New articles for Geosphere posted online in June

Geological Society of America

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Locations studied include the Turkish-Iranian Plateau and the Zagros Mountain Belt; West Kunlun, Northwestern China; and the Little Cottonwood stock, Utah, USA. You can find these articles at https://geosphere.geoscienceworld.org/content/early/recent .

Seismic attenuation tomography of the Sn phase beneath the Turkish-Iranian Plateau and the Zagros mountain belt

Ayoub Kaviani; Eric Sandvol; Wenfei Ku; Susan L. Beck; Niyazi Türkelli …

Abstract: The Turkish-Iranian Plateau and the Zagros highlands are among the most prominent physiographic features in the Middle East and were formed as a result of continental collision between the Arabian and Eurasian plates. To better understand the nature of the lithospheric mantle and the origin of the observed seismic anomalies in this region, we investigated seismic attenuation of the uppermost mantle by detailed measurements of the quality factor of the Sn seismic phase (Sn Q). To that end, we collected a large data set consisting of 30 years (1990–2020) of waveforms recorded by 1266 permanent and temporary seismic stations, applying both the two-station method (TSM) and reverse two-station method (RTM) to measure path-averaged Sn Q. Finally, we performed a tomographic inversion on the path-averaged Sn Q to map the lateral variations of the upper-mantle attenuation across the northern Middle East. Our Sn attenuation maps show moderately low Q (Q values (>350) beneath the Zagros and northern edge of the Arabian plate. Furthermore, our Sn Q model is broadly consistent with seismic velocity models in the region suggesting that most of the seismic anomalies are the result of thermal rather than compositional effects.

View article: https://pubs.geoscienceworld.org/gsa/geosphere/article/doi/10.1130/GES02503.1/614763/Seismic-attenuation-tomography-of-the-Sn-phase

Mafic magma-driven magmatic processes and compositional variation in granitic pluton construction: The Buya intrusion of West Kunlun, Northwestern China

Chao Wang; Liang Liu; Wen-qiang Yang; Yu-ting Cao; R. Hugh Smithies

Abstract: To investigate the direct evidence for a number of physico-chemical processes related to pluton construction and growth, we examine the Buya pluton of West Kunlun in Northwestern China, which emplaced within the 455–460 Ma time frame. Field observations, geochemical data, and thermodynamic modeling show that mafic dikes of the Buya pluton were conduits for magma chamber replenishment during pluton construction. These mafic inputs, and the enclaves that resulted from them, induced compaction of the semi-consolidated, crystal-rich, felsic mushes below them. The accumulation of highly silicic, fine-grained granite at the top of the Buya pluton is the result of episodic melt segregation events from these mushes. This sequence of events may reflect a common process that promotes compositional variation in granite suites. Combined geochemical and Hf- and Nd-isotopic data suggest that parental magmas of the mafic sheet and enclave are similar to sanukitoid, which is potentially consistent with a mantle peridotitic source metasomatized by slab melts. These mafic magmas intruded the lower crust where the original magma was modified by mafic lower-crust melt. Following emplacement at shallow crustal levels of the mafic inputs (~3.7 kbar, ~5.3 km, constrained by amphibole geobarometry), the felsic mush evolved through the extraction of interstitial melts driven by hybridization with episodic inputs of mafic magmas as well as crystal consequent accumulation and fractional crystallization of plagioclase, hornblende, and accessory phases such as allanite, apatite, and zircon. This fractional crystallization process may also provide an explanation for the apparently high Sr/Y features in some silicic high-K, calc-alkaline magmas.

View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02451.1/614732/Mafic-magma-driven-magmatic-processes-and

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

Collin G. Jensen; Eric H Christiansen; Jeffrey D. Keith

Abstract: Many 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.

View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02369.1/614461/Multi-stage-construction-of-the-Little-Cottonwood

Seismostratigraphic analysis of Lake Cahuilla sedimentation cycles and fault displacement history beneath the Salton Sea, California, USA

Daniel S. Brothers; Neal W. Driscoll; Graham M. Kent; Robert L. Baskin; Alistair J. Harding …

Abstract: The Salton Trough (southeastern California, USA) is the northernmost transtensional stepover of the Gulf of California oblique-divergent plate boundary and is also where the southern terminus of the San Andreas fault occurs. Until recently, the distribution of active faults in and around the Salton Sea and their displacement histories were largely unknown. Subbottom CHIRP (compressed high-intensity radar pulse) surveys in the Salton Sea are used to develop a seismic facies model for ancient Lake Cahuilla deposits, a detailed map of submerged active faults, and reconstructed fault displacement histories during the late Holocene. We observe as many as fourteen Lake Cahuilla sequences in the Salton Sea (last ~3 k.y.) and develop a chronostratigraphic framework for the last six sequences (last ~1200 yr) by integrating CHIRP data and cone penetrometer logs with radiocarbon-dated stratigraphy at an onshore paleoseismic site. The Salton Sea contains northern and southern subbasins that appear to be separated by a tectonic hinge zone, and a subsidence signal across hinge-zone faults of 6–9 mm/yr (since ca. A.D. 940) increases toward the south to >15 mm/yr. The faults mapped to the south of the hinge zone appear to accommodate transtension within the San Andreas–Imperial fault stepover. We identify 8–15 distinct growth events across hinge-zone faults, meaning growth occurred at least once every 100 yr since Lake Cahuilla sedimentation began. Several faults offset the top of the most recent Lake Cahuilla highstand deposits, and at least two faults have offset the Salton Sea flood deposits. Active faults and folds were also mapped to a limited extent within the northern subbasin and display growth, but their kinematics and rupture histories require further study. The broad distribution of active faulting suggests that strain between the San Andreas, San Jacinto, and Imperial faults is highly distributed, thus discrepancies between geologic and geodetic slip-rate estimates from these major fault systems are to be expected.

View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02468.1/614460/Seismostratigraphic-analysis-of-Lake-Cahuilla

U-Pb and fission-track data from zircon and apatite resolve latest- and post-Alleghanian thermal histories along the Fall Line of the Atlantic margin of the southeastern United States

William H. Craddock; Paul B. O’Sullivan; Ryan J. McAleer

Abstract: Although the Atlantic continental margin of the eastern United States is an archetypal passive margin, episodes of rejuvenation following continental breakup are increasingly well documented. To better constrain this history of rejuvenation along the southern portion of this continental margin, we present zircon U-Pb (ZUPb) age, zircon fission-track (ZFT) age, apatite U-Pb (AUPb) age, and apatite fission-track (AFT) age and length data from six bedrock samples. The samples were collected along the boundary between the exposed Appalachian hinterland (Piedmont province) and the updip limit of passive margin strata (Coastal Plain province). The samples were collected from central Virginia southward to the South Carolina–Georgia border. ZUPb age distributions are generally consistent with geologic mapping in each of the sample areas. The AUPb data are highly discordant owing to high common-Pb abundances, but for two plutons at the northern and southern ends of the sample area, they define a discordia regression line that indicates substantial Permo-Triassic exhumation-driven cooling. ZFT age distributions are highly dispersed but define central values ranging from Permian to Jurassic. AFT data mostly appear to define a singular underlying cooling age, generally approximately Jurassic or Early Cretaceous. Apatite fission tracks are moderately long (mean lengths in the range of ~13.5 µm), however track lengths for one sample in central North Carolina are shorter (~12.5 µm). To interpret the post-breakup thermal history, we present inverse models of time-temperature history for the five plutonic samples. The models show a history of (1) rapid cooling (>10 °C/m.y.) from deep-crustal to near-surface temperatures by the Triassic, (2) hundreds of degrees of Triassic reheating, (3) Jurassic–Early Cretaceous cooling (at rates of 1–10 °C/m.y.), and (4) slow Late Cretaceous–Cenozoic cooling (~1 °C/m.y.). An additional suite of forward models is presented to further evaluate the magnitude of maximum Triassic reheating at one sample site that is particularly well constrained by thermal maturity data. The model results and geologic reasoning suggest that the inverse models may overestimate Triassic paleotemperatures but that other aspects of the inverse modeling are robust. Overall, this thermal history can be reconciled with several aspects of the lithostratigraphy of distal parts of the continental margin, including the lack of Jurassic–earliest Cretaceous strata beneath the southern Atlantic coastal plain and Cretaceous–Cenozoic grain-size trends.

View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02447.1/614326/U-Pb-and-fission-track-data-from-zircon-and

GEOSPHERE articles are available at https://geosphere.geoscienceworld.org/content/early/recent

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