The Midcontinent Rift system (ca. 1.1 Ga) is a 2000-km-long series of elongated volcanic and sedimentary troughs and associated intrusive centers exposed chiefly in the Lake Superior region of North America. The rift system represents a long history of intense magmatism and subsequent sedimentation that was arrested by far-field tectonic events before sea-floor spreading was established. The premature cessation preserved a record of processes related to the beginning of continental rifting. The rift system under Lake Superior has been long studied using seismic-reflection data collected as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE). We reexamine GLIMPCE Line C by developing a detailed velocity model for time to depth conversion constrained by other legacy data. We corroborate the model and develop a geologic interpretation using gravity and magnetic modeling and ties to geology mapped onshore. We recognize superposed subsiding sedimentary and volcanic basins for the southern half of the Line C depth section. This interpretation differs from previous paradigms that show major crustal faults that bound half-grabens or full grabens. We conclude that high-velocity (6.9 km/s) intrusive zones rather than major crustal faults border the sides of the basins. We speculate that the volcanic basin represents the initiation of seaward dipping reflectors. The syn-magmatic subsidence can be explained by dike injection and volcanic loading. Discrete lava basins throughout the region likely subsided at different times in a disorganized manner along the rift trend, raising questions about the long-term role of lithospheric thinning and melt generation.
The spatial extents of buried Precambrian basement terranes and tectonic features are poorly known below the Allegheny Plateau of New York and Pennsylvania, USA. Geological features around the plateau periphery and geophysical variations within the plateau imply the occurrence of both Mesoproterozoic Grenville province terranes and Neoproterozoic basins associated with Iapetan rifting of the Laurentian margin. Cornell University explored geothermal energy reservoirs below Ithaca, New York, by drilling borehole ESH 1 to 2978 m depth, ending 119 m below the Cambrian-basement unconformity. The substantial dataset for the upper part of the basement enables characterization of the lithologic sequence, which is interpreted to be the fill of a Neoproterozoic basin, metamorphosed to greenschist grade. Petrophysical properties differ among four intervals 18−44 m thick, further divided into meter-scale intervals of repetitively variable properties. Wireline logs and drill cuttings for all four intervals demonstrate high abundance of minerals containing structural water (e.g., phyllosilicates) and potassium (muscovite, biotite, K-feldspar). Cuttings and six sidewall cores reveal fine-grained metamorphic rocks derived from both sedimentary and volcanic protoliths. K-feldspar dominates the upper petrophysical zone while phyllosilicates and quartz dominate the lower three intervals, consistent with alteration of the shallowest basement. Downhole patterns of density and gamma variations are similar to those of sedimentary fills of documented rift basins elsewhere and distinctly different than patterns in high-grade metamorphic and plutonic basement. Applying to other regional wells the criteria developed here may improve mapping of basement boundaries and facilitate identifying Iapetan basins below the Allegheny Plateau.
The 50-km-long Thousand Lake fault (TLF) aligns with a series of west-dipping normal faults forming an ∼200-km-long structural discontinuity along the eastern edge of the Basin and Range in south-central Utah. Quantifying a fault’s slip rate and slip rate variations is important for characterizing earthquake hazards and understanding how deformation within fault networks is accommodated through time. This study uses cross-section analyses to determine that the central TLF has displaced Cenozoic volcanic rocks by 1200−2200 m since fault initiation at 15−10 Ma, implying a long-term slip rate of ∼0.1−0.2 mm/a. In contrast, displacement measurements across a Late Pleistocene fan in Bicknell, Utah, along with luminescence and 3He geochronology, allow us to infer that the fault slip rate is only 0.03−0.08 mm/a over the late Pleistocene. The TLF is an example of a Basin and Range normal fault that has experienced significant slip rate variation over geologic time. Trenching of the fan’s primary fault scarp demonstrates that the last two earthquakes have occurred since 52.8 ± 8.5 ka, and the most recent earthquake occurred after 19.7 ± 4.7 ka. Finally, lidar-based geomorphic and fault scarp mapping along the entire fault shows that the last earthquake clearly post-dates the Last Glacial Maximum, rupturing the ground surface with vertical displacements of ∼0.8−1.4 m for ∼32 km in an ∼Mw 6.8 earthquake.
Between 27 and 30 third-order Paleocene and Eocene eustatic cycles of falling sea level are recorded on various published global sea-level charts. Several were of sufficient magnitude to cause erosion in the Sacramento Basin, California, and create regional unconformities. Four are major erosion surfaces that floor fluvial valleys that cut down a few hundred meters into the Cretaceous delta plain. The others are low-angle disconformities that record more moderate erosional events. The unconformities divide the strata into seven stratigraphic sequences. The valleys trend west or southwest to a deep graben that formed in the southwestern corner of the basin when crustal extension caused subsidence of the subduction complex that had flanked the basin during the Cretaceous. Normal faults in the Kirby Hill fault system became active and created a horst block that separated the graben into the shallow Rio Vista basin on the east from the deeper Suisun basin to the west. North-south sediment transport in the Cretaceous was replaced by northeast-southwest transport, a new shelf edge formed along the horst block, and Paleocene rivers cut submarine canyons through the shelf edge, feeding submarine fans into the Suisun basin. Planktic Foraminifera and calcareous nannoplankton in valley deposits indicate that valley filling took place in the middle Paleocene (Selandian Stage), late Paleocene (Thanetian Stage), early Eocene (Ypresian Stage), and latest Eocene−early Oligocene (Rupelian Stage) during planktic foraminiferal and nannoplankton zones P3 (CP3), P5−P6a (CP9a), P7 (CP9b−CP10), and P18 (CP16−CP17). This implies that incision occurred in CP2−early CP3 (62−61 Ma), CP8b (56−55 Ma), and CP15 (38−37 Ma), when some sea-level charts record major sea-level falls of >250 ft (75 m). Downcutting in the Sierra Nevada likely occurred during those events. Sea level began rising after these lowstands and reached highstands in the latest Thanetian and Ypresian, when the valleys and the eroded delta plain were buried by transgressive marine shale and shoreline sand bodies were plastered against the Midland Fault scarp in the Rio Vista graben. Less than 200 ft (60 m) of strata were removed during early and late Paleocene (Danian and Thanetian Stages) and early Eocene (early and late Ypresian Stages) erosion. These events took place during medium sea-level falls of between 80 and 250 ft (25−75 m). They were succeeded by widespread transgressions to or east of the present boundary between the basin and Sierra Nevada when sea level rose. For example, the disconformity between the middle Ypresian Capay Shale and late Ypresian Domengine Formation formed when a river flowing from the Sierra eroded 100−125 ft (30−40 m) into the Capay. During the Capay transgression in CP10−CP11 (53−50 Ma), the crystalline bedrock in the Sierra was undergoing deep chemical weathering, creating a thick kaolinitic-quartzose lateritic soil that was eroded when sea level began falling in CP12a between 50 and 49 Ma. Fault displacement on the eastern margin of the Rio Vista graben accelerated during the middle Eocene, causing the shelf edge to backstep to the Midland Fault. No new submarine canyon was cut through the shelf, but arkosic sediment was shed from an uplift southwest of the basin during CP14a and was deposited in a submarine fan that spread eastward through the Suisun graben into the Rio Vista graben. It was dammed against normal faults in the Denverton Creek and Midland fault systems and subsequently buried by a delta that prograded to the southwest from an andesitic source northeast of the Sierra Nevada. This river cut a deep valley through its delta in the late Eocene and filled it in the early Oligocene. Another unconformity formed in the middle Oligocene after nannoplankton zone CP19. It is overlain by Miocene alluvial fans along the eastern edge of the basin and by channel deposits in the Sierra Nevada. These rivers carried rhyolitic silt, sand, and gravel from calderas in central Nevada but did not reach the Rio Vista graben. This unconformity may be partly due to sea-level fall at ca. 30 Ma, but the development of coarse-grained alluvial fans implies steeper stream gradients and major uplift east of the basin after the Eocene.
Field observations plus sanidine compositional and 40Ar/39Ar age data collected on rocks from the Sour Creek dome (SCD) area of Yellowstone volcano (United States) serve to identify five new ignimbrite packages with ages (average 631.5 ± 0.9 ka) that are analytically indistinguishable from those of the Lava Creek Tuff (LCT), challenging the current framework of two LCT eruptive members. Newly recognized package 1 occurs as LCT-aged recycled clasts within a proximal lag breccia, reflecting a major time break in the LCT eruption sequence. Conformably above, the scoria-bearing ignimbrite of package 2 is extensively found in the dome, having been previously mis-mapped as Huckleberry Ridge Tuff and LCT member A. To the south, package 3 is a crystal-rich, densely welded ignimbrite (inferred to be younger than package 2) that is conformably overlain by package 4, a fine-grained ignimbrite that contains abundant clasts of recycled tuff. Lastly, the topographically highest parts of the dome are made up of package 5, a multicomponent ignimbrite containing scoria, recycled clasts, and pumices, potentially correlative with package 2 and/or package 4. Cathodoluminescence imaging and zoning assessment was integrated with sanidine geochemistry from all five packages, including their recycled clasts. Four of the five packages were sourced from separate magmatic bodies within a larger plumbing system, each defined by their sanidine Ba distributions and componentry. Although the SCD was originally mapped as a resurgent dome and is actively deforming today, faults cutting the region show comparable offsets for NW-SE “resurgent faulting” as for orthogonal NE-SW faulting, the latter of which also cuts the 162 ka Elephant Back lava flow. We infer that faulting in the SCD well postdates eruption and that the SCD instead represents a domical accumulation of ignimbrite, not a resurgent structure. The abundance of recycled materials (and absence of Absaroka country rocks) in the SCD ignimbrites implies that vent locations were well inboard of the mapped caldera boundary, with some potentially along the line of the Yellowstone River, requiring a revision of the currently accepted LCT caldera outline. The LCT eruptive events were thus far more complex than currently recognized, with multiple deposits fed from multiple magma bodies over a time scale that cannot be resolved by radiometric dating.
An understanding of the distribution, transport direction, geometry, composition, and age of Campanian through lower Oligocene strata in the subsurface of the Sacramento Basin, California, provides an essential framework for interpreting depositional, erosional, and tectonic events in the adjacent Sierra Nevada. These clastic strata were deposited in an alternating series of marine transgressions and regressions in the Great Valley forearc basin, some of which were due to rising and falling sea levels that were global in extent. Fluvial deposits are an important component of 11 different formations, but stream deposits in Cretaceous fluvial-deltaic systems differ in several important aspects from deposits in Paleogene fluvial-deltaic systems. These differences are key to distinguishing sea-level highstands from lowstands, identifying source areas, developing paleogeographic reconstructions, and interpreting the uplift history of the Sierra Nevada. As sea level rose in the late Santonian, the shoreline retreated to the north and east, possibly as far as the Oregon border by ca. 84 Ma. Remnants of this shoreline (Chico Formation) are locally exposed northeast of the city of Chico along Big Chico and Butte Creeks. The abundance of plutonic and metamorphic grains and absence of muscovite in this sandstone indicates that it was derived from the Sierra Nevada. Sediment transport to submarine canyons along the shelf edge was cut off and submarine fans (Guinda Formation) on the floor of the basin were abandoned and blanketed by several hundred feet of hemipelagic mud (Dobbins Shale). When sea-level rise slowed in the early Campanian, transgression ended and regression began. A high-sinuosity river flowing into the basin from Idaho deposited an elongate, fluvial-dominated delta (Kione Formation) north of the city of Sacramento. Over time, the delta prograded south and reached the southeast-trending Willows Fault. This southwest-dipping normal fault had offset the crystalline basement and Dobbins Shale and created the shelf edge. As the river prograded to the fault and migrated to the southeast along it, it fed sediment to the prodelta slope and submarine fans of the Forbes Formation south of the fault. The Kione and Forbes formations are quartzofeldspathic and rich in volcanic grains, plagioclase, and muscovite derived from the Idaho batholith, but metamorphic grains from the Sierra are lacking. Tabular, laterally extensive channel deposits of high-sinuosity trunk streams and linear, narrower, low-sinuosity distributaries are present in the Kione and most of the younger Cretaceous delta systems. As sea-level rise accelerated again in the late Campanian, the shoreline transgressed to the east of the early Campanian shoreline and the Sacramento Shale blanketed the Kione and Forbes formations. That shoreline has been eroded but was at least 10 mi (16 km) east of westernmost exposures of Sierran bedrock. Regression resumed when sea level reached a new highstand and the river began building a new delta in the northeastern San Joaquin Basin, 50 mi (80 km) south of the Kione delta. By this time, the river had apparently migrated farther east into the area of the present Sierran foothills, but its deposits have not been preserved. In the late Campanian and Maastrichtian, the basin continued to subside and tilt to the south and sea level rose in stepwise fashion but was overtaken by sediment input. Regression resumed and the river began migrating back to the west, building cuspate to lobate delta lobes (Starkey and Mokelumne River formations) and submarine fans that migrated westward across the Sacramento Basin and prograded southward into the San Joaquin Basin. The arkosic sand in these deltas and fans is also rich in volcanic grains derived from the magmatic arc northeast of the basin, but metamorphic grains like those in the Chico Formation are absent. The data indicate that the western foothills of the Sierra Nevada were submerged in the late Campanian and the range was a relatively minor sediment source. Southward sediment transport from Idaho continued to be dominant.
Grotta Romanelli (Romanelli Cave) stands as a key site of the European Paleolithic, preserving traces of human occupation since the middle Pleistocene. To date, little attention has been devoted to the carbonate bedrock that hosts the cavity, and to the role that tectonic structures played in its formation. To fill this gap, we conducted combined structural, petrophysical, and stratigraphic analyses, supported by the interpretation of a high-resolution 3-D virtual outcrop model of the cave and its surroundings. The results indicate that the cave was part of a system of Cretaceous extension-related structures affecting synkinematic carbonates. In particular, the cave opens within, and aligns with, an extensional fault−related anticline, which developed in the footwall of an embryonic extensional fault zone. The overall morphology of the cave and its progressive widening were strongly governed by exfoliation along preexisting discontinuities affecting synkinematic strata, namely, bedding planes and fault surfaces. Furthermore, the presence of porous, poorly lithified synkinematic Upper Cretaceous limestones, particularly at the thin entrance roof, raises critical stability concerns. The results of this integrated analysis, which provides the first geologically constrained 3-D model of the bedrock limestones of Grotta Romanelli, offer a robust basis for assessing the structural control, stability, and long-term evolution of the site. Results therefore enhance both scientific interpretation and management of limestone-hosted caves.
Quaternary glaciogenic deposits, which serve as vital groundwater aquifers, are often partially cemented with carbonate minerals, but the timing of emplacement of these cements and their geochemical conditions of formation remain understudied. Here, we integrated petrography, δ18O, Mg/Ca, and clumped isotope (Δ47) data from two calcite-cemented outcrops in western Michigan (USA) to investigate the origin of glacial sediment cements. The two sites have contrasting topographic and textural settings reflecting distinct formation conditions. The elevated Wabasis Lake deposit records sequential phreatic-to-vadose cementation, while the Cherry Valley Sands site, very close to the modern water table, shows a mixed phreatic and vadose overprinted cement fabric. The Cherry Valley Sands geochemical data are essentially as expected for cement formation under modern conditions. In contrast, Wabasis Lake phreatic cements have lower calcite δ18O (−8.3‰ ± 0.5‰ relative to Vienna Peedee belemnite), show a clear correlation between δ18O and Mg/Ca, and, via a Δ47 temperature of 9 ± 2 °C, yield a precipitation water δ18O value of −9.7‰ ± 0.5‰ relative to Vienna standard mean ocean water, i.e., ∼2‰ more negative than modern meteoric water. Rather than near-modern, we conclude these data are instead consistent with preserved ancient conditions in phreatic cements, reflecting late glacial to early Holocene recharge. These results highlight coarse, well-cemented glaciogenic deposits as valuable and overlooked potential paleohydroclimate proxies. They also place constraints on the conditions of cement formation in glacial aquifers.
The Proterozoic crust of southern Laurentia has been considered for >30 yr to be an archetype of continental growth via the accretion of exotic crustal fragments. In a broadly accepted model, Laurentia grew by the sequential addition of the ca. 1.8- 1.7 Ga Yavapai and ca. 1.7- 1.6 Ga Mazatzal provinces. These provinces were originally defined in central Arizona, southwestern United States, where the Yavapai and Mazatzal provinces are distinguished by marked differences in lithology, metamorphic grade, and structural style across a proposed suture between the Moore Gulch and Slate Creek shear zones. Here, we tested the hypothesis that rocks of the Mazatzal province are allochthonous with respect to the Yavapai province and that the Moore Gulch shear zone marks the northern edge of the province suture zone. We present: (1) new combined zircon U-Pb and Hf-isotope analyses of plutonic rocks older than 1.7 Ga across the Moore Gulch shear zone, (2) new field and microstructural analysis of the Moore Gulch shear zone, and (3) a compilation of published detrital zircon U-Pb and Hf-isotope data from quartzite successions characteristic of the Mazatzal province. Plutonic rocks on either side of the Moore Gulch shear zone are isotopically juvenile at ca. 1.75- 1.74 Ga, with epsilon Hf(t) values near +10. Detrital zircon data from post- 1.7 Ga quartzites are indistinguishable across the province boundary zone, and stretching lineations in the northeast- striking Moore Gulch shear zone plunge steeply to the east- northeast and record top- to- the- southwest shearing in an oblique sinistral reverse fault geometry. These results do not support the presence of a crustal suture between the Yavapai and Mazatzal provinces. Instead, we favor the interpretation that metasedimentary rocks of the Mazatzal province were deposited unconformably across those of the Yavapai province, and that rocks of the Mazatzal province are para- autochthonous with respect to the Yavapai province. Correlation of Mazatzal Group quartzites across block-bounding shear zones of the Arizona transition zone indicates that deformation attributed to the Mazatzal orogeny in its type locale, and juxtaposition of different crustal levels across block-bounding shear zones, actually occurred during the Mesoproterozoic (ca. 1.49-1.35 Ga) Picuris orogeny. The Mazatzal province is not distinct from the Yavapai province; the two are continuous.
Authigenic potassium feldspar (K-feldspar) occurs throughout the North American midcontinent in late Proterozoic crystalline basement and in the overlying sequence of Paleozoic sedimentary strata often in proximity to the Great Unconformity. Age determinations for this authigenic K-feldspar range from Paleoproterozoic (ca. 1750 Ma) to Mississippian (ca. 360 Ma), commonly interpreted as emplaced by orogeny-driven brines. The accuracy of these formation ages is often complicated by occurrences of authigenic K-feldspar on primary igneous or detrital K-feldspar, where mixing of these components during analysis is unavoidable. In this study, we examined an occurrence of authigenic K-feldspar associated with a late fault that crosscuts Proterozoic crystalline basement rocks and the Reef Deposit, a Paleoproterozoic gold and copper occurrence in northern Wisconsin. The major fault that cuts the Reef Deposit was previously interpreted as associated with the ca. 1100 Ma Midcontinent Rift. However, analysis of K-feldspar microveins hosted within Precambrian quartz veins, devoid of primary K-feldspar, using in situ 40Ar/39Ar methods yielded an age consistent with faulting and fluid circulation at 499.7 +/- 0.8 Ma. This age predates known Paleozoic Laurentian collisional tectonic events. We propose that these K-feldspar microveins reflect a continuation of rifting and related isostatic adjustments along the Iapetus margin, prolonging the duration of extension by similar to 25 m.y. If correct, this occurrence of authigenic K-feldspar would represent the most distal observation of early Paleozoic deformation and fluid migration in the midcontinent of North America. Moreover, this work highlights the potential role of extensional tectonics in the formation and distribution of authigenic K-feldspar in midcontinent North America.
The stable isotopic composition of clay minerals, volcanic glass, and leaf waxes preserved in paleochannels of the northern Sierra Nevada (California, USA) shows higher than modern elevations in the Paleogene, yet this result remains debated due to difficulty reconciling the topography and geomorphology of the modern and ancient landscapes. Here, we present new leaf wax hydrogen isotope data (delta H-2(nC29)) from individual fossil leaves preserved in Eocene-Oligocene alluvial gravels of the northern Sierra Nevada to reconstruct past precipitation delta H-2. We use modern water samples and the Orographic Precipitation and Isotopes model to estimate the distribution of precipitation isotopes across the western North American Cordillera and use this result as a basis for interpreting Eocene precipitation isotopes after removing the Coast Ranges and correcting for a warmer Eocene climate. In combination, these data are used to constrain changes to Sierra Nevada topography between the Eocene and present. We show the following: (1) Eocene delta H-2(nC29) values decrease systematically across the ancient Sierra Nevada, consistent with Rayleigh distillation during orographic lifting and rainout. (2) Bulk sediment and individual leaf delta H-2(nC29) show good agreement, and heterogeneity within individual leaf delta H-2(nC29) data indicates isotopic signatures were locally derived and primary rather than reset. (3) The topography of the northern Eocene Sierra Nevada was similar to 30% higher than today. To estimate paleotopography, we evaluate the observed hydrogen isotope fractionation in the Eocene relative to the present (Phi(lift)) and show that this is proportional to the ratio of the past topography to present topography, and we base our conclusion on the average Phi(lift) values of similar to 1.29. New stable isotope and modeling results closely align with previous estimates and are consistent with a high Late Cretaceous magmatic arc that decayed through the Cenozoic and experienced renewed uplift in the late Cenozoic.
Emergency assessments of postfire debris-flow hazards that are performed by the U.S. Geological Survey (USGS) provide estimates of debris-flow likelihood and rainfall triggering conditions that are used for evaluating and managing runoff-generated debris-flow hazards in recently burned areas throughout the western United States. Although the immediate postfire period, within roughly one year after fire, is typically the most susceptible to runoff-generated debris flows, the hazard evolves in time and space as the burned area recovers. The recovery trajectory a given burned area will take depends on local climate and weather and can be difficult to predict. Some burned areas recover quickly, whereas others experience debris flows for multiple years after fire. As a result, extending our ability to update debris-flow likelihood estimates and rainfall thresholds based on observed recovery of the burned area would be beneficial. We present a method for multi-year runoff-generated debris-flow hazard assessment that leverages the USGS "M1" debris-flow likelihood model and integrates updated, satellite-derived, normalized burn ratio data to estimate vegetation recovery. We predict recovery-aware rainfall thresholds and validate them against a multi-year inventory of debris-flow observations from 12 fires in the western United States. We find that recovery-aware rainfall thresholds perform better than more risk-averse, recovery-unaware thresholds developed just after fire when susceptibility is highest. In addition, recovery-aware thresholds show the potential to reduce warning fatigue by reducing false positives. The methodology we present here advances multi-year debris-flow hazard prediction and could be adapted for use with other debris-flow models that incorporate burn severity data.
Detailed structural mapping, U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS), and sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) analyses were carried out on the Nashoba Formation of the northwestern Nashoba terrane, eastern Massachusetts, to better constrain metamorphic conditions and structural timing in the terrane and provide an initial test for channel flow and ductile extrusion hypotheses. The Nashoba terrane is an early Paleozoic arc/back-arc complex on the trailing edge of Gondwana-derived Ganderia. High-grade metamorphism and partial melting occurred during the latest Silurian to Devonian due to accretion of the New England Avalon terrane. The partially migmatitic rocks of the NW-dipping Nashoba terrane are separated by shear/fault zones from lower-grade rocks of the Avalon terrane to the southeast and the Merrimack belt to the northwest. The Nashoba Formation is characterized by NW-dipping isoclinal folds, overprinted by NW-side-down asymmetric folds, shear zones, and faults. The isoclinal folds formed during a ca. 430-410 Ma period of initial partial melting and extensive plutonism in the Nashoba terrane. Crustal thickening occurred between ca. 419 Ma and ca. 367 Ma. High-grade metamorphic conditions in the Nashoba terrane prevailed between ca. 410 Ma and ca. 370 Ma, while plutonic rocks intruded mainly in the overlying Merrimack belt. Between ca. 370 Ma and ca. 360 Ma, NW-side-down folding and subsequent localized shearing occurred in the Nashoba Formation, while the terrane started cooling, based on new ages of fine-grained dikes and previous 40Ar/39Ar hornblende ages. These data are interpreted in a model of channel flow and ductile extrusion similar to the Himalaya-Tibetan Plateau system.
The Ruby metamorphic core complex domain encompasses an similar to 10,000 km(2) area that includes the Ruby Mountains, East Humboldt Range, Wood Hills, Windermere Hills, and Pequop Mountains in northeast Nevada, USA. The domain contains metamorphic and mylonitized mid-crustal rocks that were exhumed by an intricate array of poorly understood Late Cretaceous-Cenozoic normal faults. This study establishes the first holistic structural architecture and sequence of normal faulting in the Ruby metamorphic core complex domain. The architecture and sequence of faulting are established by synthesizing new and published geologic mapping, and geochronology and sedimentology of synextensional basin fill. Our synthesis shows that after Mesozoic thrust faulting via the Windermere-Angel Lake and Independence thrusts, including a period of Barrovian metamorphism, the core complex sustained four phases of exhumation accommodated by normal faulting. The first phase of normal faulting produced the top-to-the-west to-northwest Pequop fault and was active at some time between 84 Ma and 41 Ma. The second phase began between 38 Ma and 35 Ma and was accommodated by the newly recognized, top-to-the-northwest Ruby-East Humboldt (REH)-Holborn fault. The REH-Holborn fault was a rolling-hinge-style, ductile-to-brittle normal fault that created a synextensional basin filled with sediment of the Clover Creek formation from at least 35-17 Ma. Basin filling was followed by the extinction of the fault between 17 Ma and 15 Ma. The third phase of exhumation was accomplished by N-striking horst-and-graben-style normal faults of the 16 Ma to >3 Ma east-dipping Thousand Springs and west-dipping Knoll-Ruby fault systems, whose synextensional basins filled with sediment of the Humboldt Formation. The modern range-bounding normal faults are accomplishing the fourth and ongoing phase of exhumation. This study has implications for previous work that attributes high pressures recorded by metamorphic rocks to tectonic overpressure in that it shows that significant thrust faulting was a prominent contributor to the development of high pressures. Moreover, this study shows that significant Eocene-Oligocene exhumation of mid-crustal rocks-previously attributed to diapirism in the absence of regional extension-occurred during extension and was accommodated by the REH-Holborn normal fault. This indicates that diapirism was not the primary mode of exhumation.
The thermal structure of the lithosphere affects plate boundary evolution and seismogenesis, but obtaining extensive constraints on the subsurface thermal gradient can be challenging. Bottom simulating reflectors (BSRs), the expression of the base of the gas hydrate stability zone (GHSZ) in seis-mic reflection data, can be used to calculate subsurface thermal gradients in the depth range of the GHSZ (typically within -1 km of the seafloor). The Queen Charlotte plate boundary (QCPB), a transpressive system separating the North American and Pacific plates, has hosted multiple magnitude 7+ earthquakes and is undergoing subduction initiation in its southern segment. To investigate the thermal state of the QCPB, we map BSRs within a 2021 regional multichannel seismic reflection dataset and use them to calculate subsurface thermal gradients. BSRs are abundant in the southern half of the QCPB, where compressional deformation dominates, and sparse in the north, found only overlying isolated thrust faults. We find an along-strike thermal gradient trend that corresponds to the increasing age of the Pacific plate. Deviations from the along-strike trend are likely related to focused fluid flow, topography, and erosion. Perpendicular to the QCPB we find a rapid decrease in thermal gradient landward of the deformation front similar to subduction zone prisms and a thermal gradient increase approaching the strike-slip Queen Charlotte Fault, which may reflect fluid flow or shear heating. Differ-ences between previous thermal models and the BSR-derived constraints suggest that the temperature of the Haida Gwaii thrust, and therefore the size of the seismogenic zone, may be smaller than suggested by past models while also varying along-strike.
The active spreading ridge between the Woodlark and Australia plates is currently subducting northward beneath the Solomon Islands. We determined the shape and extent of the upper mantle slab window that has developed as a result of subduction of this ridge, the Woodlark spreading center. Seafloor magnetic anomalies on the Woodlark and Australia plates, formed at the Woodlark spreading center, indicate the geometry of now-subducted conjugates of the spreading center, both spreading segments and transform faults. Our synthesis of active seismicity, marine geophysical surveys, and on-land geological studies shows that the portion of the Solomon Islands above the Woodlark spreading center subduction region currently acts as a microplate. Thus, we determined relative velocities of the three plates involved at their triple junction and then constructed the boundaries of the Woodlark slab window accordingly. To first order, the slab window extends from similar to 157.0 degrees W to at least 160.0 degrees W and coincides with the southwestward-subducting slab of Ontong-Java Plateau lithosphere (i.e., Pacific plate) beneath the Solomon Islands block. We infer that heat advected into the slab window upper mantle by ridge subduction lowers mantle viscosity there to similar to 2 x 10(18) Pa.s, thus facilitating southwestward subduction and incipient subduction of the thick crust and lithosphere of the Ontong-Java Plateau beneath the Solomon Islands block.
New geochemical, U-Pb geochronology, and Sr-Nd-Hf isotope data provide evidence for the tectonic evolution of the Seventymile terrane in interior Alaska, USA. Ultramafic and mafic rocks of the Seventymile terrane are thought to represent components of a dismembered ophiolite and provide unique constraints on regional terrane evolution and accretion. The Seventymile ophiolite represents fragments of the Devonian to Permian Slide Mountain Ocean (SMO) that separated allochthonous and parautochthonous continental fragments of western North America. It now occurs as multiple thrust sheets containing Permian mafic and ultramafic rocks overlying and/or possibly imbricated with amphibolite-facies supracrustal rocks of allochthonous Yukon-Tanana terrane and parautochthonous North America. Seventymile klippen contain variably serpentinized peridotite, primarily harzburgite, low-grade meta-mafic rocks, and minor oceanic sedimentary rocks (argillite, chert, limestone, and metasandstone). Mafic rocks include gabbro to diabase, typically as dikes, veinlets, or rare massive stocks intruding peridotite. Mafic rocks also include greenstones of the Seventymile assemblage in klippen structurally underlying, and in shear zone contact with, ultramafic klippen. New trace element and radiogenic isotope data from mafic magmatic rocks associated with the Seventymile ultramafic bodies show evidence for a weakly subduction-modified mantle source, like the mantle source of normal mid-ocean-ridge basalt (N-MORB) or back-arc basin basalt (BABB). Seventymile assemblage greenstones are more heterogeneous. They range from N-MORB to enriched mid-ocean-ridge basalt (E-MORB) and ocean-island basalt (OIB), with a subset of samples indicative of continental arc affinity. These geochemistry results indicate that distinct tectonic environments are represented by at least two, and possibly three, lithological and structural units comprising the Seventymile terrane. Hf-Nd isotope systematics are consistent with a depleted MORB mantle (DMM)-like component that overlaps with Pacific MORB. Primary zircon is rare, but new in situ U-Pb data for gabbro and greenstone indicate ca. 274-272 Ma peak zircon and titanite crystallization. Scattered younger zircons define a ca. 255 Ma zircon peak and correspond to secondary crystallization associated with baddeleyite reaction of high-Si fluids during low-grade metamorphism. If Seventymile suites are contemporaneous, obduction associated with the closure of the SMO resulted in the stacking of ophiolitic packages representing distinct tectonomagmatic settings across the transition from pericontinental, to epicontinental, to distal ocean back-arc. Intrusions hosted in klippe of ultramafic rocks, plus the least subduction-modified greenstones underlying them, geologically and compositionally resemble Slide Mountain rocks of the Campbell Range formation in eastern Yukon and may provide a new piercing point across the Tintina fault.
The Bellingham basin, spanning onshore and offshore regions of northwest deforming under north- south shortening in the north Cascadia forearc. Accommodating the regional strain are Holocene- active faults within the basin that have been previously mapped both onshore by paleoseismic investigations and inferred through LiDAR imagery and aeromagnetic anomalies. In this study, we add new gravity measurements to an existing database and interpret previously acquired aeromagnetic and legacy marine seismic reflection data to define better the structure of the southern Bellingham basin and to assess Holocene and recently active faults within it. A regional- residual separation method reveals prominent northwest- and northeast- oriented contacts created by magnetic field gradients that align with fault traces, documented Holocene fault displacement, and basin structures discovered through geologic mapping and exploratory wells. One of these magnetic contacts is associated with the Sandy Point fault and extends northwest toward a cluster of seismicity near Point Roberts. The magnetic contact suggests that the fault may be longer than previously thought and thus capable of producing a larger earthquake and posing a greater earthquake hazard to the cities of Bellingham (Washington), Vancouver (British Columbia), and surrounding communities. The study also presents a model for the possible interplay among the Holocene- active Boulder Creek fault, Lummi Island fault, and Macaulay Creek thrust fault, the latter of which was responsible for the 1990 M 5 Deming earthquake and was the likely source of previous historic earthquakes.
The Dachaidan fault, an important active fault within the northern marginal fault system of the Qaidam basin along the northeastern margin of the Tibetan Plateau, has received limited scientific attention in the past. In recent years, several M>6 earthquakes near the fault have drawn significant research interest to its seismic potential. This study investigates the seismically active Dachaidan fault, a critical geological hazard threatening Dachaidan in western China's Qinghai Province. Absence of paleoseismic studies on this fault has resulted in structurally critical data gaps, constraining our understanding of large-magnitude earthquake recurrence, spatial segmentation, and rupture intensity distribution along the fault. To address this problem, we conducted trench excavation, analysis, and stratigraphic radiocarbon ages dating based on high-resolution satellite imagery and geomorphic field surveys. Five paleo-earthquakes were identified: E-I without specific age, E-II within 7800-7200 yr B.P., E-III within 6300-4800 yr B.P., E-IV within 3900 yr B.P. to similar to 2900 yr B.P., and E-V after 1900 yr B.P., which indicates four events since the Holocene with an average recurrence interval of 2000 +/- 300 yr in the past similar to 8000 yr B.P. Fault landforms and trenches suggest the most recent event occurred with a coseismic dextral offset of similar to 1.3 m. Empirical equations between coseismic offset and earthquake magnitude reveal that the magnitude of one event is 7.1 +/- 0.2.
In the North American Cordillera, a system of margin-parallel strike-slip faults and ductile shear zones accommodated northward translation of accreted terranes between the Cretaceous and Eocene. Constraining the timing of displacement on these structures is important for understanding the evolution of strain accommodation along the Cordilleran margin. The western Idaho shear zone (WISZ) is a north-striking, dextral-transpressional shear zone that deformed the boundary between the North American margin and accreted terranes to the west. At the latitude of the Salmon River canyon (45 degrees 25 ' N), available timing constraints demonstrate that shearing in the WISZ most likely initiated at ca. 108-104 Ma and continued after ca. 91 Ma. To more precisely constrain the timing of ductile shearing, we present new U-Pb zircon crystallization ages from granitoid dikes that intruded the WISZ along the Salmon River canyon. Dikes that crystallized between ca. 112 Ma and 101 Ma were deformed by the steeply east-dipping ductile fabrics that were generated during WISZ shearing. Undeformed dikes that crosscut these steeply east-dipping ductile fabrics yielded crystallization ages of ca. 85 Ma, which brackets the timing of cessation of WISZ shearing. This is consistent with published ca. 83-81 Ma 40Ar/39Ar biotite ages from the WISZ that approximate the timing of cooling through the quartz crystal-plastic transition. Our new timing constraints are consistent with published estimates for the timing of shearing further to the south in the WISZ and support correlation of the WISZ with dextral-transpressional shear zones along-strike in northwestern Nevada and the Sierra Nevada batholith that share similar kinematics and timing (ca. 105-83 Ma).
