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 El Jaralito batholith of Central Sonora is located in the footwall of two large-scale low-angle normal faults with opposite vergences. These faults structurally control the two southernmost metamorphic core complexes of the Basin and Range Province. To investigate the presence of previously proposed fracture zones that might dissect the range and to constrain the timing of rapid cooling and exhumation since the middle Miocene, we present new structural and thermochronological data. We integrate our results along with the previously published data by presenting new QTQt thermal history models for the El Jaralito batholith, enabling a refined interpretation of the timing of the extensional event responsible for its exhumation. Our models indicate that rapid cooling occurred between 25 Ma and 16 Ma along the southwest-dipping Mazatán−Puerta del Sol detachment, which we interpret as the active period of this detachment fault. In contrast, along the northeast-dipping El Amol low-angle normal fault, rapid cooling took place between ca. 25 Ma and 20 Ma and again from ca. 10 Ma to ca. 5 Ma. We attribute this younger cooling phase to renewed slip along high-angle normal faults and increased surface processes such as erosion. We also find no evidence supporting the existence of previously proposed E−W strike-slip transfer zones dissecting the range. Instead, our results reveal increasing metamorphic and exhumation gradients along both major normal faults—from southwest to northeast along the El Amol fault and from northeast to southwest along the Mazatán−Puerta del Sol fault. The lateral overlap of these fault tips defines a high-relief accommodation zone, which we interpret as the result of a scissor-like opening geometry during extension.

A series of cross sections, field relationships, and geochronologic and thermochronologic constraints demonstrate the existence of a Late Cretaceous−Miocene hybrid thick- and thin-skinned fold-and-thrust belt in the Western Cordillera of southern Peru (15°S−16°S). Shortening in the eastern half of the central Andes has been thoroughly investigated, but in southern Peru, the timing, position, and style of deformation within the western slope during Andean orogenesis remain poorly constrained. Thermal modeling of zircon fission-track results and the zircon and apatite (U-Th)/He data from the hanging wall of a northeast-verging basement-involved reverse fault in the Western Cordillera demonstrate rapid cooling from ca. 75 Ma to 55 Ma. We propose this cooling history reflects the onset, or continuation, of contractional deformation and is the earliest quantitative record of shortening in southern Peru. In the footwall of this structure, we documented a thin-skinned duplex and a fault-bend fold, attesting to the linkage within this hybrid thick- and thin-skinned fold-and-thrust belt. In the forearc, crosscutting relationships reveal a reactivated normal fault that was inverted during the Oligocene, part of a southwest-verging domain that constitutes the other flank of this fold-and-thrust belt and highlights the role of selective reactivation of preexisting structures. Timing constraints on other structures reveal spatially distributed deformation spanning the forearc and Western Cordillera during the Eocene to Miocene. The resulting interpretations highlight diverse structural styles, inherited structures, and distributed out-of-sequence deformation across the northern central Andes of southern Peru, which have implications for the spatiotemporal distribution and style of deformation throughout the central Andes.

It sure doesn’t seem like it, but this August marks Geosphere’s 20th year of publication. In 2005, when the Geological Society of America (GSA) first began publishing the online-only journal, scholarly journals were still psychologically tied to paper copies as the true form of “publication.” GSA’s bold, forward-thinking move into online-only publishing provided Geosphere authors with the ability to vault over color charges, static images, and extended printing times. When publishing in Geosphere, both then and now, full color is always an option; images are not only mobile, but also potentially interactive; and after a manuscript is typeset and proofread, it is immediately available. One feature that continues to distinguish Geosphere is its capacity for interactive publication, something that was novel at its launch and has matured into a defining characteristic (see Interactive Publication section). With a 2024 5-yr impact factor of 2.6, work published in Geosphere gets seen and cited.

Abstract We summarize evidence suggesting that magmatic accreted crust (subaerially accreted crust and submarine accreted oceanic crust) underlies a much larger portion of the Gulf of Mexico basin than has been appreciated previously. This conclusion suggests that traditional models of the Jurassic tectonic development of the basin, with wide areas of thinned continental crust underlying the salt basins, require significant modification. Using an updated compilation of long-offset, deep-penetrating offshore and reprocessed onshore seismic reflection profiles, we produced a new plate kinematic interpretation for the Gulf of Mexico linked to a process-based understanding of key tectonic events, their timing, and the distribution and structure of crustal types and pre-salt sediments observed across the Gulf of Mexico. The near-onshore and offshore Gulf of Mexico region is interpreted to be underlain by accreted magmatic crust formed during two phases of seafloor spreading: (1) an older rim of subaerial seafloor spreading marked by seaward-dipping reflectors that grade laterally into thin, accreted crust of an enigmatic nature overlain by an undeformed pre-salt sedimentary succession, and (2) younger production of more normal submarine Penrose crust. Continental breakup was diachronous, initiating at 200–190 Ma and becoming younger to the east, and marked by easterly trending extensional propagators preserved as basin systems along the western margin of Florida: the Mississippi Salt Basin, Apalachicola Basin, and Tampa Embayment. These propagators formed successively from north to south and west to east as the Gulf of Mexico spreading system adjusted to Yucatan rotation, before the spreading axis shifted southward into the Florida Straits. Phase 1 breakup initiated north of the present coast along the Houston magnetic anomaly, with little local evidence for upper-crustal faulting. Any crustal thinning there would thus have been a consequence of lower-crustal, depth-dependent continental extension. Regionally, unextended continental crust may be evidence of exploitation of preexisting Alleghanian-Ouachita weaknesses, of which the western continuation of the Suwannee shear zone is a prime candidate. Between phase 1 breakup (200–190 Ma) and 169 Ma, Yucatan migrated southeastward with South America (Gondwana) and rotated ~15° counterclockwise. This gradual southward shift of Gulf of Mexico accretion may have resulted from the region's extension axis encountering rheological strength barriers related to the Central Atlantic and proto–Caribbean Ocean margins. Each successive line of breakup was characterized by an initial phase of subaerial extrusions and development of seaward-dipping reflectors. Evidence suggests that these extension systems in the eastern Gulf of Mexico occurred in a widening and propagating basin network below global sea level, where continental sediments were deposited in subaerial and/or lacustrine environments and ultimately capped by evaporites. In phase 2, between 169 Ma and 140 Ma, Yucatan rotated an additional ~52° counterclockwise. Evaporites started forming in Bajocian (169 Ma) time during transient connection(s) to the global ocean. Fully marine conditions were established in Callovian (164 Ma) time as rotation continued, resulting in submarine accretion of Penrose crust. A major implication of this work is confirmation that prolific hydrocarbon systems can develop on “oceanic” (accreted) crust if ambient depositional environments are favorable.

The most recent models for the Mesoproterozoic (ca. 1.5−1.35 Ga) Picuris-Baraboo-Pinware orogeny call on transpression resulting from oblique, diachronous convergence at the southern margin of Laurentia to explain the patterns of deformation and magmatism along this transcontinental belt. The Marqueñas Formation metaconglomerate provides a rare opportunity to directly study the strain and kinematics of deformation within the intraplate Picuris segment of the orogen. Statistical analysis of deformed quartzite pebble and boulder dimensions shows flattening strain at the outcrop to map scale (kilometers to meters). Quartz crystallographic preferred orientation (CPO) records a combination of flattening and non-coaxial shear at the intraclast scale (millimeters to micrometers). Kinematic vorticity axes, determined by crystallographic vorticity analysis on quartzite clasts, are well preserved despite widespread static recrystallization and align with principal strain axes determined from quartz CPO. The compatibility of strain and kinematic vorticity data indicates that flattening was produced in progressive, 3-D general shear. Outcrop-scale and map-scale structural relations link Marqueñas Formation flattening strain to oblique slip, with components of north-directed thrusting and dextral shear, on the Plomo-Pecos shear zone. Quartz flattening CPO yields predominantly crossed-girdle c-axis figures with opening angles of 69°−92° and a mean of 80°. Quartz c-axis opening angle thermometry yields deformation temperatures of 601 ± 50 °C, suggesting that flattening was synchronous with prograde to peak metamorphism during the second phase of deformation (D2) in the Picuris Mountains. We conclude that flattening of the Marqueñas Formation records inclined transpression within the Picuris orogen, consistent with oblique convergence along the Mesoproterozoic Laurentian plate margin.

Many mountain belts are built through repeated collision, and in the case of orogenies closely spaced in time, determining when one orogeny ends and another begins can be challenging. The southern Appalachian mountains were formed by three mountain-building events closely spaced in time, including the Taconic (ca. 480−440 Ma), Neoacadian (ca. 375−340 Ma), and Alleghanian (ca. 330−265 Ma) orogenies. Notably, the end of the Neoacadian and the beginning of the Alleghanian are only separated by ∼10 m.y., and some published dates record deformation and metamorphism in the eastern Blue Ridge during this interval, blurring the boundary between these two discrete events. The Toxaway dome, located along the North Carolina−South Carolina, USA, border at the eastern edge of the eastern Blue Ridge, is a structural dome cored by Mesoproterozoic Toxaway Gneiss and surrounded by the younger Tallulah Falls Formation. Previous ages constraints from the Toxaway dome (343 and 338 Ma U-Pb zircon ages) make it an ideal location to explore whether there was continuous deformation during this period of supposed quiescence between the Neoacadian and Alleghanian orogenies. We used optical microscopy and electron backscatter diffraction in quartz to determine deformation temperatures, thermobarometry to determine metamorphic pressure-temperature conditions, and monazite petrochronology to determine the timing of deformation. Quartz and feldspar recrystallization fabrics parallel to dome-defining fabrics indicate deformation occurred at amphibolite-facies conditions, which is corroborated by our pressure-temperature estimates of 0.67−0.8 ± 0.12 GPa and 661−689 ± 25 °C. Monazite grains that record the timing of reactions of garnet growth and breakdown range from 342 ± 4.8 Ma to 296 ± 10.8 Ma, bridging the interval between the Neoacadian and Alleghanian orogenies. Three samples from the nearby Tallulah Falls dome, which occupies a similar structural position along the edge of the eastern Blue Ridge in Georgia, record monazite dates of 334 ± 4.2 Ma to 304 ± 4.8 Ma, indicating there was tectonic activity in this region before the commonly defined beginning of the Alleghanian orogeny. We propose (1) there was no period of quiescence between the Neoacadian and Alleghanian orogenies in the eastern Blue Ridge, and (2) deformation during this time was at higher temperatures and pressures than previously reported.

Abstract The Mineral King pendant is an ~15-km-long, northwest-striking assemblage of Permian to mid-Cretaceous metavolcanic and metasedimentary rocks that form a steeply dipping wall-rock screen between large mid-Cretaceous plutons of the Sierra Nevada batholith (California, USA). Pendant rocks are generally well layered and characterized by northwest-striking, steeply dipping, layer-parallel cleavage and flattening foliation and steeply northwest-plunging stretching lineation. Northwest-elongate lithologic units with well-developed parallel layering and an absence of prominent faults or shear zones suggests a degree of stratigraphic continuity. However, U-Pb zircon dating of felsic metavolcanic and volcanosedimentary rocks across the pendant indicates a complex pattern of structurally interleaved units with ages ranging from 277 Ma to 101 Ma. We utilize a compilation of 39 existing and new U-Pb zircon ages and four reported fossil localities to construct a revised geologic map of the Mineral King pendant that emphasizes age relationships rather than lithologic or stratigraphic correlations as in previous studies. We find that apparently coherent lithologic units are lensoidal and discontinuous and are cryptically interleaved at meter to kilometer scales. Along-strike facies changes and depositional unconformities combine with kilometer-scale tight folding and structural imbrication to create a complex map pattern with numerous discordant units. Discrete faults or major shear zones are not readily apparent in the pendant, although such structures are necessary to produce the structural complications revealed by our new mapping and U-Pb dating. We interpret the Mineral King pendant to be structurally imbricated by a combination of kilometer-scale tight to isoclinal folding and cryptic faulting, accentuated by, and eventually obscured by, pervasive flattening and vertical stretching that preceded and accompanied emplacement of the bounding mid-Cretaceous plutons. Deformation in the Mineral King pendant represents a significant episode of pure-shear-dominated transpression between ca. 115 Ma and 98 Ma that adds to growing evidence for a major mid-Cretaceous transpressional orogenic event affecting the western U.S. Cordillera.

Abstract The Cordillera Blanca and Cordillera Huayhuash contain some of the highest topography in the Andes and provide insight into tectonomagmatic processes associated with the onset of flat-slab subduction. These adjacent ranges shared a similar history of deformation and exhumation prior to the late Miocene, when synconvergent extension began in the Cordillera Blanca. Magmatism in the Cordillera Huayhuash has been inferred as coeval with magmatism in the Cordillera Blanca. Yet, extension, which has been correlated with magmatic heat flow, is limited to the Cordillera Blanca. New zircon U-Pb dates and trace and rare earth element concentrations from the Cordillera Blanca batholith and the Huayllapa pluton in the Cordillera Huayhuash and reassessment of existing zircon data help to characterize regional magmatic processes prior to the establishment of flat-slab subduction. Two compositionally distinct samples of the Huayllapa pluton yielded mean ages of 24.8 ± 0.4 Ma and 25.4 ± 0.8 Ma. In contrast, the Cordillera Blanca batholith has a protracted crystallization history postdating that of the Cordillera Huayhuash by up to 20 m.y. Miocene magmatism in the Cordillera Blanca began at 19 Ma and ended with injection of large volumes of geochemically distinct, mantle-derived magma from 10 to 5 Ma. We suggest that 6–5 Ma magmatism in the Cordillera Blanca promoted elevated heat flow and reduced shear strength, which facilitated extensional shearing along the western slopes of the range, whereas colder amagmatic crust in the Cordillera Huayhuash inhibited southward propagation of faulting. Our data demonstrate that the linkages between magmatism and elevated heat flow identified in the Cordillera Blanca are important driving processes in initiating extension in cordilleran-style orogenies.

Abstract The chemical and isotopic characteristics of a solidified pluton represent the integration of magmatic and sub-solidus processes operating across a range of spatial and temporal scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired requires the combination of multiple tools tracing processes at different time and length scales. We combine whole-rock oxygen and Sr-Nd isotopes, zircon oxygen isotopes and trace elements, and mineral compositions with published high-precision U-Pb zircon geochronology to evaluate differentiation within the bimodal Guadalupe Igneous Complex, Sierra Nevada, California (USA). The complex was constructed in ~300 k.y. between 149 and 150 Ma. Felsic magmas crystallized as centimeter- to meter-sized segregations in gabbros in the lower part of the complex and as granites and granophyres structurally above the gabbros. A central mingling zone separates the mafic and felsic units. Pluton-wide δ18O(whole-rock), δ18O(zircon), and Sr-Nd isotopic ranges are too large to be explained by in situ, closed-system differentiation, instead requiring open-system behavior at all scales. Low δ18O(whole-rock) and δ18O(zircon) values indicate assimilation of hydrothermally altered marine host rocks during ascent and/or emplacement. In situ differentiation processes operated on a smaller scale (meters to tens of meters) for at least ~200 k.y. via (1) percolation and segregation of chemically and isotopically diverse silicic interstitial melt from a heterogeneous gabbro mush; (2) crystal accumulation; and (3) sub-solidus, high-temperature, hydrothermal alteration at the shallow roof of the complex to modify the chemical and isotopic characteristics. Whole-rock and mineral chemistry in combination with geochronology allows deciphering open-system differentiation processes at the outcrop to pluton scale from magmatic to sub-solidus temperatures over time scales of hundreds of thousands to millions of years.