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USGS2009

Recent developments in understanding the tectonic evolution of the Southern California offshore area: Implications for earthquake-hazard analysis

During late Mesozoic and Cenozoic time, three main tectonic episodes affected the Southern California offshore area. Each episode imposed its unique structural imprint such that early-formed structures controlled or at least influenced the location and development of later ones. This cascaded structural inheritance greatly complicates analysis of the extent, orientation, and activity of modern faults. These fault attributes play key roles in estimates of earthquake magnitude and recurrence interval. Hence, understanding the earthquake hazard posed by offshore and coastal faults requires an understanding of the history of structural inheritance and modifi-cation. In this report we review recent (mainly since 1987) findings about the tectonic development of the Southern California offshore area and use analog models of fault deformation as guides to comprehend the bewildering variety of offshore structures that developed over time. This report also provides a background in regional tectonics for other chapters in this section that deal with the threat from offshore geologic hazards in Southern California. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2024

Volcanism and tectonics of young basaltic fields in the eastern California shear zone, California, USA

Circa 12 Ma, there was a fundamental reorganization of magmatism and tectonics in the Mojave Desert, California, USA, from basaltic to rhyolitic fields associated with extensional tectonics to dispersed basaltic monogenetic fields associated with the northwest- or east-striking strike-slip faults. The broad zone of strike-slip faults associated with the San Andreas transform margin stretched east to Arizona, but the high slip-rate central part that is known as the Quaternary eastern California shear zone is the locus of the post–12 Ma volcanism. We compiled a literature review of 29 basaltic fields, conducted detailed and reconnaissance study of several fields, and conducted geochemistry on many. Most of the volcanic fields are near or cut by the long fault systems, but more importantly, in nearly two-thirds of the fields that have scoria cones, the cones are <1 km from one of these faults. Eighteen volcanic fields have geochemistry data, and of the 441 analyses of major elements, 286 are new U.S. Geological Survey data that include comprehensive trace elements. The data are used to compare fields and determine the compositional variations during the formation of each field. Comparisons between fields show that several nearby volcanic fields have similar geochemical compositions and trends compared to distant fields; we suggest that six magmatic clusters formed, some with 200–300 k.y. duration. Spatial patterns change over the 12 m.y. span, with early fields forming in the north and south and the youngest in a central area from Pisgah to Amboy fields. The volume of magma also changed, most notably with a sixfold increase ca. 4 Ma followed by a 2–3 m.y. quiescence. In addition to the fracture control provided by strike-slip faults as magma conduits, we advance arguments for shear melting in the mantle along the most active faults, with secondary controls of local tectonic interactions, to explain spatial and temporal patterns in the fields.

California↗

USGS2009

Sediment yield from the tectonically active semiarid Western Transverse Ranges of California

Sediment yields from the world's rivers are generally highest from steep drainage basins with weak lithology, active tectonics, or severe land-use impacts. Here, we evaluate sediment yields from the Western Transverse Ranges of California in an attempt to explain why they are two- to tenfold greater than the surrounding areas of California. We found that suspended-sediment yields across the gauged basins of the Western Transverse Range during 1969-1999 varied by approximately an order of magnitude (740-5300 t/km 2 /yr). Similarly, fine-sediment concentrations for normalized discharge rates varied by almost two orders of magnitude (e.g., 1.3-110 g/L for the mean annual flood) for 11 previously unmonitored drainages of the Santa Ynez Mountains. Areas with high sediment yields consistently have weakly consolidated bedrock (Quaternary-Pliocene marine formations) and are associated with the highest rates of tectonic uplift of the region (>5 mm/yr). These regions are important to the sediment discharge budgets, because ???50% of the total suspended-sediment discharge from the Western Transverse Range is estimated to be generated within these regions, even though they represent only ???10% of the total watershed area. Previous estimates of suspended-sediment discharge from the Ventura River have likely been underestimated by ???50% because the gauging station is located immediately upstream of a high sediment yield region. We also found a significant and positive correlation between sediment yield and the percentage of a watershed with grassland and agricultural land use. These results suggest that there is adequate variation within the lithology, tectonics, and land use of the broader Western Transverse Range geologic province to induce large variations in sediment yield at the local scale. ?? 2009 Geological Society of America.

Geological Society of America Bulletin↗

USGS2014

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: characteristics, extent, and tectonic significance

Fault-dismembered segments of a distinctive, extensive, highly allochthonous, and tectonically significant Ordovician (ca. 480–460 Ma) basin, which contains suites of bimodal metavolcanic rocks, associated base metal deposits, and thick immature deep-water (turbiditic) metasediments, occur in parts of the southern Appalachian Talladega belt, eastern Blue Ridge, and Inner Piedmont of Alabama, Georgia, and North and South Carolina. The basin's predominantly metasedimentary strata display geochemical and isotopic evidence of a mixed provenance, including an adjacent active volcanic arc and a provenance of mica (clay)-rich sedimentary and felsic plutonic rocks consistent with Laurentian (Grenvillian) upper-crustal continental rocks and their passive-margin cover sequences. Geochemical characteristics of the subordinate intercalated bimodal metavolcanic rocks indicate formation in a suprasubduction environment, most likely a back-arc basin, whereas characteristics of metasedimentary units suggest deposition above Neoproterozoic rift and outer-margin lower Paleozoic slope and rise sediments within a marginal basin along Ordovician Laurentia's Iapetus margin. This tectonic setting indicates that southernmost Appalachian Ordovician orogenesis (Taconic orogeny) began as an extensional accretionary orogen along the outer margin of Laurentia, rather than in an exotic (non-Laurentian) arc collisional setting. B-type subduction polarity requires that the associated arc-trench system formed southeast of the palinspastic position of the back-arc basin. This scenario can explain several unique features of the southern Appalachian Taconic orogen, including: the palinspastic geographic ordering of key tectonic elements (i.e., back-arc, arc, etc.), and a lack of (1) an obducted arc sensu stricto on the Laurentian margin, (2) widespread Ordovician regional metamorphism, and (3) Taconic klippen to supply detritus to the Taconic foreland basin.

Alabama, Georgia, North Carolina, South Carolina↗

USGS2019

Late Quaternary sea-level history of Saipan, Commonwealth of the Northern Mariana Islands, USA: A test of tectonic uplift and glacial isostatic adjustment models

In 1979, S. Uyeda and H. Kanamori proposed a tectonic model with two end members of a subduction-boundary continuum: the “Chilean” type (shallow dip of the subducting plate, great thrust events, compression, and uplift of the overriding plate) and a “Mariana” type (steep dip of the subducting plate, no great thrust events, tension, and no uplift). This concept has been used to explain variable rates of Quaternary uplift around the Pacific Rim, yet no uplift rates have been determined for the Mariana Islands themselves, one of the end members in this model. We studied the late Quaternary Tanapag Limestone, which rims much of the eastern and southern coasts of Saipan, Northern Mariana Islands, with elevations of ∼13 m to ∼30 m. Samples from 12 well-preserved corals ( Acropora , Porites , and Goniastrea ) yielded U-series ages ranging from ca. 134 ka to ca. 126 ka. These ages correlate the emergent reef of the Tanapag Limestone with the last interglacial period, when sea level was several meters above present. Ages and measured reef elevations from the Tanapag Limestone, along with paleo–sea-level data, yield relatively low late Quaternary uplift rates of 0.002–0.19 m/k.y., consistent with the Uyeda-Kanamori model. A review of data from other localities near subduction zones around the Pacific Basin, however, indicates that many coastlines do not fit the model. Uplift rates along the Chilean coast are predicted to be relatively high, but field studies indicate they are low. On some coastlines, relatively high uplift rates are better explained by subduction of seamounts or submarine ridges rather than subduction zone geometry. Despite the low long-term uplift rate on Saipan, the island also hosts an emergent, low-elevation (+3.9–4.0 m) reef with corals in growth position below a notch (+4.2 m). The corals are dated to 3.9–3.1 ka. The occurrence of this young, emergent reef is likely not due to tectonic uplift; instead, it is interpreted to be the result of glacial isostatic adjustment processes after the end of the last glacial period. Our findings are consistent with similar observations on tectonically stable or slowly uplifting islands elsewhere in the equatorial Pacific Ocean and agree with numerical models of a higher-than-present Holocene sea level in this region due to glacial isostatic adjustment processes.

Northern Mariana Islands, Saipan↗

USGS2025

Re-evaluating the tectonic affinity of Proterozoic crustal provinces in the Southwest USA: Detrital zircon evidence for a Laurentian source for the Yavapai and Mojave Provinces

Models for crustal growth commonly involve the accretion of dominantly juvenile crust to continental margins. However, tracking the provenance and tectonic affinity of dominantly juvenile crustal provinces is challenging. This difficulty is highlighted by uncertainty over whether the Yavapai and Mojave Provinces, part of the >1300-km-wide system of Proterozoic orogens in southwestern Laurentia, (1) have similar crustal and tectonic histories and (2) if they formed on or near Laurentian, Australian, or Antarctic cratons. Here, we contribute new large-n detrital zircon U-Pb geochronology and Sm-Nd whole-rock isotope geochemistry to help constrain the provenance of the Yavapai Province and address these broader questions. Yavapai Province metasedimentary rocks from central Colorado in the southwestern USA have abundant pre-1.80 Ga detrital zircon grains, with ca. 1.85 Ga, 2.30 Ga, and 2.50−2.70 Ga peaks, and variable amounts of 1.79−1.78 Ga grains. Evolved whole-rock Sm-Nd isotopic compositions from these rocks, including 2.36−2.08 Ga model ages, also suggest mixing between 1.79 Ga and 1.78 Ga Yavapai Province arcs and early Proterozoic to Archean sources. Nearly identical pre-1.8 Ga detrital and inherited zircon age distributions suggest that the Yavapai and Mojave Provinces formed on and/or incorporated similar material. The Trans-Hudson orogen, and to a slightly lesser extent the Penokean orogen, provide the closest matches to the pre-1.80 Ga material in the Yavapai and Mojave Provinces. This similarity, coupled with a weaker resemblance to Australian and Antarctic sources, support a Laurentian affinity for the Yavapai and Mojave Provinces. We envision Paleoproterozoic arc formation on both oceanic crust and material of Laurentian affinity and multiple phases of arc-back-arc genesis, closure, and accretionary tectonism along the long-lived margin of the supercontinent Columbia (Nuna).

Arizona, Colorado, Idaho, New Mexico, Utah, Wyomin↗

USGS2007

Escape tectonics and the extrusion of Alaska: Past, present, and future

The North Pacific Rim is a tectonically active plate boundary zone parts of which may be characterized as a laterally moving orogenic stream. Crustal blocks are transported along large-magnitude strike-slip faults in western Canada and central Alaska toward the Aleutian-Bering Sea subduction zones. Throughout much of the Cenozoic, at and west of its Alaskan nexus, the North Pacific Rim orogenic Stream (NPRS) has undergone tectonic escape. During transport, relatively rigid blocks acquired paleomagnetic rotations and fault-juxtaposed boundaries while flowing differentially through the system, from their original point of accretion and entrainment toward the free face defined by the Aleutian-Bering Sea subduction zones. Built upon classical terrane tectonics, the NPRS model provides a new framework with which to view the mobilistic nature of the western North American plate boundary zone. ?? 2007 The Geological Society of America.

Alaska↗

USGS2016

Pre-Mississippian tectonic affinity across the Canada Basin–Arctic margins of Alaska and Canada

New and reprocessed seismic reflection data on the Alaskan and Canadian Arctic margins of the Canada Basin, together with geologic constraints from exploration wells and outcrops, reveal structural and stratigraphic relationships in pre-Mississippian rocks that constrain models of Canada Basin opening. Lithostratigraphic age and acoustic character indicate that the Devonian and older passive-margin to foreland-basin succession in the Canadian M’Clure Strait is also found on the central Alaska margin. This succession also displays similar structural geometry and relief as well as deformational age on both margins. Moreover, Middle Devonian to Early Mississippian tectonic vergence—north directed on the central Alaska margin and east directed in the Canadian M’Clure Strait—indicates a common direction of tectonic transport if the two margins were conjugate. All of these observations demonstrate that pre-Mississippian rocks of the Alaskan and Canadian Arctic margins share a common tectonic history of uplift and exhumation and that the two margins were conjugates prior to opening of the Canada Basin.

Alaska↗

USGS2023

Breaking plates: Creation of the East Anatolian fault, the Anatolian plate, and a tectonic escape system

Lateral movement of lithospheric fragments along strike-slip faults in response to collision (escape tectonics) has characterized convergent settings since the onset of plate tectonics and is a mechanism for the formation of new plates. The Anatolian plate was created by the sequential connection of strike-slip faults following ≥10 m.y. of distributed deformation that ultimately localized into plate-bounding faults. Thermochronology data and seismic images of lithosphere structure near the East Anatolian fault zone (EAFZ) provide insights into the development of the new plate and escape system. Low-temperature thermochronology ages of rocks in and near the EAFZ are significantly younger than in other fault zones in the region, e.g., apatite (U-Th)/He: 11–1 Ma versus 27–13 Ma. Young apatite (U-Th)/He ages and thermal history modeling record thermal resetting along the EAFZ over the past ~5 m.y. and are interpreted to indicate thermal activity triggered by strike-slip faulting in the EAFZ as it formed as a through-going, lithosphere-scale structure. The mechanism for EAFZ formation may be discerned from S-wave velocity images from the Continental Dynamics–Central Anatolian Tectonics (CD-CAT) seismic experiment. These images indicate that thin but strong Arabian lithospheric mantle extends ~50–150 km north beneath Anatolian crust and would have been located near the present surficial location of the Bitlis-Zagros suture zone (co-located with the EAFZ in our study area) at ca. 5 Ma. Underthrusting of strong Arabian lithosphere facilitated localization of the EAFZ and thus was a fundamental control on the formation of the Anatolian plate and escape system.

East Anatolian Fault↗

USGS2023

Tectonics, geochronology, and petrology of the Walker Top Granite, Appalachian Inner Piedmont, North Carolina (USA): Implications for Acadian and Neoacadian orogenesis

The Walker Top Granite (here formally named) is a peraluminous megacrystic granite that occurs in the Cat Square terrane, Inner Piedmont, part of the southern Appalachian Acadian-Neoacadian deformational and metamorphic core. The granite occurs as disconnected concordant to semi-concordant plutons in migmatitic, sillimanite zone rocks of the Brindle Creek thrust sheet. Locally garnet-bearing, the Walker Top Granite contains blocky alkali feldspar megacrysts 1–10 cm long in a groundmass of muscovite-biotite-quartz-plagioclase-alkali feldspar and accessory to trace zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. It varies from granite to granodiorite and contains several xenoliths of biotite gneiss, amphibolite, quartzite, and in one location encloses charnockite (here formally named Vale Charnockite). New sensitive high-resolution ion microprobe U-Pb zircon magmatic crystallization ages obtained from the plutons of the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains; 366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its type locality. Major-, trace-element, and isotopic chemistry indicates that Walker Top is a high-K, peraluminous granite, plotting as volcanic arc or syn-collisional on tectonic discrimination diagrams and suggests that it represents deep-seated anatectic magma with S- to I-type affinity. The alkali calcic, ferroan Vale Charnockite likely formed by deep crustal melting, and similar geochemical and trace-element compositions suggest a similar tectonic origin as Walker Top Granite. The discontinuous nature of the Walker Top Granite plutons precludes it intruded as a volcanic arc. Instead, the peraluminous nature, common xenoliths of surrounding country rock, and geochemical and isotopic signatures suggest it formed by partial melting of Cat Square and Tugaloo terrane rocks. Following emplacement and crystallization, Walker Top plutons were deformed into elliptical to linear shapes—SW-directed sheath folds—enveloped by partially melted, pelitic and quart-zofeldspathic rocks. Collectively, Walker Top and other plutons helped weaken the crust and facilitate lateral crustal flow in a SW-directed, tectonically driven orogenic channel during the Acadian-Neoacadian event. A comparison with the northern Appalachians recognizes a similar temporal magmatic and deformational history during the Acadian and Neoacadian orogenies, although while the Walker Top Granite intruded the lower plate during eastward subduction beneath the peri-Gondwanan Carolina superterrane, the northern Appalachian plutons intruded the upper plate during subduction of the Avalon superterrane westward beneath Laurentia. We hypothesize that a transform fault, located near the southern end of the New York promontory, accommodated oppositely directed lateral plate motion and different subduction polarity between the Carolina and Avalon superterranes during the Acadian and Neoacadian orogenies.

North Carolina↗

USGS2016

A synthesis of Jurassic and Early Cretaceous crustal evolution along the southern margin of the Arctic Alaska–Chukotka microplate and implications for defining tectonic boundaries active during opening of Arctic Ocean basins

A synthesis of Late Jurassic and Early Cretaceous collision-related metamorphic events in the Arctic Alaska–Chukotka microplate clarifies its likely movement history during opening of the Amerasian and Canada basins. Comprehensive tectonic reconstructions of basin opening have been problematic, in part, because of the large size of the microplate, uncertainties in the location and kinematics of structures bounding the microplate, and lack of information on its internal deformation history. Many reconstructions have treated Arctic Alaska and Chukotka as a single crustal entity largely on the basis of similarities in their Mesozoic structural trends and similar late Proterozoic and early Paleozoic histories. Others have located Chukotka near Siberia during the Triassic and Jurassic, on the basis of detrital zircon age populations, and suggested that it was Arctic Alaska alone that rotated. The Mesozoic metamorphic histories of Arctic Alaska and Chukotka can be used to test the validity of these two approaches. A synthesis of the distribution, character, and timing of metamorphic events reveals substantial differences in the histories of the southern margin of the microplate in Chukotka in comparison to Arctic Alaska and places specific limitations on tectonic reconstructions. During the Late Jurassic and earliest Cretaceous, the Arctic Alaska margin was subducted to the south, while the Chukotka margin was the upper plate of a north-dipping subduction zone or a zone of transpression. An early Aptian blueschist- and greenschist-facies belt records the most profound crustal thickening event in the evolution of the orogen. It may have resulted in thicknesses of 50–60 km and was likely the cause of flexural subsidence in the foredeep of the Brooks Range. This event involved northern Alaska and northeasternmost Chukotka; it did not involve central and western Chukotka. Arctic Alaska and Chukotka evolved separately until the Aptian thickening event, which was likely a result of the rotation of Arctic Alaska into central and western Chukotka. In northeastern Chukotka, the thickened rocks are separated from the relatively little thickened continental crust of the remainder of Chukotka by the oceanic rocks of the Kolyuchin-Mechigmen zone. The zone is a candidate for an Early Cretaceous suture that separated most of Chukotka from northeast Chukotka and Alaska. Albian patterns of magmatism, metamorphism, and deformation in Chukotka and the Seward Peninsula may represent an example of escape tectonics that developed in response to final amalgamation of Chukotka with Eurasia.

Arctic Alaska-Chukotka microplate↗

USGS1995

Jurassic tectonics of northeastern Nevada and northwestern Utah from the perspective of barometric studies

Jurassic tectonism in the northeastern Great Basin produced varied structures, many closely associated with widespread magmatism at ca. 155–165 Ma and with local metamorphism. Many of the plutons are of suitable mineralogy for Al-in-hornblende barometry, providing the potential for depth data. We have studied conditions of metamorphism in the Pilot Range and barometry for six Jurassic plutons across the northeastern Great Basin. All barometry results are in harmony with pressures estimated from stratigraphic data, requiring little or no tectonic thickening. On the basis of structural styles and barometric data, we divide the northeastern Great Basin into three Jurassic tectonic provinces. An eastern extensional province, largely in western Utah, is characterized by Paleozoic strata that were thrust faulted and then intruded by shallow plutons shortly after or during normal and strike-slip faulting. Extension was probably a short-lived event associated with magmatism, but its west trend indicates a total reorientation of stress at this time, perhaps within transtensional strike-slip zones. A central province of modest, and possibly locally extreme, Jurassic shortening in eastern Nevada is characterized by metamorphosed Paleozoic rocks and by thrusts and kilometer-scale southeast-vergent folds. Upper amphibolite facies, but low pressure (3–4 kbar) metamorphism is present near Jurassic plutons in the Pilot Range and Ruby Mountains, probably indicating metamorphism induced by heat from magmas. In contrast, metamorphism in other ranges, which is known only to be pre–Late Cretaceous, indicates thickening of 10–20 km. This thickening may have entirely postdated the Jurassic. A western province in north-central Nevada is characterized by preserved Jurassic volcanic rocks and shallow plutons, indicating that little erosion, and probably surface uplift, occurred during the late Mesozoic. Folds and thrust faults indicate minor Jurassic shortening but many structures are undated. The low-pressure upper-crustal conditions for demonstrably Jurassic events suggest that higher-pressure metamorphism recorded in the central province is younger (Cretaceous) in age. We suggest that Jurassic structures were caused by distributed minor crustal shortening, manifested mainly as small-scale thrust faults. Local thermal highs created by plutonism produced metamorphic zones in relatively shallow crust. Shortening in the east was manifested by zones of strike-slip, within which plutons were emplaced in tensile niches. Lack of a deep foreland basin and lack of evidence for massive erosion argue against high-relief mountain belts caused by significant crustal shortening. Paleozoic rocks metamorphosed at pressures far in excess of stratigraphic burial are restricted to narrow lenses exhumed during Late Cretaceous and Tertiary extension and are bordered by rocks that always have been part of the shallow crust. The abundant shallow-crustal rocks preserved across the region indicate that a conventional hypothesis of large-scale, regional crustal thickening causing many kilometers of surface uplift and consequent erosion is unlikely to have taken place in the Mesozoic.

Nevada, Utah↗

USGS1996

Block and shear-zone architecture of the Minnesota River Valley subprovince: Implications for late Archean accretionary tectonics

The Minnesota River Valley subprovince of the Superior Province is an Archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east-northeast-trending linear geophysical anomalies. Two of the block-bounding zones are verified regional-scale shears. The geological nature of the third boundary has not been established. Potential-field geophysical models portray the boundary zones as moderately north-dipping surfaces or thin slabs similar in strike and dip to the Morris fault segment of the Great Lakes tectonic zone at the north margin of the subprovince. The central two blocks of the subprovince (Morton and Montevideo) are predominantly high-grade quartzofeldspathic gneiss, some as old as 3.6 Ga, and late-tectonic granite. The northern and southern blocks (Benson and Jeffers, respectively) are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late-tectonic plutons. A belt of moderately metamorphosed mafic and ultramafic rocks having some attributes of a dismembered ophiolite is partly within the boundary zone between the Morton and Montevideo blocks. This and the other block boundaries are interpreted as late Archean structures that were reactivated in the Early Proterozoic. The Minnesota River Valley subprovince is interpreted as a late accretionary addition to the Superior Province. Because it was continental crust, it was not subductible when it impinged on the convergent southern margin of the Superior Craton in late Archean time, and it may have accommodated to convergent-margin stresses by dividing into blocks and shear zones capable of independent movement.

Minnesota↗

USGS2014

Shale hydrocarbon reservoirs: Some influences of tectonics and paleogeography during deposition

Continuous hydrocarbon accumulations in shale reservoirs appear to be characterized by common paleotectonic and paleogeographic histories and are limited to specific intervals of geologic time. In addition, most North American self-sourced shale correlates with geologic time periods of calcitic seas and greenhouse conditions and with evolutionary turnover of marine metazoans. More knowledge about the relations among these controls on deposition is needed, but conceptual modeling suggests that integrating tectonic histories, paleogeographic reconstructions, and eustatic curves may be a useful means by which to better understand shale plays already in development stages and potentially identify new organic-carbon-rich shale targets suitable for continuous resource development. Upwelling and anoxic waters are commonly cited to explain the accumulation and preservation, respectively, of marine organic carbon. In addition, and perhaps alternatively, the broad correlation of self-sourced shale with macroevolutionary trends in land plants and marine metazoans suggests that reduced consumption of organic matter by benthos during periods of high terrestrial and marine organic productivity was responsible. Fundamental to any of the processes that acted during deposition, however, was active tectonism. Basin type can often distinguish self-sourced shale plays from other types of hydrocarbon source rocks. The deposition of North American self-sourced shale was associated with the assembly and subsequent fragmentation of Pangea. Flooded foreland basins along collisional margins were the predominant depositional settings during the Paleozoic, whereas deposition in semirestricted basins was responsible along the rifted passive margin of the U.S. Gulf Coast during the Mesozoic. Tectonism during deposition of self-sourced shale, such as the Upper Jurassic Haynesville Formation, confined (re)cycling of organic materials to relatively closed systems, which promoted uncommonly thick accumulations of organic matter.

Book chapter↗

USGS1989

Eustatic and tectonic controls on deposition of hybrid siliciclastic/carbonate basinal cycles: Discussion with examples

Changes in sea level represent one of the dominant controls on basinal sedimentation adjacent to mixed carbonate siliciclastic sediment-source areas. Sedimentary responses to sea level change of the siliciclastic and carbonate components of these hybrid systems commonly result in deposition of alternating siliciclastic and shelf-derived carbonate basinal deposits. Such deposition is particularly pronounced adjacent to carbonate platforms, where alternate immersion/exposure of the flat platform top results in carbonate highstand deposition and siliciclastic lowstand deposition. In contrast, small to moderate-amplitude sea level drops in distally steepened ramp settings may expose only the inner part of the gently sloping shelf, resulting in lowstand deposition of mixed sil ciclastic/allodapic (FOOTNOTE 3) carbonate sediments. Along active margins, tectonic uplift and subsidence of sediment-source areas act as additional major controls on deposition of siliciclastic/carbonate basinal cycles. Two of the most important tectonic effects are (1) uplift of siliciclastic sediment-source areas, resulting in increased erosion and swamping of carbonate shelf sources and (2) exposure or immersion of shelf-carbonate sediment-source areas due to vertical tectonic movements. Most published seismic-stratigraphic studies of sea level control on deep-sea sedimentation focus on lowstand deposition of siliciclastic sediment and commonly neglect the contribution of allodapic carbonate sediment deposited during sea level highstands. However, the examples discussed in this paper indicate that highstand carbonate deposits may make up a significant proportion of many hybrid carbonate/siliciclastic basinal sequences.

American Association of Petroleum Geologists Bulle↗

USGS1985

Character and regional significance of Great Falls tectonic zone, east-central Idaho and west-central Montana

The Great Falls tectonic zone, here named, is a belt of diverse northeast-trending geologic features that can be traced from the Idaho batholith in the Cordilleran miogeocline, across thrust-belt structures and basement rocks of west-central and southwestern Montana, through cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. Geologic mapping in east-central Idaho and west-central Montana has outlined a continuous zone of high-angle faults and shear zones. These structures (1) extend more than 150 km (93 mi) northeastward from near Salmon, Idaho, toward Anaconda, Montana, (2) had recurrent movement from middle Proterozoic to Holocene time, (3) controlled the intrusion and orientation of Late Cretaceous to early Tertiary dike swarms, and (4) ontrolled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent fault movement in this zone and strong structural control over igneous intrusion suggest a fundamental tectonic feature that has influenced the tectonic development of the Idaho-Montana area from at least middle Proterozoic time to the present.

Idaho, Montana↗

USGS2014

Seismic‐wave attenuation determined from tectonic tremor in multiple subduction zones

Tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground‐motion prediction and hazard mapping. Traditionally, recorded earthquakes of magnitude ∼3–8 are used to develop ground‐motion prediction equations; however, typical earthquake records may be sparse in areas of high hazard. In this study, we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor, which is plentiful in some regions. Tectonic tremor occurs in the frequency band of interest for ground‐motion prediction (i.e., ∼2–8 Hz) and is located on the subducting plate interface, at the lower boundary of where future large earthquakes are expected. We empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones: Nankai, Japan; Cascadia, United States–Canada; Jalisco, Mexico; and southern Chile. With the large amount of data available from tremor, we show that in the upper plate, the lower crust is less attenuating than the upper crust. We apply the same analysis to intraslab events in Nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic crust. This suggests that high pore‐fluid pressure is present in the tremor source region. These differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface.

Cascadia↗

USGS2018

Evaluation of ground motion models for USGS seismic hazard forecasts: Induced and tectonic earthquakes in the Central and Eastern U.S.

Ground motion model (GMM) selection and weighting introduces a significant source of uncertainty in United States Geological Survey (USGS) seismic hazard models. The increase in moderate moment magnitude induced earthquakes (Mw 4 to 5.8) in Oklahoma and Kansas since 2009, due to increased wastewater injection related to oil and gas production (Keranen et al., 2013; 2014; Weingarten et al., 2015; McNamara et al., 2015a), provides useful near-source (< 40 km) instrumental ground-motion observations for comparisons between central and eastern US (CEUS) induced (Rennolet et al., 2017) and tectonic (Goulet et al., 2014) earthquakes. In this study, we evaluate over 50 GMMs using two well-established probabilistic scoring methods: log likelihood (LLH) (Scherbaum et al., 2004; 2009) and multivariate LLH (MLLH) (Mak et al., 2017). The LLH approach compares the mean and standard deviation (σ) of the observed and modeled ground motions. The MLLH approach advances the LLH method by considering the variability (φ,τ) of multiple correlated variables namely intra- (within) and inter- (between) event residuals. For the probabilistic scoring GMM evaluation methods (LLH, MLLH), we compute horizontal component peak ground acceleration (PGA) and 1s period pseudo spectral acceleration (PSA1.0) total residuals using GMM software (nshmp-haz) recently implemented by the USGS National Seismic Hazard Model Project (NSHMP). We observe from LLH and MLLH scores that: 1) newer GMMs with lower standard deviations (σ,φ,τ) score better than older GMMs with higher published uncertainty; 2) 2014 CEUS GMMs score better for CEUS tectonic earthquakes than induced earthquakes; 3) NGA-West2, G17 and A15 GMMs score well for CEUS induced earthquake ground motions; and 4) NGA-East GMMs score well for CEUS tectonic earthquake ground motions. We also use the LLH and MLLH scores to evaluate GMM weights applied in past USGS seismic hazard forecasts and to inform weighting of GMMs in future seismic hazard forecasts.

Bulletin of the Seismological Society of America↗