped by Gates (1954) and Gates (1959) , respectively. Based on these discrepancies, a new NCGMP project was proposed to remap the Woodbury and Roxbury 7.5 min quadrangles, commencing in the fall of 2016. The expected USGS product will be a two-quadrangle, 1:24,000-scale Scientific Investigations Map (SIM). This field guide is not meant as a comprehensive review of all of the geologic research done in this area of Connecticut; rather, it looks at previous bedrock geologic mapping from the perspective of new and recent mapping in the four-quadrangle area and discusses the structural, stratigraphic, and nomenclatural revisions necessary for the next revision of the state geologic map.

Connecticut↗

USGS2021

Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

GSA Memoirs↗

USGS2021

Late Cenozoic paleogeographic reconstruction of the San Francisco Bay Area from analysis of stratigraphy, tectonics, and tephrochronology

The Neogene stratigraphic and tectonic history of the Mount Diablo area is a consequence of the passage of the Mendocino Triple Junction (MTJ) by the San Francisco Bay area between 12 and 6 Ma, volcanism above a slab-window trailing the MTJ, and crustal transpression beginning ~8-6 Ma, when the Pacific Plate and Sierra Nevada microplate began to converge obliquely. Between ~12-6 Ma, parts of the Sierra Nevada microplate were displaced by faults splaying from the main trace of the San Andreas Fault and incorporated into the Pacific Plate. The Mount Diablo anticlinorium was formed by crustal compression within a left-stepping, restraining bend of the eastern San Andreas Fault system (SAF), with southwest-verging thrusting beneath, and with possible clockwise rotation between faults on its southeast and northwest. At ~10,5 Ma, a drainage divide formed between the northern Great Central Valley (GCV) and the ocean. Regional uplift accelerated at ~6 Ma with onset of transpression between the Pacific and North American plates. Marine deposition ceased in the eastern Coast Range basins as a consequence of the regional uplift accompanying passage of the MTJ, and trailing slab-window volcanism. From ~11 to ~5 Ma, andesitic volcanic intrusive rocks and lavas were erupted along the northwest crest of the central to northern Sierra Nevada and were deposited on its western slope, providing abundant sediment to northern Great Central Valley (GCV) and the northeastern Coast Ranges. Sediment filled the GCV, overtopped the Stockton fault and arch forming one large, south-draining system that flowed into a marine embayment at its southwestern end, the ancestral San Joaquin Sea. This marine embayment shrunk with time and by ~2.3 Ma was eventually cut off from the ocean. Fluvial drainage continued southwest in GCV until it was cut off in turn, probably by some combination of sea level fluctuations and transpression along the SAF that uplifted, lengthened and narrowed the outlet channel. As a consequence, a great lake, Lake Clyde, formed in the GCV at ~1.4 Ma, occupying all of the ancestral San Joaquin Valley and part of ancestral Sacramento Valley. The lake rose and fell with global glacial and interglacial cycles. After a long, extreme glacial period, Marine Oxygen Isotope Stage (MOIS) 16, it overtopped Carquinez sill at 0.63 Ma and drained via San Francisco valley (now Bay) and the Colma gap, into the Merced marine embayment of the Pacific Ocean. Later, a new outlet for GCV drainage formed between ~75 and ~130 ka ago., when the Colma gap closed due to transpression and right-slip on the SAF, and Duxbury Point at the south end of Pt. Reyes Peninsula moved sufficiently northwest along the SAF to unblock a bedrock notch, the feature we now call the Golden Gate.

California↗

USGS2022

Tectonic influence on axial-transverse sediment routing in the Denver Basin

Detrital zircon U-Pb and (U-Th)/He ages from latest Cretaceous–Eocene strata of the Denver Basin provide novel insights into evolving sediment sourcing, recycling, and dispersal patterns during deposition in an intracontinental foreland basin. In total, 2464 U-Pb and 78 (U-Th)/He analyses of detrital zircons from 21 sandstone samples are presented from outcrop and drill core in the proximal and distal portions of the Denver Basin. Upper Cretaceous samples that predate uplift of the southern Front Range during the Laramide orogeny (Pierre Shale, Fox Hills Sandstone, and Laramie Formation) contain prominent Late Cretaceous (84–77 Ma), Jurassic (169–163 Ma), and Proterozoic (1.69–1.68 Ga) U-Pb ages, along with less abundant Paleozoic through Archean zircon grain ages. These grain ages are consistent with sources in the western U.S. Cordillera, including the Mesozoic Cordilleran magmatic arc and Yavapai-Mazatzal basement, with lesser contributions of Grenville and Appalachian zircon recycled from older sedimentary sequences. Mesozoic zircon (U-Th)/He ages confirm Cordilleran sources and/or recycling from the Sevier orogenic hinterland. Five of the 11 samples from syn-Laramide basin fill (latest Cretaceous–Paleocene D1 Sequence) and all five samples from the overlying Eocene D2 Sequence are dominated by 1.1–1.05 Ga zircon ages that are interpreted to reflect local derivation from the ca. 1.1 Ga Pikes Peak batholith. Corresponding late Mesoproterozoic to early Neoproterozoic zircon (U-Th)/He ages are consistent with local sourcing from the southern Front Range that underwent limited Mesozoic–Cenozoic unroofing. The other six samples from the D1 Sequence yielded detrital zircon U-Pb ages similar to pre-Laramide units, with major U-Pb age peaks at ca. 1.7 and 1.4 Ga but lacking the 1.1 Ga age peak found in the other syn-Laramide samples. One of these samples yielded abundant Mesozoic and Paleozoic (U-Th)/He ages, including prominent Early and Late Cretaceous peaks. We propose that fill of the Denver Basin represents the interplay between locally derived sediment delivered by transverse drainages that emanated from the southern Front Range and a previously unrecognized, possibly extraregional, axial-fluvial system. Transverse alluvial-fluvial fans, preserved in proximal basin fill, record progressive unroofing of southern Front Range basement during D1 and D2 Sequence deposition. Deposits of the upper and lower D1 Sequence across the basin were derived from these fans that emanated from the southern Front Range. However, the finer-grained, middle portion of the D1 Sequence that spans the Cretaceous-Paleogene boundary was deposited by both transverse (proximal basin fill) and axial (distal basin fill) fluvial systems that exhibit contrasting provenance signatures. Although both tectonic and climatic controls likely influenced the stratigraphic development of the Denver Basin, the migration of locally derived fans toward and then away from the thrust front suggests that uplift of the southern Front Range may have peaked at approximately the Cretaceous-Paleogene boundary.

Colorado↗