K. Ogata, A. Weert, P. Betlem, T. Birchall, K. Senger
Sedimentary injectites are increasingly documented in many hydrocarbon plays at various scales, either interpreted as potential risks (e.g., top-seal bypass, a drilling hazard) or benefits (e.g., reservoir interconnection, increased hydrocarbon volumes) for production operations. As such, they have potential critical implications for the assessment of suitability for CO2 injection and sequestration. Detailed characterization of such units, especially in terms of diagenesis and (paleo) fluid flow, is directly achievable at outcrop scale, overcoming dimensional and time constraints otherwise unresolvable at seismic scale. Two sedimentary injection complexes have been recognized in the succession of the Middle Jurassic–Lower Cretaceous Agardhfjellet Formation exposed at Deltaneset, central Spitsbergen, Norway, at different stratigraphic levels. The upper complex comprises two main clastic dikes characterized by different orientation and consolidation, tapering out vertically (upward and downward) within a stratigraphic thickness and lateral extent of more than 50 m and 200 m, respectively. The lower complex is coarser grained, made up by a network of interconnected dikes and sills, shooting off from isolated lenticular and morphologically articulated bodies, interpreted as sedimentary intrusions linked to seafloor extrusion (sand volcano). Petrographic and micromorphological analyses were used to identify the underlying lithologies of the Late Triassic to Middle Jurassic Wilhelmøya Subgroup as the possible source of this remobilized material for both the upper and lower complexes. This subsurface remobilization and consequent intrusion were first achieved in the lower complex during the Late Jurassic at shallow burial conditions, and then at higher confinement pressure for the upper complex, probably during the Late Cretaceous. These results highlight how field data can be used to constrain long-lived spatiotemporal relationships of sedimentary intrusions, allowing a finely tuned upscaling of seismic data and interpretations.
{"title":"Shallow and deep subsurface sediment remobilization and intrusion in the Middle Jurassic to Lower Cretaceous Agardhfjellet Formation (Svalbard)","authors":"K. Ogata, A. Weert, P. Betlem, T. Birchall, K. Senger","doi":"10.1130/ges02555.1","DOIUrl":"https://doi.org/10.1130/ges02555.1","url":null,"abstract":"Sedimentary injectites are increasingly documented in many hydrocarbon plays at various scales, either interpreted as potential risks (e.g., top-seal bypass, a drilling hazard) or benefits (e.g., reservoir interconnection, increased hydrocarbon volumes) for production operations. As such, they have potential critical implications for the assessment of suitability for CO2 injection and sequestration. Detailed characterization of such units, especially in terms of diagenesis and (paleo) fluid flow, is directly achievable at outcrop scale, overcoming dimensional and time constraints otherwise unresolvable at seismic scale.\u0000 Two sedimentary injection complexes have been recognized in the succession of the Middle Jurassic–Lower Cretaceous Agardhfjellet Formation exposed at Deltaneset, central Spitsbergen, Norway, at different stratigraphic levels. The upper complex comprises two main clastic dikes characterized by different orientation and consolidation, tapering out vertically (upward and downward) within a stratigraphic thickness and lateral extent of more than 50 m and 200 m, respectively. The lower complex is coarser grained, made up by a network of interconnected dikes and sills, shooting off from isolated lenticular and morphologically articulated bodies, interpreted as sedimentary intrusions linked to seafloor extrusion (sand volcano). Petrographic and micromorphological analyses were used to identify the underlying lithologies of the Late Triassic to Middle Jurassic Wilhelmøya Subgroup as the possible source of this remobilized material for both the upper and lower complexes. This subsurface remobilization and consequent intrusion were first achieved in the lower complex during the Late Jurassic at shallow burial conditions, and then at higher confinement pressure for the upper complex, probably during the Late Cretaceous. These results highlight how field data can be used to constrain long-lived spatiotemporal relationships of sedimentary intrusions, allowing a finely tuned upscaling of seismic data and interpretations.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":"24 11","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41294929","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
D. Rouby, A. Loparev, D. Chardon, Flora Bajolet, M. Dall’asta, F. Paquet, C. Fillon, J. Roig, J. Ye
Sediment routing systems of cratonic domains have not been studied extensively because their relief and erosion rates are very low, although their vast dimensions allowed them to contribute to a significant proportion of the sediments exported to the global ocean. To gain further insights into the behavior of cratonic sediment routing systems at geological time scales, we investigated the Guiana Shield and its Atlantic rifted margin (i.e., the Guiana-Suriname and Foz do Amazonas Basins, northern South America) over the Meso-Cenozoic with an emphasis on paleoenvironment and accumulation histories of the offshore sediments. We show that the basins of the Guiana Shield rifted margin record (1) periods of very low siliciclastic supply concomitant with the development of carbonate platforms, alternating with (2) phases of higher siliciclastic supply associated with sand-dominated clastic deposits and turbidites. Low siliciclastic supplies reflect either very limited rift-related relief growth and erosion such as during the Central Atlantic rifting in the Late Jurassic or intense lateritic weathering of the cratonic source area during Paleogene–Miocene climate optima. Higher siliciclastic supplies correspond either to (1) periods of rapid rift-related relief growth and erosion such as during the Equatorial Atlantic rifting (Early Cretaceous), (2) periods of drainage reorganization over a steadily eroding cratonic domain (Late Cretaceous), or (3) periods of tapping of sediments stored in the Andean retro-foreland basins via the present-day Orinoco and Amazon Rivers (Plio-Pleistocene).
{"title":"Sediment routing systems to the Atlantic rifted margin of the Guiana Shield","authors":"D. Rouby, A. Loparev, D. Chardon, Flora Bajolet, M. Dall’asta, F. Paquet, C. Fillon, J. Roig, J. Ye","doi":"10.1130/ges02561.1","DOIUrl":"https://doi.org/10.1130/ges02561.1","url":null,"abstract":"Sediment routing systems of cratonic domains have not been studied extensively because their relief and erosion rates are very low, although their vast dimensions allowed them to contribute to a significant proportion of the sediments exported to the global ocean. To gain further insights into the behavior of cratonic sediment routing systems at geological time scales, we investigated the Guiana Shield and its Atlantic rifted margin (i.e., the Guiana-Suriname and Foz do Amazonas Basins, northern South America) over the Meso-Cenozoic with an emphasis on paleoenvironment and accumulation histories of the offshore sediments.\u0000 We show that the basins of the Guiana Shield rifted margin record (1) periods of very low siliciclastic supply concomitant with the development of carbonate platforms, alternating with (2) phases of higher siliciclastic supply associated with sand-dominated clastic deposits and turbidites. Low siliciclastic supplies reflect either very limited rift-related relief growth and erosion such as during the Central Atlantic rifting in the Late Jurassic or intense lateritic weathering of the cratonic source area during Paleogene–Miocene climate optima. Higher siliciclastic supplies correspond either to (1) periods of rapid rift-related relief growth and erosion such as during the Equatorial Atlantic rifting (Early Cretaceous), (2) periods of drainage reorganization over a steadily eroding cratonic domain (Late Cretaceous), or (3) periods of tapping of sediments stored in the Andean retro-foreland basins via the present-day Orinoco and Amazon Rivers (Plio-Pleistocene).","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45171727","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Aaron Conley, Samantha Ramirez, J. Ricketts, R. Langford, T. Pavlis, M. Heizler
Rocks exposed in the southern Indio Mountains provide an important geologic record of the transition from Laramide contraction to Rio Grande rift extension. These rocks include (1) a package of folded and faulted Mesozoic rocks deformed during Laramide compression, (2) Eocene volcanic and sedimentary rocks that are tilted (but not folded) and fill a paleovalley, and (3) Miocene conglomerate deposited within a Rio Grande rift half-graben. We present a combination of geologic mapping, sedimentological and structural analysis, and geothermochronology to document the timing and nature of this transition in western Texas as an important comparison with the more thoroughly studied tectonic and erosional history of New Mexico and Colorado. Detailed geologic mapping and cross-section reconstruction reveal a highly irregular angular unconformity overlying Mesozoic rocks and that post-Laramide topography funneled local Eocene deposition through an EW-trending paleovalley. New and existing 40Ar/39Ar geochronology constrains the timing of paleovalley deposition to 38.1–36.6 Ma. The presence of megabreccia units, coupled with paleoflow analysis, argue that major Laramide topographic relief in western Texas persisted into the Middle Eocene and that detritus was shed toward the Tornillo basin to the east. These data, when viewed within the context of regional tectonic patterns, suggest that Laramide deformation in western Texas had ceased by 38.1 Ma, although they do not preclude translation along an underlying buried thrust. Eocene paleovalley cutting and filling are reminiscent of widespread Eocene erosion and fluvial deposition that occurred in Colorado and northern New Mexico and suggest that this event may have been more widespread than previously thought, extending into Chihuahua and western Texas. Subsequent Rio Grande rift extension occurred primarily along several NW-striking normal faults that dissected the older structures and the paleovalley and led to deposition of conglomerate within a half-graben. Apatite (U-Th)/He thermo- chronology applied to normal fault footwall rocks indicates exhumational cooling was occurring by 27 Ma, and detrital sanidine 40Ar/39Ar geochronology of basin fill indicates a maximum depositional age of 11.9 Ma. Clast count data from the conglomerate show a prominent unroofing trend, wherein clasts include locally derived Mesozoic units and Eocene volcanic rocks.
{"title":"Reconstructing the erosional and tectonic record of Laramide contraction to Rio Grande rift extension, southern Indio Mountains, western Texas, USA","authors":"Aaron Conley, Samantha Ramirez, J. Ricketts, R. Langford, T. Pavlis, M. Heizler","doi":"10.1130/ges02620.1","DOIUrl":"https://doi.org/10.1130/ges02620.1","url":null,"abstract":"Rocks exposed in the southern Indio Mountains provide an important geologic record of the transition from Laramide contraction to Rio Grande rift extension. These rocks include (1) a package of folded and faulted Mesozoic rocks deformed during Laramide compression, (2) Eocene volcanic and sedimentary rocks that are tilted (but not folded) and fill a paleovalley, and (3) Miocene conglomerate deposited within a Rio Grande rift half-graben. We present a combination of geologic mapping, sedimentological and structural analysis, and geothermochronology to document the timing and nature of this transition in western Texas as an important comparison with the more thoroughly studied tectonic and erosional history of New Mexico and Colorado. Detailed geologic mapping and cross-section reconstruction reveal a highly irregular angular unconformity overlying Mesozoic rocks and that post-Laramide topography funneled local Eocene deposition through an EW-trending paleovalley. New and existing 40Ar/39Ar geochronology constrains the timing of paleovalley deposition to 38.1–36.6 Ma. The presence of megabreccia units, coupled with paleoflow analysis, argue that major Laramide topographic relief in western Texas persisted into the Middle Eocene and that detritus was shed toward the Tornillo basin to the east. These data, when viewed within the context of regional tectonic patterns, suggest that Laramide deformation in western Texas had ceased by 38.1 Ma, although they do not preclude translation along an underlying buried thrust. Eocene paleovalley cutting and filling are reminiscent of widespread Eocene erosion and fluvial deposition that occurred in Colorado and northern New Mexico and suggest that this event may have been more widespread than previously thought, extending into Chihuahua and western Texas.\u0000 Subsequent Rio Grande rift extension occurred primarily along several NW-striking normal faults that dissected the older structures and the paleovalley and led to deposition of conglomerate within a half-graben. Apatite (U-Th)/He thermo- chronology applied to normal fault footwall rocks indicates exhumational cooling was occurring by 27 Ma, and detrital sanidine 40Ar/39Ar geochronology of basin fill indicates a maximum depositional age of 11.9 Ma. Clast count data from the conglomerate show a prominent unroofing trend, wherein clasts include locally derived Mesozoic units and Eocene volcanic rocks.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47396779","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or break-away zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligocene- aged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal fault-slip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slab-driven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere pre-conditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transten
加利福尼亚州内华达山脉的隆起历史是一个长期存在分歧的话题，其中大部分集中在内华达山脉南部东侧正断层边界范围内滑动的时间和性质上。正断层滑动的历史对于表征内华达山脉的隆起历史以及表征驱动并继续驱动正断层的地质和地球动力学因素非常重要。为了解决这些问题，我们完成了新的结构研究和广泛的磷灰石（U-Th）/He（AHe）热测年法，这些样品是从内华达山脉南部正断层（SNNF）向东倾斜的下盘中的三个垂直断面采集的。我们对基岩断层面的结构研究表明，SNNF是一条陡峭（~70°）的东倾正断层。新的AHe数据揭示了两个海拔不变的AHe年龄阵列，表明了两个不同的冷却和剥露期，我们将其解释为在约28-27 Ma沿SNNF开始的正断层，在约17-13 Ma开始的第二阶段正断层。我们认为，从渐新世晚期开始，SNNF标志着现在长期稳定的西部界限或断裂带，盆地和山脉。沿SNNF的滑动，以及下盘的相关卸载，可能导致内华达山脉在新生代晚期出现两个时期的隆起。受北美板块沿法拉隆-北美俯冲带边界向西运动的驱动，以及重力不稳定的北部和南部盆地和山脉对寒冷的内华达山脉的推动，海沟退缩，可能驱动了沿SNNF的渐新世晚期正常滑动，以及盆地和山脉内类似的但通常是局部和较小的延伸。我们推测，通过用热浮力软流圈替换变陡的Farallon板块，厚的原始盆地和山脉岩石圈为渐新世晚期的伸展做好了准备。虽然到渐新世晚期，Farallon板块的变陡尚未到达内华达山脉南部，但我们推测，随着内华达山脉地块因海沟退缩和重力势能而被拉向西部，沿SNNF的渐新世晚期滑动重新激活了白垩纪晚期右旋剪切带。沿SNNF的主要中新世中期正断层滑动历史与沿盆地和山脉（包括东部相邻的Inyo和White山脉）边界正断层记录的高震级滑动是同一时期的，这表明这一伸展时期是一个重大的区域构造事件。我们推断，沿着Juan de Fuca-北美俯冲带边界的板块驱动的海沟后退和叠加在大陆岩石圈上的南部祖先Cascade山脉的顺时针旋转共同驱动了这一中新世中期正滑事件，沿着SNNF向东延伸穿过盆地和山脉。沿着太平洋-北美洲板块边界的跨张性板块运动，很可能是一个不断增长的板块窗口，继续推动沿着SNNF和西部盆地和山脉的延伸，但直到大约11 Ma，当门多西诺三重交汇点到达我们最北（U-Th）/He样带的纬度时。
{"title":"Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range","authors":"Jeffrey Lee, D. Stockli, A. Blythe","doi":"10.1130/ges02574.1","DOIUrl":"https://doi.org/10.1130/ges02574.1","url":null,"abstract":"The uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or break-away zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligocene- aged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal fault-slip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slab-driven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere pre-conditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transten","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48435968","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xuxuan Ma, Zhiqin Xu, A. Lusk, S. Erdmann, Xijie Chen, Shiwei Ma
The Gangdese belt of the southern Lhasa terrane (southern Tibet) records a Chilean-type accretionary orogeny driven by subduction of Neotethyan oceanic lithosphere, prior to Indo-Asian collision and formation of the Tibetan Plateau. We present detailed structural analysis of outcrops and a drill core in the Jiama copper ore district along with 40Ar-39Ar cooling ages from white mica, plagioclase, and potassium feldspar and zircon U-Pb geochronology of granitoids and sandstone. These data add new constraints to the formation of a major angular unconformity, deformation along and within the footwall of the Gangdese décollement, and the coupling between deformation and magmatism. Structural analysis indicates that top-to-the-south motion along the décollement produced south-vergent folding and thrusting of Upper Jurassic to Cretaceous strata in the Gangdese back-arc basin. A synthesis of new and compiled age data reveals that the décollement and associated south-vergent deformation occurred between ca. 90 and 65 Ma, contemporaneous with the formation of a major ca. 85–69 Ma angular unconformity between the overlying Paleocene–Eocene Linzizong Formation and the underlying Upper Cretaceous Shexing Formation. We posit that this deformation in the Gangdese belt resulted from flat-slab subduction of the Neotethyan oceanic slab beneath the southern margin of the Asian continent. A flat-slab subduction geometry is consistent with previously documented synchronous thrusting in the forearc and back-arc basins as well as the observed arc magmatic lull of the Gangdese belt between ca. 80 and 65 Ma.
{"title":"Late Cretaceous tectonothermal events of the Gangdese belt, southern Tibet","authors":"Xuxuan Ma, Zhiqin Xu, A. Lusk, S. Erdmann, Xijie Chen, Shiwei Ma","doi":"10.1130/ges02602.1","DOIUrl":"https://doi.org/10.1130/ges02602.1","url":null,"abstract":"The Gangdese belt of the southern Lhasa terrane (southern Tibet) records a Chilean-type accretionary orogeny driven by subduction of Neotethyan oceanic lithosphere, prior to Indo-Asian collision and formation of the Tibetan Plateau. We present detailed structural analysis of outcrops and a drill core in the Jiama copper ore district along with 40Ar-39Ar cooling ages from white mica, plagioclase, and potassium feldspar and zircon U-Pb geochronology of granitoids and sandstone. These data add new constraints to the formation of a major angular unconformity, deformation along and within the footwall of the Gangdese décollement, and the coupling between deformation and magmatism. Structural analysis indicates that top-to-the-south motion along the décollement produced south-vergent folding and thrusting of Upper Jurassic to Cretaceous strata in the Gangdese back-arc basin. A synthesis of new and compiled age data reveals that the décollement and associated south-vergent deformation occurred between ca. 90 and 65 Ma, contemporaneous with the formation of a major ca. 85–69 Ma angular unconformity between the overlying Paleocene–Eocene Linzizong Formation and the underlying Upper Cretaceous Shexing Formation. We posit that this deformation in the Gangdese belt resulted from flat-slab subduction of the Neotethyan oceanic slab beneath the southern margin of the Asian continent. A flat-slab subduction geometry is consistent with previously documented synchronous thrusting in the forearc and back-arc basins as well as the observed arc magmatic lull of the Gangdese belt between ca. 80 and 65 Ma.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45223875","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Carter, R. McAleer, C. Holm-Denoma, M. Occhi, B. E. Owens, J. Vazquez
Field relations as well as geochemical and petrologic studies of meta-igneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.
{"title":"Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA","authors":"M. Carter, R. McAleer, C. Holm-Denoma, M. Occhi, B. E. Owens, J. Vazquez","doi":"10.1130/ges02546.1","DOIUrl":"https://doi.org/10.1130/ges02546.1","url":null,"abstract":"Field relations as well as geochemical and petrologic studies of meta-igneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44209188","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Sacramento–San Joaquin Delta (Delta) in California (USA) is an important part of the state’s freshwater system and is also a major source of agricultural and natural resources. However, the Delta is traversed by a series of faults that make up the easternmost part of the San Andreas fault system at this latitude and pose seismic hazard to this region. In this study, we use new high-resolution chirp subbottom data to map and characterize the shallow expression of the Kirby Hills fault, where it has been mapped to cross the Sacramento River at the western extent of the Delta. The fault is buried here, but we document a broad zone of deformation associated with the eastern strand of the fault that changes in character, along strike, across ~600 m of the river channel. Radiocarbon dates from sediment cores collected in the Sacramento River provide some minimum constraints on the age of deformation. We do not observe evidence of the western strand as previously mapped. We also discuss difficulties of conducting a paleoseismologic study in a fluvial environment.
{"title":"Shallow deformation on the Kirby Hills fault, Sacramento–San Joaquin Delta, California (USA), revealed from high-resolution seismic reflection data and coring in a fluvial system","authors":"S. Klotsko, J. Maloney, J. Watt","doi":"10.1130/ges02525.1","DOIUrl":"https://doi.org/10.1130/ges02525.1","url":null,"abstract":"The Sacramento–San Joaquin Delta (Delta) in California (USA) is an important part of the state’s freshwater system and is also a major source of agricultural and natural resources. However, the Delta is traversed by a series of faults that make up the easternmost part of the San Andreas fault system at this latitude and pose seismic hazard to this region. In this study, we use new high-resolution chirp subbottom data to map and characterize the shallow expression of the Kirby Hills fault, where it has been mapped to cross the Sacramento River at the western extent of the Delta. The fault is buried here, but we document a broad zone of deformation associated with the eastern strand of the fault that changes in character, along strike, across ~600 m of the river channel. Radiocarbon dates from sediment cores collected in the Sacramento River provide some minimum constraints on the age of deformation. We do not observe evidence of the western strand as previously mapped. We also discuss difficulties of conducting a paleoseismologic study in a fluvial environment.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-04-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41995506","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The tectonic stress field of the southwestern Ordos Basin during the Late Triassic is controversial. The major controversy is whether the southwest- ern Ordos Basin was a compressional basin throughout the Late Triassic or whether it transformed from an extensional into a compressional basin during this period. We divided the Late Triassic into the early to middle and late to terminal periods. Two paleotectonic stress field simulation models of the southwestern Ordos Basin were constructed using finite-element software (ANSYS 10). Our results showed high consistency with regional geologic correlations, suggesting the credibility of the models. We found that the southwestern Ordos Basin was dominated by NE-SW extensional stress and strain during the early to middle Late Triassic, associated with strike-slip faulting along the western margin of the Ordos block. This is consistent with the development of syndepositional normal faults and was probably induced by the scissor collision from east to west between the North China craton and Yangtze block. The tectonic stress field of the southwestern Ordos Basin during the late to terminal Late Triassic mainly manifested as NE-SW compressive stress and strain. The dominant tectonic dynamics for the Ordos block during this period may have changed to northward compression of the Songpan-Ganzi and Qiangtang terranes. The southwestern Ordos Basin was characterized by compressional deformation and northeastward migration of the depocenter. The southwestern Ordos Basin transformed from an extensional basin associated with strike-slip faulting during the early to middle Late Triassic into a compressional depression basin during the late to terminal Late Triassic.
{"title":"Late Triassic tectonic stress field of the southwestern Ordos Basin and its tectonic implications: Insights from finite-element numerical simulations","authors":"Lijun Song, Zeng-Zhen Wang","doi":"10.1130/ges02557.1","DOIUrl":"https://doi.org/10.1130/ges02557.1","url":null,"abstract":"The tectonic stress field of the southwestern Ordos Basin during the Late Triassic is controversial. The major controversy is whether the southwest- ern Ordos Basin was a compressional basin throughout the Late Triassic or whether it transformed from an extensional into a compressional basin during this period. We divided the Late Triassic into the early to middle and late to terminal periods. Two paleotectonic stress field simulation models of the southwestern Ordos Basin were constructed using finite-element software (ANSYS 10). Our results showed high consistency with regional geologic correlations, suggesting the credibility of the models. We found that the southwestern Ordos Basin was dominated by NE-SW extensional stress and strain during the early to middle Late Triassic, associated with strike-slip faulting along the western margin of the Ordos block. This is consistent with the development of syndepositional normal faults and was probably induced by the scissor collision from east to west between the North China craton and Yangtze block. The tectonic stress field of the southwestern Ordos Basin during the late to terminal Late Triassic mainly manifested as NE-SW compressive stress and strain. The dominant tectonic dynamics for the Ordos block during this period may have changed to northward compression of the Songpan-Ganzi and Qiangtang terranes. The southwestern Ordos Basin was characterized by compressional deformation and northeastward migration of the depocenter. The southwestern Ordos Basin transformed from an extensional basin associated with strike-slip faulting during the early to middle Late Triassic into a compressional depression basin during the late to terminal Late Triassic.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45032706","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The formation consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meter-scale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.
{"title":"Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA","authors":"S. Rowland, Slava Korolev, J. Hagadorn, K. Ghosh","doi":"10.1130/ges02514.1","DOIUrl":"https://doi.org/10.1130/ges02514.1","url":null,"abstract":"We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The formation consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meter-scale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-03-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46108543","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Cenozoic strata on the Great Plains are the products of a long-lived, continental sediment routing system, and yet strikingly little is known about these ancient rivers. This article details the discovery of ~3100 fluvial ridges—erosionally inverted alluvial-fan, channel-fill, channel-belt, and valley-fill deposits—extending from the Rocky Mountain front to the eastern margin of the Great Plains. The direct detection of these channel bodies reveals new insights into late Eocene–Pliocene drainage evolution. Late Eocene–Oligocene streams were morphologically diverse. Alluvial fans adjacent to the Rocky Mountain front changed eastward to parallel or downstream-divergent, fixed, single-thread, straight to slightly sinuous (S = 1.0–1.5) streams <50 m in width. At ~100 km from the Rocky Mountain front, streams became sinuous and laterally mobile, forming amalgamated channel bodies as much as 3 km in width. Streamflow in all these systems was highly dispersed (southeast to northeast) and temporally variable. These characteristics reveal a nascent Great Plains alluvial apron hosting small, poorly integrated drainages undergoing abrupt changes. By the Miocene, more uniform streamflow generally trended east-northeast. Channel deposits are identifiable 500 km from the Rocky Mountain front. Middle Miocene valley fills gave way to fixed, multithread channels a few kilometers in width by the late Miocene. These patterns evince a mature alluvial apron hosting bigger rivers in well-integrated drainages. We interpret the systematic changes between fixed and mobile channel styles to record spatially and temporally variable aggradation rates. The widening of channels in the late Miocene likely reflects increased discharge relating to wetter climates upstream or the integration of once-isolated Rocky Mountain drainage basins into a continental-scale drainage system.
{"title":"Exhumed fluvial landforms reveal evolution of late Eocene–Pliocene rivers on the Central and Northern Great Plains, USA","authors":"J. Korus, R. Joeckel","doi":"10.1130/ges02587.1","DOIUrl":"https://doi.org/10.1130/ges02587.1","url":null,"abstract":"Cenozoic strata on the Great Plains are the products of a long-lived, continental sediment routing system, and yet strikingly little is known about these ancient rivers. This article details the discovery of ~3100 fluvial ridges—erosionally inverted alluvial-fan, channel-fill, channel-belt, and valley-fill deposits—extending from the Rocky Mountain front to the eastern margin of the Great Plains. The direct detection of these channel bodies reveals new insights into late Eocene–Pliocene drainage evolution. Late Eocene–Oligocene streams were morphologically diverse. Alluvial fans adjacent to the Rocky Mountain front changed eastward to parallel or downstream-divergent, fixed, single-thread, straight to slightly sinuous (S = 1.0–1.5) streams <50 m in width. At ~100 km from the Rocky Mountain front, streams became sinuous and laterally mobile, forming amalgamated channel bodies as much as 3 km in width. Streamflow in all these systems was highly dispersed (southeast to northeast) and temporally variable. These characteristics reveal a nascent Great Plains alluvial apron hosting small, poorly integrated drainages undergoing abrupt changes. By the Miocene, more uniform streamflow generally trended east-northeast. Channel deposits are identifiable 500 km from the Rocky Mountain front. Middle Miocene valley fills gave way to fixed, multithread channels a few kilometers in width by the late Miocene. These patterns evince a mature alluvial apron hosting bigger rivers in well-integrated drainages. We interpret the systematic changes between fixed and mobile channel styles to record spatially and temporally variable aggradation rates. The widening of channels in the late Miocene likely reflects increased discharge relating to wetter climates upstream or the integration of once-isolated Rocky Mountain drainage basins into a continental-scale drainage system.","PeriodicalId":55100,"journal":{"name":"Geosphere","volume":" ","pages":""},"PeriodicalIF":2.5,"publicationDate":"2023-03-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47923386","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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