Geological Society of America Bulletin最新文献

{"title":"On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt","authors":"Marco Filippi, Fabrice Jouffray, J. Lardeaux, Massimo Tiepolo, M. Spalla","doi":"10.1130/b37010.1","DOIUrl":"https://doi.org/10.1130/b37010.1","url":null,"abstract":"The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"1 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140091031","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Jura-Cretaceous synorogenic magmatism and relations to supercontinental rifting in the northwestern U.S. Cordillera","authors":"Keith D. Gray, K. Johnson, David A. Foster, V. Isakson","doi":"10.1130/b37203.1","DOIUrl":"https://doi.org/10.1130/b37203.1","url":null,"abstract":"Changes in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780−485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150−121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ∼35 km). Early pluton emplacement during right-oblique arc−continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane−continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastward-impinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"9 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139957676","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Upper crustal seismic velocity structure of the Hayward fault zone, San Francisco Bay, California, USA: Results from the 2016 East Bay Seismic Experiment (EBSI-16)","authors":"Luther Strayer, R. Catchings, Joanne Chan, Mark Goldman, Adrian McEvilly, John Suppe","doi":"10.1130/b36919.1","DOIUrl":"https://doi.org/10.1130/b36919.1","url":null,"abstract":"We developed Vp, Vs, Vp/Vs ratio, and Poisson’s ratio models of the uppermost crust (<4 km depth) from the eastern San Francisco (SF) Bay (California, USA) to near the Calaveras fault along a 15-km-long, linear profile. Upper crustal velocities are highly variable beneath, west, and well east of the Hayward fault. We observe eight notable features, from west to east: (1) Near San Francisco Bay, there is an ∼2-km-wide structure with high Vp/Vs ratios (up to 5) and Poisson’s ratios (up to 0.48) extending from the surface to the base of our model, which we suggest the structure is a near-vertical fault that lies along a straight-line projection between the Silver Creek fault to the south and the Point Richmond fault to the north. The structure may be part of an ∼90-km-long fault along the eastern SF Bay. (2) The western East Bay Plain, the lower lying area between the bay and the hills, includes up to 800 m of low-velocity sediments (Vp ∼1600−3000 m/s, Vs ∼500 m/s to ∼1000 m/s), underlain by higher velocity basement rocks (Vp ∼3000−5800 m/s; Vs ∼1000−1500 m/s). (3) Between ∼1 km and 3 km east of the Bay shoreline, sediments thin in a series of steps (likely faults) toward the Hayward fault. (4) Between ∼3 km west and ∼1 km east of the Hayward fault (at the East Chabot fault) at depths greater than 1 km, basement Vp (up to 6000 m/s) and Vs (up to 2800 m/s) are high, and Vp/Vs ratios (<2) and Poisson’s ratios (<0.3) are low, suggesting crystalline rocks. Furthermore, a near-vertical zone of low Vp/Vs ratios and Poisson’s ratios is between near-surface traces of the Hayward and East Chabot faults, likely corresponding to the San Leandro Gabbro of Ponce et al. (2003). (5) Eastward of the East Chabot fault in the upper 1.5 km, basement Vp (∼3000 m/s to ∼4200 m/s) and Vs (∼1200−2000 m/s) are lower than those west of the fault. (6) In the eastern Hayward/Oakland Hills, there are zones of laterally varying, high- and low-velocity (Vp ∼2500−3000 m/s) Jurassic−Cretaceous and Tertiary sediments in the shallow subsurface that likely extend much deeper than imaged. (7) Seismic energy that propagates westward from sources east of the Hayward fault (HF) appear weaker than energy that propagates eastward from sources west of the HF, suggesting that the HF acts as a partial barrier to shallow seismic energy propagation into the more populated eastern SF Bay area. (8) Unlike many fault zones, it appears that the active trace of the Hayward fault (in our study area) is not cored by a prominent, low-velocity zone relative to rocks to the east and west of the active trace. However, the active trace does mark a prominent change from relatively higher velocities to the west and lower velocities to the east.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"73 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139526626","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Insights into Caribbean tectonics from a detrital zircon U-Pb provenance study of siliciclastic strata in western and central Cuba","authors":"Patricia Ascanio-Pellon, D. Stockli, Daniel Ruiz-Arriaga, L. Stockli","doi":"10.1130/b37114.1","DOIUrl":"https://doi.org/10.1130/b37114.1","url":null,"abstract":"The tectonic affinity and origins of the Jurassic, siliciclastic San Cayetano and Constancia formations exposed in western and central Cuba in the Caribbean region remain debated. The scarcity of modern geochronologic studies on these Cuban units hampers both sedimentary provenance and palinspastic reconstructions, resulting in Caribbean models that tend to oversimplify the formation of Cuba and correlate the Jurassic strata to various regions such as North America, South America, the Yucatán margin, or the conceptual Caribeana domain. Using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS), we conducted a detailed detrital zircon (DZ) U-Pb provenance study of these Cuban siliciclastic strata that provides critical insights into understanding the formation of Cuba during the Early Jurassic rifting stages of Pangea. Results from 19 San Cayetano Formation samples show a dominant Oaxaquia (ca. 1 Ga) and Chiapas batholith (ca. 250 Ma) signature, while six Constancia samples display variable ca. 1 Ga and ca. 250 Ma grains. The Lower Cretaceous Polier Formation and the Paleocene Moncada Formation also display the same ca. 1 Ga and ca. 250 Ma signatures. After comparing these Cuban data with data from other regional DZ studies, we propose that that the San Cayetano and Constancia formations are correlative to the Todos Santos Formation located in the southwestern Yucatán region of Mexico. These Cuban units were predominantly deposited adjacent to the Chiapas batholith during the Early Jurassic in northwest−southeast-trending basins created by Pangean rifting. They were eventually sheared during eastward migration of the Caribbean plate and transported northward until they collided with the North American continent in the Paleogene. This DZ study provides new constraints on the tectonic provenance of western and central Cuba and improves plate tectonic reconstructions of the Caribbean.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"29 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139527058","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Late Mesozoic subduction-accretion in the southern Qiangtang: Insights from the Sumxi igneous complex of west-central Tibet","authors":"Chao Wang, Lin Ding, Fulong Cai, Deng Zeng, Jinxiang Li, Liyun Zhang, Yahui Yue","doi":"10.1130/b36945.1","DOIUrl":"https://doi.org/10.1130/b36945.1","url":null,"abstract":"Knowledge of the evolution of the Bangong-Nujiang Tethyan Ocean is crucial for reconstructing the paleography of the Tethyan Realm, given its significance as a key component of the eastern Tethys. Nonetheless, there has been uncertainty regarding both the timing and the processes involved in the closure of this ocean. This study focused on a 110−106 Ma igneous complex comprising basalts−basaltic andesites, trachyandesites, and granodiorites from the Sumxi area in the western part of the Qiangtang terrane of west-central Tibet. The basalts−basaltic andesites have SiO2 contents of 52.5−58.7 wt% and MgO contents of 2.89−4.63 wt%, and exhibit some arc-like geochemical signatures. However, these rocks also have elevated Nb contents (>10 ppm) and Nb/La ratios (>0.5), as well as enriched Sr-Nd isotopic composition [εNd(t) = −7.40 to −6.00], implying that they are products of a mantle source metasomatized by adakitic melts. The trachyandesites are characterized by intermediate compositions (SiO2 = 63.6−65.2 wt%), high Mg number (40−60), and more enriched εNd(t) values (−8.37 to −7.49). Comparing their geochemical composition to that of mélange rocks, it is postulated that these trachyandesites were formed through the partial melting of a mantle source including mélange matrix rocks within a subduction zone. The granodiorites exhibit adakitic geochemical features (Sr = 830.14−1032.70 ppm, Y = 14.86−15.37 ppm, Sr/Y = 54−68), indicating that they originated from the partial melting of a thickened lower crust in a continental arc setting. Our results, in combination with a synthesis of tectonomagmatism along the Bangong-Nujiang suture zone, provide convincing evidence for subduction of an oceanic plateau and subsequent slab roll-back. The Sumxi igneous complex, with its clear arc affinity, suggests that the Bangong-Nujiang Tethyan Ocean, or at least its western part, remained open until the late Early Cretaceous (ca. 106 Ma).","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"7 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139532663","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hydrologic windows into the crystalline basement and their controls on groundwater flow patterns across the Paradox Basin, western USA","authors":"M. Person, J.C. McIntosh, J.-H. Kim, C. Noyes, L. Bailey, S. Lingrey, R. Krantz, D. Lucero, P. Reiners, G. Ferguson","doi":"10.1130/b37063.1","DOIUrl":"https://doi.org/10.1130/b37063.1","url":null,"abstract":"Conceptual models of sedimentary basin groundwater flow systems typically assume that the crystalline basement acts as an impermeable boundary and can be neglected. In this study, we use hydrologic models constrained by isotopic and geochemical datasets to argue that the La Sal Mountains, Utah, USA, act as a hydrologic window into the Paradox Basin’s lower aquifer system and underlying crystalline basement. We conducted a sensitivity study in which we varied crystalline basement/laccolith permeability as well as fault zone connectivity along a cross-sectional transect from the La Sal Mountains to Lisbon Valley. When the crystalline basement/laccolith units are set at relatively permeable levels (10−14 m2), simulated tracers that include total dissolved solids, oxygen isotopic composition of pore fluids (δ18O), and groundwater residence times are in closest agreement with field measurements. Model results indicate that pore fluids in the basal aquifer system underlying the Paradox Formation confining unit are a mixture of relatively young meteoric fluids and older Paradox Formation brines. The presence of faults did not significantly modify fluid exchange between the upper and lower aquifer systems. This was due, in part, to underpressuring within the Paradox Formation. Our study concludes that the Paradox Basin represents a regional recharge area for the Colorado Plateau, with groundwater discharge occurring along the Colorado River within the Grand Canyon some 375 km away to the southwest. This is only possible with a permeable crystalline basement. Our findings help explain the genesis of Mississippi Valley-type ore deposits of the US Midcontinent, where the presence of a permeable basement may be useful in addressing issues related to solute mass and energy balance.","PeriodicalId":508784,"journal":{"name":"Geological Society of America Bulletin","volume":"43 9","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139532336","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}