Range Faulting Research Articles

Fairweather transform boundary Oligocene to present orogenesis: Fairweather Range vertical extrusion and rotation of the Yakutat microplate at ca. 3 Ma

Oblique-slip along transform fault boundaries is often partitioned between a strike-slip system and thrust faults that accommodate contraction. However, topography along the Yakutat-North American transform (Fairweather fault), is asymmetric with low-terrain above active thrusts on the western, Yakutat side of the transform and high topography on the continental side with peaks >4500 m (Mount Fairweather: 4671 m) to the west of the Border Ranges fault, limited recorded earthquakes >M4, and no apparent reverse faults to generate the highest terrain. In this study we compile, for the first time, published U-Pb zircon, 40Ar/39Ar and K-Ar (hornblende, muscovite, and biotite) and U-Th/He and fission-track (zircon and apatite) bedrock ages (109) from 75 samples to investigate the exhumation history of the Fairweather Range region, complemented by a published detrital sample (ZFT and AFT) and 13 new 40Ar/39Ar (hornblende, biotite, and K-feldspar) ages on 9 bedrock samples from both sides of the Fairweather fault. Additionally, we examined published seismicity and geodetic data of the Fairweather region and assessed if plate paleo-vectors correlate with the cooling history of the Fairweather Range. Cooling age, seismic, and block-motion patterns indicate the Fairweather Range has been vertically extruded between the Fairweather and the Border Ranges faults as a coherent block since ca. 25 Ma. The pre-6 Ma Pacific plate motion (N30°W) aligns with the N33°W strike of the Fairweather Fault whereas a hypothetical pre-6 Ma Yakutat microplate paleo-vector of (N39°W) does not: indicating a post-6 Ma timing for Yakutat microplate counter-clockwise rotation (9°). We infer that rotation and impingement of the Yakutat microplate along the Fairweather fault at ca. 3 Ma led to the development of the Fairweather restraining bend and increased cooling rates. The resultant thickened Fairweather welt and the ∼30 km thick southeast end of the Yakutat microplate compounded double-indenter tectonics into Alaska's southeast convergent corner

Multi-stage construction of the Little Cotton wood stock, Utah, USA: Origin, intrusion, venting, mineralization, and mass movement

AbstractMany porphyry molybdenum deposits are hosted in multi-phase plutons, but it is unclear in some deposits how these magmas originated and whether the pluton intruded as it fractionated or was intruded by new batches of magma. New mapping has clarified field relationships between units in the White Pine porphyry Mo system hosted in the Little Cottonwood stock, Utah (western United States), including the White Pine intrusion, the Red Pine porphyry, rhyolite dikes, and phreatomagmatic pebble dikes. Geologic relations and geochemistry show the system formed in a continental arc setting during rollback of the subducting Farallon slab rather than during extension related to orogenic collapse. Whole-rock geochemistry shows distinct fractionation trends for each of the major intrusive units in the composite pluton, suggesting they formed separately, which is supported by new U-Pb zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) ages of ca. 30 Ma for the Little Cottonwood stock, 27 Ma for the White Pine intrusion, and 26 Ma for the previously undated Red Pine porphyry. Mineral textures, cross-cutting relationships, and alteration mineralogy indicate that intrusion of the youngest phase led to a fluid-saturated magmatic system and triggered venting, including emplacement of pebble dikes. In the adjacent east Traverse Mountains, pebble dikes contain clasts that have similar mineral assemblages, textures, and ages as the major igneous units in the White Pine deposit. This indicates that the pebble dikes in east Traverse Mountains and in the pluton are the upper and lower parts of the same magmatic-hydrothermal system, which was decapitated by a mega-landslide that was likely facilitated by alteration in the Oligocene hydrothermal system and by later Basin and Range faulting.

Tectonic Underplating and Dismemberment of the Maclaren‐Kluane Schist Records Late Cretaceous Terrane Accretion Polarity and ~480 km of Post‐52 Ma Dextral Displacement on the Denali Fault

AbstractTerrane accretion introduces irregular geometry and allochthonous material to obliquely convergent margins, which create opportunities to quantify strike‐slip displacement along otherwise margin‐parallel fault systems. We present new bedrock geologic mapping and U‐Pb and40Ar/39Ar geochronology from the Alaska Range suture zone in the eastern Alaska Range, which confirm a long‐hypothesized correlation between the Maclaren Glacier metamorphic belt (Alaska, USA) and the Kluane metamorphic assemblage (Yukon Territory, Canada) across the right‐lateral Denali fault. The new data inform a palinspastic reconstruction showing that the dissected metamorphic belts and associated plutons record ~480 km of dextral displacement along the Denali fault since ca. 52 Ma. Before strike‐slip separation, the Maclaren‐Kluane schist formed by west‐vergent forearc underplating in the waning stage of the ca. 100–90 Ma arc built upon the Yukon‐Tanana terrane. The prograde structural and metamorphic evolution of the Maclaren‐Kluane schist records the final collision of the Wrangellia composite terrane at ca. 75–65 Ma along a set of east‐dipping thrust shear zones, which we infer to record the polarity of the Late Cretaceous plate boundary between the composite terrane and North America. Paleogene extension partially exhumed the schists to the upper crust and may be a consequence of regionally distributed strike‐slip faulting at that time. Localization of the modern Denali fault after ca. 52 Ma dismembered the schists and four neighboring belts of plutonic, metasedimentary, and volcanic rocks. The transition to Yakutat oblique flat slab subduction at ca. 30–25 Ma marks the onset of transpressional deformation in the Denali fault system, which reactivated Late Cretaceous collisional structures bounding the Maclaren schist. Neogene reactivation of the Totschunda fault reduced strike‐slip motion on the Denali fault east of the Denali‐Totschunda intersection and continues to transfer residual plate boundary slip onto the Denali fault west of the intersection. Key outcomes of our synthesis include: (a) Much of the ~480 km of displacement on the Denali fault accumulated after strike‐slip on the neighboring Tintina and Border Ranges fault systems had largely shut down; (b) The modern Denali fault system should not be grouped with strike‐slip faults credited with large‐scale margin‐parallel transport of Cordilleran terranes in the Cretaceous. Instead, a poorly understood proto‐Denali fault system may be a candidate for large‐scale Cretaceous translation; and (c) the longevity (≥33 Myr) of the highly localized Denali fault master strand (≤1 km wide) implies that it occupies a major mechanical boundary that penetrates the lithosphere.

Strain Partitioning and Interseismic Fault Behavior Along the Caribbean‐South American Transform Plate Boundary

AbstractWe combine Global Positioning System and Interferometric Synthetic Aperture Radar (InSAR) data to characterize the interseismic behavior (i.e., locked or creeping), and strain partitioning for the faults along the Caribbean‐South American transform plate boundary. Interseismic strain is distributed mainly on three faults, the San Sebastian, El Pilar, and Central Range faults, but partitioning occurs across multiple faults in the west (San Sebastian and La Victoria faults) and east (Sub‐Tobago Terrane, Central Range, and South Coast faults). In northern Venezuela, slip is partitioned on the San Sebastian (16.4 ± 1.7 mm/yr) and La Victoria (4.3 ± 0.9 mm/yr) faults. In north‐eastern Venezuela, the El Pilar fault accommodates slip at a rate of 18.6 ± 1.8 mm/yr. In Trinidad and Tobago, slip is partitioned between the Sub‐Tobago Terrane (3.0 ± 0.1 mm/yr), Central Range (14.5 ± 2.0 mm/yr), and South Coast (3.0 ± 0.1 mm/yr) faults. The La Victoria, San Sebastian, the western El Pilar segment, and Sub‐Tobago Terrane faults are locked to depths of 16.2 ± 4.0 km, 7.7 ± 5.2 km, 6.7 ± 2.8 km, and 8.0 ± 0.2 km, respectively. The eastern segment of the El Pilar, the Central Range, and the South Coast faults all creep. Our new InSAR results indicate that the entire Central Range Fault is creeping. The locked western segment of this transform plate boundary is capable of producing a Mw 8 earthquake, which is a significant finding regarding seismic hazard and risk.

Voluminous Paleogene volcanism in the southern Mesa Central, Mexico: Unravelling the fissure-fed origin of rhyolitic ignimbrites of the Villa Garcia-Loreto Volcanic Complex

The southern portion of the Mesa Central (MC) physiographic province is covered by numerous Oligocene to Miocene volcanic successions that are part of the Sierra Madre Occidental silicic large igneous province. The MC merges with the Sierra Madre Occidental and the southern Basin and Range extensional province. The voluminous Villa Garcia-Loreto Volcanic Complex (VGLVC) is located in the southern portion of the MC and consists of predominantly rhyolitic lava flows, and voluminous products of explosive rhyolitic volcanism, mainly as lava-like to rheomorphic ignimbrite sheets. The volcanism in the area appears in two events, first, at 60.4 ± 0.3 Ma (K–Ar whole-rock age) with the fissure-fed eruption of an andesitic lava and second, with the fissure-fed eruption of two densely-welded, crystal-poor ignimbrites at 31.59 ± 0.14 Ma and 28.66 ± 0.07 Ma (40Ar–39Ar sanidine ages). Chondrite-normalized rare earth element patterns and primitive mantle normalized multi-element patterns show enrichment in light rare earth elements (LREE) and high field strength elements (HFSE). Geochemical analysis and mineral chemistry show that the andesitic magmas were generated by partial melting of the uppermost mantle in a subduction-related setting and ascended directly through the brittle crust and erupted along N-S trending faults. Rhyolitic magmas were generated by partial melting of the lower crust in an intra-plate and extensional setting, associated with an intense phase of the southern Basin and Range extension. During the ascent and storage in a shallow magma chamber fractional crystallization of mainly feldspars and pyroxene was the major process taking place in the evolution of the magma. Geophysical results suggest that the volcanic rocks of the VGLVC were originated by fissure-fed eruptions associated with the Loreto Graben fault. The major Basin and Range faults locally reach a depth of 7 km and trending in NNW-SSE direction. The combined results show that the Basin and Range extension favored the formation, ascent, and eruption of rhyolitic and andesitic magmas in the southern MC.

Quaternary activity and seismogenic potential of the Sierra Chica Fault System, Pampean Ranges of Argentina

The Sierra Chica Fault System (SCFS) is a west-vergent thrust bounding the easternmost basement uplift in the Pampean Ranges of Argentina. Neogene-Recent activity along this structure is responsible for ongoing mountain-building in the broken foreland of the Andes, some 800 km away from the Pacific-South American subduction zone. We characterize Quaternary activity along the SCFS and provide the first numerical age constraints at sites where crystalline rocks of the Sierra Chica range overthrust Quaternary sediments as recent as ~50 ka. Detailed trench logging of the deformation zone defines average slip rates along the SCFS to 0.01–0.36 mm/a over the past ~90 ka. Historical earthquakes of magnitudes ≤7.4 occur along Pampean Range faults without any primary surface ruptures. Thus, evidence for surface ruptures suggests that late Pleistocene slip along the SFCF was likely associated with prehistoric earthquakes with magnitudes >7, although the application of global scaling laws to this tectonic setting tends to underpredict seismogenic potential along the SCFS. Our study aims to highlight the challenges of evaluating the seismic hazard of a fault system that has been active during the late Quaternary, but lack diagnostic geomorphic expression of past surface ruptures. This observation underscores that caution is needed when applying global scaling laws to intracontinental deformation in the broken forelands of subduction zones.

Rock and age relationships within the Talkeetna forearc accretionary complex in the Nelchina area, southern Alaska

Subduction zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report the results of field mapping and petrographic, geochemical, and geochronological analyses of the McHugh Complex accretionary prism mélange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the mélange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this mélange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the mélange have both mid-ocean ridge basalt and intra-plate geochemical affinities, attesting to the complex relations of subduction-zone inputs in an alternating erosive–accretionary margin. A new zircon U–Pb age and geochemical analyses of a set of felsic dikes that cross-cut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin during the Paleocene–Eocene. The McHugh section and cross-cutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma.

Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

AbstractThe Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling, and inferred exhumation patterns as proxies for the region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 40Ar/39Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 40Ar/39Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ∼10 km of the Castle Mountain fault suggest ∼2–3 km of vertical displacement and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-N or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east–sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics.

Quantifying the Epistemic Uncertainty in the Probabilistic Seismic Hazard from Two Major Faults in Western Nevada

The probabilistic seismic hazards in the larger cities of western Nevada are dominated by the Mount Rose and Carson Range faults. These are normal faults; Reno and Carson City are on the hanging wall. This paper finds the sensitivity of the hazard posed by these faults to epistemic uncertainty of dip, slip rate, magnitude, and choice of ground motion prediction equation (GMPE) for an earthquake that ruptures the length of each fault. SA(0.01 s) with exceedance probability of 2% in 50 years was determined for each branch of a logic tree for those properties. This paper uses tools of probability theory to characterize this logic tree distribution with a lognormal distribution function, and to suggest an estimate of uncertainty in the mean hazard. In this case, which should not be considered general, on the hanging wall, the uncertainties in fault dip and the choice of the GMPE are the dominant contribution to the standard deviation of the lognormal distribution function. Elsewhere nearby, uncertainties in fault slip rate dominate, while at larger distances uncertainties in the magnitude dominate.

The timing and origin of pre- and post-caldera volcanism associated with the Mesa Falls Tuff, Yellowstone Plateau volcanic field

We present new sanidine 40Ar/39Ar ages and paleomagnetic data for pre- and post-caldera rhyolites from the second volcanic cycle of the Yellowstone Plateau volcanic field, which culminated in the caldera-forming eruption of the Mesa Falls Tuff at ca. 1.3Ma. These data allow for a detailed reconstruction of the eruptive history of the second volcanic cycle and provide new insights into the petrogenesis of rhyolite domes and flows erupted during this time period. 40Ar/39Ar age data for the biotite-bearing Bishop Mountain flow demonstrate that it erupted approximately 150kyr prior to the Mesa Falls Tuff. Integrating 40Ar/39Ar ages and paleomagnetic data for the post-caldera Island Park rhyolite domes suggests that these five crystal-rich rhyolites erupted over a centuries-long time interval at 1.2905±0.0020Ma (2σ). The biotite-bearing Moonshine Mountain rhyolite dome was originally thought to be the downfaulted vent dome for the pre-caldera Bishop Mountain flow due to their similar petrographic and oxygen isotope characteristics, but new 40Ar/39Ar dating suggest that it erupted near contemporaneously with the Island Park rhyolite domes at 1.2931±0.0018Ma (2σ) and is a post-caldera eruption. Despite their similar eruption ages, the Island Park rhyolite domes and the Moonshine Mountain dome are chemically and petrographically distinct and are not derived from the same source. Integrating these new data with field relations and existing geochemical data, we present a petrogenetic model for the formation of the post-Mesa Falls Tuff rhyolites. Renewed influx of basaltic and/or silicic recharge magma into the crust at 1.2905±0.0020Ma led to [1] the formation of the Island Park rhyolite domes from the source region that earlier produced the Mesa Falls Tuff and [2] the formation of Moonshine Mountain dome from the source region that earlier produced the biotite-bearing Bishop Mountain flow. These magmas were stored in the crust for less than a few thousand years before being erupted contemporaneously along a 30km long, structurally controlled vent zone related to extracaldera Basin and Range faults. These data highlight the rapidity with which magma can be generated and erupted over large distances at Yellowstone.

Basement control of alkalic flood rhyolite magmatism of the Davis Mountains volcanic field, Trans-Pecos Texas, U.S.A.

Voluminous silicic lava flows, erupted ~37.4Ma from widespread centers within the Davis Mountains Volcanic Field (DMVF), covered approximately 10,000km2 with an initial volume as great as 1000km3. Lava flows form three major stratigraphic units: the Star Mountain Rhyolite (minimum 220km3) of the eastern Davis Mountains and adjacent Barilla Mountains, the Crossen Formation (~75km3) of the southern Davis Mountains, and the Bracks Rhyolite (~75km3) of the Rim Rock region west of the Davis Mountains proper. Similar extensive rhyolite lava also occurs in slightly younger units (Adobe Canyon Rhyolite, ~125km3, 37.1Ma), Sheep Pasture Formation (~125km3, ~36Ma) and, less voluminously, in the Paisano central volcano (~36.9Ma) and younger units in the Davis Mountains. Individual lava flows from these units formed fields as extensive as 55km and 300-m-thick.Flood rhyolite lavas of the Davis Mountains are marginally peralkaline quartz trachyte to low-silica rhyolite. Phenocrysts include alkali feldspar, clinopyroxene, FeTi oxides, and apatite, and, rarely, fayalite, as well as zircon in less peralkaline units. Many Star Mountain flows may be assigned to one of four geochemical groupings. Temperatures were moderately high, ranging from ~911 to 860°C in quartz trachyte and low silica rhyolite. We suggest that flood rhyolite magma evolved from trachyte magma by filter pressing processes, and trachyte from mafic magma in deeper seated plutons.The Davis Mountains segment of Trans-Pecos Texas overlies Grenville basement and is separated from the older Southern Granite and Rhyolite Province to the north by the Grenville Front, and from the younger Coahuila terrane to the south by the Ouachita Front. We suggest that basement structure strongly influenced the timing and nature of Trans-Pecos magmatism, probably in varying degrees of impeding the ascent of mantle-derived mafic magmas, which were produced by upwelling of asthenospheric mantle above the foundered Farallon slab. Basalt was able to penetrate Coahuila crust in the Big Bend region. Thicker Grenville crust under the Davis Mountains retarded ascent of mafic magmas, allowing mafic plutons to differentiate into silicic magma that was eventually erupted as flood lava. North of the Grenville Front, magmatism was further delayed and thicker, older crust there may have helped concentrate magmatism under the Davis Mountain region. Only after the onset of Basin and Range faulting was true basalt erupted over much of the Trans Pecos.

Thermochronology in southeast Alaska and southwest Yukon: Implications for North American Plate response to terrane accretion

This study presents the first comprehensive dataset of low-temperature thermochronology from 43 bedrock samples collected north of the active Yakutat–North American plate boundary. Our apatite and zircon (U–Th)/He and fission-track data reveal the cooling history of the inboard Wrangellia Composite Terrane that is dominated by rapid cooling after Late Jurassic to Early Cretaceous arc magmatism followed by very little cooling and exhumation until today. Deformation resulting in rock exhumation due to the collision of the Yakutat microplate is spatially very limited (20–30 km) and is concentrated mainly in the Chugach–Prince William Terrane and rocks near the Border Ranges Fault. Focused exhumation from greater depths of ca. 10 km with very high rates (>5 km/Myr) is localized at the syntaxis region, starting ca. 10 Ma and shifted south through time. The rapid exhumation rates are explained by the development of strong feedbacks between tectonically driven surface uplift and erosion, which started already before glaciation of the area. The shift in the location towards the south is a consequence of continuous readjusting between tectonics and climate, which is changing on local and global scales since the Late Miocene.

Geology of the High Rock caldera complex, northwest Nevada, and implications for intense rhyolitic volcanism associated with flood basalt magmatism and the initiation of the Snake River Plain–Yellowstone trend

We present the geologic history of the High Rock caldera complex (HRCC; Nevada, USA), a major mid-Miocene silicic center associated with flood basalt volcanism. Based on 70 40Ar/39Ar ages and new 1:24,000- and 1:100,000-scale geologic mapping, we document that between ca. 16.5 and 15.5 Ma a minimum of ∼700 km3 of rhyolitic magma erupted from the HRCC, covering an area of ∼8300 km2 in northwestern Nevada and southern Oregon (USA). The volcanism immediately followed eruption of the Steens flood basalt in the region, and was contemporaneous with eruption of compositionally similar rhyolites from the McDermitt volcanic field (MVF) to the east. The HRCC and MVF together mark the starting point for the eastward-younging trend of voluminous rhyolitic calderas of the Snake River Plain–Yellowstone trend. The HRCC comprises 4 major calderas, 24–40 km in diameter, that young from north-northeast to south-southwest: the Virgin Valley caldera formed on eruption of the 16.38 Ma high-silica alkali rhyolite Idaho Canyon Tuff; the overlapping Badger Mountain caldera collapsed due to eruption of the 16.34 Ma crystal-rich, low-silica rhyolite Summit Lake Tuff; eruption of the moderately peralkaline high-silica alkali rhyolite Soldier Meadow Tuff at 16 Ma resulted in formation of the Hanging Rock caldera; and the Cottonwood Creek caldera formed on eruption of the newly recognized, 15.70 Ma tuff of Yellow Rock Canyon, which is zoned from high-silica alkali rhyolite to trachyte. The four calderas contain caldera fill deposits, including pumice and ash falls, lahars, phreatomagmatic deposits, and well-bedded lacustrine deposits, which preserve diverse mid-Miocene fossil fauna and flora. Au and U mineralization developed along ring fractures of the Virgin Valley and Cottonwood Creek calderas. After silicic volcanism largely ceased, trachyte, trachyandesite, and alkalic basaltic lavas erupted through the caldera lakes. Intense silicic volcanism at the HRCC during the interval 16.4–15.5 Ma overlapped the eruption of the Steens and Columbia River Basalts, strongly suggesting a petrogenetic link. We propose that the HRCC and MVF caldera centers are localized where dikes of Steens flood basalt encountered transitional crust west of the craton with a composition and thickness that allowed significant partial melting, based on the O and Nd isotopic values of the rhyolites, which require involvement of crustal melts in their origin. Steens Basalt eruptions largely ceased in the area by the time the oldest caldera-forming ignimbrites erupted at both the HRCC and MVF, indicating that once large silicic magma bodies aggregated in the crust, they intercepted flood basalt dikes. We suggest that the roots of the HRCC and MVF are composed of large volumes of gabbroic intrusions and cumulates formed by fractional crystallization of HRCC magmas, which strengthened the middle crust beneath the calderas; major basin-bounding normal faults are diverted around them, but outflow ignimbrites are prominently offset by Basin and Range faults.

Evidence for a long‐lived accommodation/transfer zone beneath the Snake River Plain: A possible influence on Neogene magmatism?

AbstractGeochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the Snake River Plain‐Yellowstone Province (SRP‐Y). Magmatism, crustal flow, metamorphism, and extensional exhumation of core complexes north of the SRP occurred mostly between 55 and 42 Ma as compared to 42–25 Ma south of the SRP, with final exhumation of the southern complexes occurring only during younger Miocene (20–0 Ma) Basin and Range faulting. These significant differences in the timing of events suggest that the now lava‐covered SRP, which is at a high angle to Cordilleran trends, may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south. Following previous suggestions, we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab (shallower slab angle to the south) used to explain both the locus of Late Cretaceous‐Paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the SRP during the Cenozoic. The details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused Miocene SRP‐Y hot spot magmatism.

The tectonic significance of the Early Cretaceous forearc-metamorphic assemblage in south-central Alaska based on detrital zircon U–Pb dating of sedimentary protoliths

A complex array of faulted arc rocks and variably metamorphosed forearc accretionary complex rocks form a mappable arc–forearc boundary in southern Alaska known as the Border Ranges fault (BRF). We use detrital U–Pb zircon dating of metasedimentary rocks within the Knik River terrane in the western Chugach Mountains to show that a belt of Early Cretaceous amphibolite-facies metamorphic rocks along the BRF was formed when older mélange rocks of the Chugach accretionary complex were reworked in a sinistral-oblique thrust reactivation of the BRF during a period of forearc plutonism. The metamorphic subterrane of the Knik River terrane has a maximum depositional age (MDA) of 156.5 ± 1.5 Ma and a detrital zircon age spectrum that is indistinguishable from the Potter Creek assemblage of the Chugach accretionary complex, supporting correlation of these units. These ages contrast strongly with new and existing data that show Triassic to earliest Jurassic detrital zircon ages from metamorphic screens in the plutonic subterrane of the Knik River terrane, a fragmented Early Jurassic plutonic assemblage generally interpreted as the basement of the Peninsular terrane. Based on these findings, we propose the following new terminology for the Knik River terrane: (1) “Carpenter Creek metamorphic complex” for the Early Cretaceous “metamorphic subterrane”, (2) “western Chugach trondhjemite suite” for the Early Cretaceous forearc plutons within the belt, (3) “Friday Creek assemblage” for a transitional mélange unit that contains blocks of the Carpenter Creek complex in a chert–argillite matrix, and (4) “Knik River metamorphic complex” in reference to metamorphic rocks engulfed by Early Jurassic plutons of the Peninsular terrane that represent the roots of the Talkeetna arc. The correlation of the Carpenter Creek metamorphic complex with the Chugach mélange indicates that the trace of the BRF lies ∼1–5 km north of the map trace shown on geologic maps, although, like other segments of the BRF, this boundary is blurred by local complexities within the BRF system. Ductile deformation of the mélange is sufficiently intense that few vestiges of its original mélange fabric exist, suggesting the scarcity of rocks described as mélange in the cores of many orogens may result from misidentification of rocks that have been intensely overprinted by younger, ductile deformation.

Yellowstone plume trigger for Basin and Range extension, and coeval emplacement of the Nevada–Columbia Basin magmatic belt

Widespread extension began across the northern and central Basin and Range Province at 17–16 Ma, contemporaneous with magmatism along the Nevada–Columbia Basin magmatic belt, a linear zone of dikes and volcanic centers that extends for >1000 km, from southern Nevada to the Columbia Basin of eastern Washington. This belt was generated above an elongated sublithospheric melt zone associated with arrival of the Yellowstone mantle plume, with a north-south tabular shape attributed to plume ascent through a propagating fracture in the Juan de Fuca slab. Dike orientation along the magmatic belt suggests an extension direction of 245°–250°, but this trend lies oblique to the regional extension direction of 280°–300° during coeval and younger Basin and Range faulting, an ∼45° difference. Field relationships suggest that this magmatic trend was not controlled by regional stress in the upper crust, but rather by magma overpressure from below and forceful dike injection with an orientation inherited from a deeper process in the sublithospheric mantle. The southern half of the elongated zone of mantle upwelling was emplaced beneath a cratonic lithosphere with an elevated surface derived from Late Cretaceous to mid-Tertiary crustal thickening. This high Nevadaplano was primed for collapse with high gravitational potential energy under the influence of regional stress, partly derived from boundary forces due to Pacific–North American plate interaction. Plume arrival at 17–16 Ma resulted in advective thermal weakening of the lithosphere, mantle traction, delamination, and added buoyancy to the northern and central Basin and Range. It was not the sole cause of Basin and Range extension, but rather the catalyst for extension of the Nevadaplano, which was already on the verge of regional collapse.

Estimation of Local Magnitude in Northeastern Sonora, Mexico, Using Empirical Relations Based on Recorded Duration

Research Article| March 11, 2015 Estimation of Local Magnitude in Northeastern Sonora, Mexico, Using Empirical Relations Based on Recorded Duration Gina P. Villalobos‐Escobar; Gina P. Villalobos‐Escobar Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Search for other works by this author on: GSW Google Scholar Raúl R. Castro Raúl R. Castro Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Search for other works by this author on: GSW Google Scholar Author and Article Information Gina P. Villalobos‐Escobar Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Raúl R. Castro Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), División Ciencias de la Tierra, Departamento de Sismología, Carretera Ensenada‐Tijuana No. 3918, Ensenada, Baja California, 22860 Méxicovillalobos@geociencias.unam.mx Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2015 by the Seismological Society of America Seismological Research Letters (2015) 86 (3): 870–875. https://doi.org/10.1785/0220140027 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gina P. Villalobos‐Escobar, Raúl R. Castro; Estimation of Local Magnitude in Northeastern Sonora, Mexico, Using Empirical Relations Based on Recorded Duration. Seismological Research Letters 2015;; 86 (3): 870–875. doi: https://doi.org/10.1785/0220140027 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Most of northern Mexico is tectonically and morphologically situated in the southern Basin and Range Province (Suter and Contreras, 2002). The region of Sonora is particularly interesting, because the largest historical earthquake (3 May 1887, Mw 7.5) occurred in this province. The town of Bavispe was destroyed during this earthquake (Aguilera, 1888), and the seismic source generated the longest recorded normal‐fault surface rupture (101.8 km) in historic time (Suter, 2006). The major fault zone of this earthquake includes (from south to north) three neighboring segments: Otates (length l=18.9 km), Teras (l... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Constraints on the evolution of vertical deformation and Colorado River incision near eastern Lake Mead, Arizona, provided by quantitative structural mapping of the Hualapai Limestone

The 12–6 Ma Hualapai Limestone was deposited in a series of basins that lie in the path of the Colorado River directly west of the Colorado Plateau and has been deformed by an en-echelon normal fault pair (Wheeler and Lost Basin Range faults). Therefore, this rock unit represents an opportunity to study the sedimentological and structural setting over which the Colorado River first flowed after integration through western Grand Canyon and Lake Mead. In this study, we quantify the structural geometry of the Hualapai Limestone and separate the deformation into syn- and postdepositional episodes. Both the Wheeler and Lost Basin Range faults were active during Hualapai Limestone deposition, as shown by thickening of strata and fanning of time lines toward half-graben faults that bound the Hualapai subbasins. The structure is characterized by a prominent reverse-drag fold and broad, shallow syncline adjacent to the Lost Basin Range fault, and a small-magnitude reverse-drag fold and short-wavelength normal-drag fold adjacent to the Wheeler fault. We find ∼450 m of throw between the footwall and hanging-wall Hualapai Limestone sections, suggesting faulting was ongoing after Hualapai Limestone deposition ceased and during Colorado River incision. To investigate a range of possible fault geometries that may have been responsible for Hualapai Limestone deformation, we compared our structural results against surface deflections calculated by a two-dimensional (2-D) geomechanical model. While nonunique, our results are consistent with a scenario in which the Wheeler fault was surface rupturing, or nearly surface rupturing throughout deposition of the Hualapai Limestone, but was inundated at ca. 6 Ma by coalescing paleolakes in Gregg and Grand Wash Basins as sedimentation kept pace with deformation. In contrast, we find evidence suggesting the Lost Basin Range fault was deeply buried by the Hualapai Limestone and likely propagated upward and laterally to break the surface sometime after 6 Ma. Therefore, we interpret the landscape over which the Colorado River first flowed to be of low relief within the terrain bounded by the Grand Wash Cliffs, the Hiller Mountains, and subtle topographic highs to the north and south of our field area. This original low-relief depositional surface was deflected into the structure exposed today by continuing deformation by the Wheeler and Lost Basin Range faults, allowing for calculation of apparent incision rates of the modern Colorado River drainage system that spatially vary between 33 and 42 m/m.y. in the hanging wall and between 108 and 115 m/m.y. in the footwall. Hanging-wall incision rate values are similar to, but faster than, a previously published point measurement, and footwall values are similar to measured incision rates in the western Grand Canyon, suggesting the Wheeler fault system may resolve as much as ∼410 m of Colorado Plateau uplift in the last 6 m.y.

Eocene to Quaternary mafic-intermediate volcanism in San Luis Potosí, central Mexico: The transition from Farallon plate subduction to intra-plate continental magmatism

The San Luis Potosí Volcanic Field (SLPVF) of central Mexico includes volcanic sequences of felsic, intermediate and basic compositions that were erupted as discrete episodes from the Eocene to the Pleistocene. Volcanism was dominated by widespread and voluminous rhyolitic ignimbrites of the mid-Tertiary Ignimbrite Flare-up. However, the complete volcanic history must consider basaltic and andesitic Eocene–Pleistocene volcanic successions that provide key evidence for understanding the geochemical evolution of the volcanism in the SLPVF during this time span. Five sequences are recognized according to their geochemical characteristics, each comprising a volcano-tectonic episode. The first episode comprises basaltic andesites and andesites erupted during three intervals, 45–42Ma, 36–31Ma, and 31–30Ma. The oldest was derived from subduction magmatism, whereas the youngest has an intra-plate magmatic signature and this represents the transition from the end of a long lasting subduction regime of the Farallon plate to the initiation of intra-plate continental extension in the North American plate. The second episode, at 29.5–28Ma, comprises a bimodal succession of high-silica rhyolites and alkaline basalts (hawaiites) that are interpreted as magmatism generated in an intra-plate continental extension regime during the Basin and Range faulting. The third episode, at 21Ma, is characterized by trachybasalts and trachyandesites that represent mantle basaltic melts that were contaminated through assimilation of the lower crust during advanced stage of intra-plate extension that started at Oligocene. The fourth episode includes 12Ma alkaline basalts and andesites that were erupted from fissures. These mantle derived magmas evolved to andesites by crustal anatexis and crystal fractionation within a continued, extensional, intra-plate regime. Lastly, the fifth episode comprises 5.0 to 0.6Ma alkaline basalts (basanites) containing mantle xenoliths, that were erupted from maars and tuff cones, which are the youngest manifestations of mantle-derived intra-plate extensional events. Based upon this volcanic record, the last subduction manifestations of the extinct Farallon plate occurred at about 42Ma, this was followed by a transition to intra-plate magmatism between 42 and 31Ma, and an extensional, intra-plate tectonic setting from 31Ma to almost Present.

Constraints on the Structure of the Border Ranges Fault System, Kenai Peninsula, Alaska from the Preliminary Results of the 3D Inversion Model of Gravity Data

SUMMARY We tested plausible initial 3D density models across the eastern Cook Inlet basin to constrain the geometry of the Border Ranges fault system (BRFS). The BRFS forms the structural boundary between the forearc-arc structures and accretionary terranes of the Aleutian subduction zone in southern Alaska. We used 2D density cross-sections as starting “a priori” models to constrain the 3D inversion model. We computed the theoretical gravity based on the inversion, compared it to the observed gravity, and determined the gravity misfit. We tested the 3D inversion solutions on the lowland regions where the densest spacing of gravity points is located using three surfaces defining the topographic ground level, the top and base of the sediment layers within Cook Inlet basin to build the preliminary 3D models. Preliminary results show the 3D inversion is able to constrain the simple shape of Cook Inlet basin with geologically reasonable densities. We will produce several alternative starting models for the entire Cook Inlet forearc basin and the BRFS. The most reasonable models will be used as starting models for the structure of the entire study area.