Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Locations and topics studied this month include the Teton normal fault; the Middle to Late Jurassic provenance Paradox Basin; the history of Ganderian crustal fragments in southeastern New England; the central Appalachian Mountains; and the Bone Spring Formation, Texas. You can find these articles at https://geosphere.geoscienceworld.org/content/early/recent.
Diffuse spreading, a newly recognized mode of crustal accretion in the southern Mariana Trough backarc basin
Jonathan D. Sleeper; Fernando Martinez; Patricia Fryer; Robert J. Stern; Katherine A. Kelley ...
Abstract: South of the latitude of Guam, the Mariana Trough exhibits both trench-parallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deep- and shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20–40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano- tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02360.1/606478/Diffuse-spreading-a-newly-recognized-mode-of
Postglacial slip distribution along the Teton normal fault (Wyoming, USA), derived from tectonically offset geomorphological features
Andrea Hampel; Ralf Hetzel; Maria-Sophie Erdmann
Abstract: Along the eastern front of the Teton Range, northeastern Basin and Range province (Wyoming, USA), well-preserved fault scarps that formed across moraines, river terraces, and other geomorphological features indicate that multiple earthquakes ruptured the range-bounding Teton normal fault after the Last Glacial Maximum (LGM). Here we use high-resolution digital elevation models derived from lidar data to determine the vertical slip distribution along strike of the Teton fault from 54 topographic profiles across tectonically offset geomorphological features along the entire Teton Range front. We find that offset LGM moraines and glacially striated surfaces show higher vertical displacements than younger fluvial terraces, which formed at valley exits upstream of LGM terminal moraines. Our results reveal that the tectonic offsets preserved in the fault scarps are post-LGM in age and that the postglacial slip distribution along strike of the Teton fault is asymmetric with respect to the Teton Range center, with the maximum vertical displacements (27–23 m) being located north of Jenny Lake and along the southwestern shore of Jackson Lake. As indicated by earlier three-dimensional numerical models, this asymmetric slip distribution results from postglacial unloading of the Teton fault, which experienced loading by the Yellowstone ice cap and valley glaciers in the Teton Range during the last glaciation.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02370.1/606304/Postglacial-slip-distribution-along-the-Teton
Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia
John I. Ejembi; Sally L. Potter-McIntyre; Glenn R. Sharman; Tyson M. Smith; Joel E. Saylor ...
Abstract: Middle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time. This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02264.1/606305/Detrital-zircon-geochronology-and-provenance-of
Spatially variable syn- and post-Alleghanian exhumation of the central Appalachian Mountains from zircon (U-Th)/He thermochronology
Luke C. Basler; Jaclyn S. Baughman; Michelle L. Fame; Peter J. Haproff
Abstract: To assess spatial and temporal patterns of Phanerozoic orogenic burial and subsequent exhumation in the central Appalachian Mountains, we present mid-temperature zircon (U-Th)/He (ZHe; closure temperature [TC] = 140–200 °C) dates for 10 samples along a 225 km, strike-perpendicular transect spanning the Appalachian Plateau, Valley and Ridge, Blue Ridge, and Piedmont physiographic provinces in West Virginia and western Virginia. Ranges of single-grain ZHe dates exhibit an eastward younging trend from 455–358 Ma in the Pennsylvanian Appalachian Plateau to 336–209 Ma in the Valley and Ridge, 298–217 Ma in the Blue Ridge, and 186–121 Ma in the Piedmont. Within the Pennsylvanian Appalachian Plateau, detrital ZHe dates are older than corresponding depositional ages, thus limiting postdepositional burial temperatures to less than 160 °C. These ZHe dates capture predepositional mid-Paleozoic cooling signatures, indicating provenance from either recycled Taconic or Acadian basin strata or mid-Paleozoic Appalachian terranes. Across the Valley and Ridge and western Blue Ridge provinces, reset Permian detrital ZHe dates feature flat date-effective uranium correlations that suggest rapid Alleghanian cooling initiating prior to 270 Ma. ZHe dates within the Valley and Ridge are more than 100 m.y. older than previously reported regional apatite fission-track dates, reflecting a protracted period of stable post-Alleghanian thermal conditions within the foreland. By contrast, post-Triassic single-grain ZHe dates in the interior Piedmont document rapid postrift cooling, likely resulting from both the relaxation of an elevated geothermal gradient and exhumation from rift-flank uplift. The spatial discontinuity between stable synrift thermal conditions in the Valley and Ridge and rapid cooling in the Piedmont suggests that rift-flank uplift and cooling were concentrated outboard of the foreland within the Piedmont province.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02368.1/602754/Spatially-variable-syn-and-post-Alleghanian
Constraints on the paleoelevation history of the Eastern Cordillera of Colombia from its palynological record
Peter Molnar; Lina C. Pérez-Angel
Abstract: We attempted to make an objective assessment of whether fossil pollen assemblages from the Sabana de Bogotá require surface uplift of ~2000 m since 6–3 Ma, as has been argued. We relied on recently published elevation ranges of plants for which fossil pollen has been found in sites 2000–2500 m high in the Sabana de Bogotá. The elevation ranges of fossil plants do not overlap, suggesting that those ranges may be too narrow. By weighting these elevation ranges by percentages of corresponding fossil pollen and summing them, we estimated probability density functions for past elevations. These probability distributions of past elevations overlap present-day elevations and therefore do not require surface uplift since deposition of the pollen. Fossil pollen assemblages include pollen from some plant taxa for which we do not know present-day elevation ranges, and therefore, with a more complete knowledge of elevation distributions, tighter constraints on elevations should be obtainable. The elevation of the oldest assemblage, from Tequendama, which lies at the southern edge of the Sabana de Bogotá and is thought to date from 16 to 6 Ma, is least well constrained. Although our analysis permits no change in elevation since the pollen was deposited, we consider 1000–2000 m of elevation gain since 15 Ma to be likely and consistent with an outward growth of the Eastern Cordillera.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02328.1/602755/Constraints-on-the-paleoelevation-history-of-the
Reconstructing the tectono-sedimentary evolution of the Early–Middle Jurassic Tlaxiaco Basin in southern Mexico: New insights into the crustal attenuation history of southern North America during Pangea breakup
Mildred Zepeda-Martínez; Michelangelo Martini; Luigi A. Solari; Claudia C. Mendoza-Rosales
Abstract: During Pangea breakup, several Jurassic extensional to transtensional basins were developed all around the world. The boundaries of these basins are major structures that accommodated continental extension during Jurassic time. Therefore, reconstructing the geometry of Jurassic basins is a key factor in identifying the major faults that produced continental attenuation during Pangea breakup. We reconstruct the tectono-sedimentary evolution of the Jurassic Tlaxiaco Basin in southern Mexico using sedimentologic, petrographic, and U-Pb geochronologic data. We show that the northern boundary of the Tlaxiaco Basin was an area of high relief composed of the Paleozoic Acatlán Complex, which was drained to the south by a set of alluvial fans. The WNW-trending Salado River–Axutla fault is exposed directly to the north of the northernmost fan exposures, and it is interpreted as the Jurassic structure that controlled the tectono-sedimentary evolution of the Tlaxiaco Basin at its northern boundary. The eastern boundary is represented by a topographic high composed of the Proterozoic Oaxacan Complex, which was exhumed along the NNW-trending Caltepec fault and was drained to the west by a major meandering river called the Tlaxiaco River. Data presented in this work suggest that continental extension during Pangea breakup was accommodated in Mexico not only by NNW-trending faults associated with the development of the Tamaulipas–Chiapas transform and the opening of the Gulf of Mexico, but also by WNW-trending structures. Our work offers a new perspective for future studies that aim to reconstruct the breakup evolution of western equatorial Pangea.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02309.1/602756/Reconstructing-the-tectono-sedimentary-evolution
Age and tectonic setting of the Quinebaug-Marlboro belt and implications for the history of Ganderian crustal fragments in southeastern New England, USA
Gregory J. Walsh; John N. Aleinikoff; Robert A. Ayuso; Robert P. Wintsch
Abstract: Crustal fragments underlain by high-grade rocks represent a challenge to plate reconstructions, and integrated mapping, geochronology, and geochemistry enable the unravelling of the temporal and spatial history of exotic crustal blocks. The Quinebaug-Marlboro belt (QMB) is an enigmatic fragment on the trailing edge of the peri-Gondwanan Ganderian margin of southeastern New England. SHRIMP U-Pb geochronology and geochemistry indicate the presence of Ediacaran to Cambrian metamorphosed volcanic and intrusive rocks dated for the first time between ca. 540–500 Ma. The entire belt may preserve a cryptic, internal stratigraphy that is truncated by subsequent faulting. Detrital zircons from metapelite in the overlying Nashoba and Tatnic Hill Formations indicate deposition between ca. 485–435 Ma, with provenance from the underlying QMB or Ganderian crust. The Preston Gabbro (418 ± 3 Ma) provides a minimum age for the QMB. Mafic rocks are tholeiitic with trace elements that resemble arc and E-MORB sources, and samples with negative Nb-Ta anomalies are similar to arc-like rocks, but others show no negative Nb-Ta anomaly and are similar to rocks from E-MORB to OIB or backarc settings. Geochemistry points to a mixture of sources that include both mantle and continental crust. Metamorphic zircon, monazite, and titanite ages range from 400 to 305 Ma and intrusion of granitoids and migmatization occurred between 410 and 325 Ma. Age and chemistry support correlations with the Ellsworth terrane in Maine and the Penobscot arc and backarc system in Maritime Canada. The arc-rifting zone where the Mariana arc and the Mariana backarc basin converge is a possible modern analog.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02295.1/602757/Age-and-tectonic-setting-of-the-Quinebaug-Marlboro
Precision and accuracy of modal analysis methods for clastic deposits and rocks: A statistical and numerical modeling approach
Pierre-Simon Ross; Bernard Giroux; Benjamin Latutrie
Abstract: Quantifying the proportions of certain components in rocks and deposits (modal analysis or componentry) is important in earth sciences. Relevant methods for cross-sections (two- dimensional exposures) of clastic rocks include point counts or line counts. The accuracy of these methods has been supposed to be good in the literature but not necessarily verified empirically. Natural materials are inappropriate for assessing accuracy because the true proportions of each component are unknown. The precision of modal analysis methods has traditionally been evaluated from statistical models (primarily the normal approximation to the binomial distribution) but again rarely verified in practice because it is also extremely difficult to obtain different slices through the same material at outcrop scale. Here we create a set of numerical models of red and blue spheres with different proportions and sizes and cut 60 slices through the models, on which we perform point counts and line counts. We show that both of these methods are indeed able to retrieve the correct volumetric proportions of components, on average, when enough fragments are counted or intersected. As already known, precision is controlled by component abundance and the number of points counted or clasts intersected. However, we show that other important factors include differences between slices, which are relevant for our unequal-size models, and the proportion of voids, matrix, and/or cement in the rock. We present empirical precision charts for clast counts and line counts based on our models and make recommendations for future field studies.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02374.1/602758/Precision-and-accuracy-of-modal-analysis-methods
Progradational slope architecture and sediment distribution in outcrops of the mixed carbonate-siliciclastic Bone Spring Formation, Permian Basin, west Texas
Wylie Walker; Zane R. Jobe; J.F. Sarg; Lesli Wood
Abstract: Sediment transport and distribution are the keys to understanding slope-building processes in mixed carbonate-siliciclastic sediment routing systems. The Permian Bone Spring Formation, Delaware Basin, west Texas, is such a mixed system and has been extensively studied in its distal (basinal) extent but is poorly constrained in its proximal upper-slope segment. Here, we define the stratigraphic architecture of proximal outcrops in Guadalupe Mountains National Park in order to delineate the shelf-slope dynamics of carbonate and siliciclastic sediment distribution and delivery to the basin. Upper-slope deposits are predominantly fine-grained carbonate lithologies, interbedded at various scales with terrigenous (i.e., siliciclastic and clay) hemipelagic and gravity-flow deposits. We identify ten slope-building clinothems varying from terrigenous-rich to carbonate-rich and truncated by slope detachment surfaces that record large-scale mass wasting of the shelf margin. X-ray fluorescence (XRF) data indicate that slope detachment surfaces contain elevated proportions of terrigenous sediment, suggesting that failure is triggered by changes in accommodation or sediment supply at the shelf margin. A well-exposed terrigenous-rich clinothem, identified here as the 1st Bone Spring Sand, provides evidence that carbonate and terrigenous sediments were deposited contemporaneously, suggesting that both autogenic and allogenic processes influenced sediment accumulation. The mixing of lithologies at multiple scales and the prevalence of mass wasting acted as primary controls on the stacking patterns of terrigenous and carbonate lithologies of the Bone Spring Formation, not only on the shelf margin and upper slope, but also in the distal, basinal deposits of the Delaware Basin.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02355.1/600740/Progradational-slope-architecture-and-sediment
Detrital zircon geochronology of modern river sediment in south-central Alaska: Provenance, magmatic, and tectonic insights into the Mesozoic and Cenozoic development of the southern Alaska convergent margin
Cooper R. Fasulo; Kenneth D. Ridgway
Abstract: New and previously published detrital zircon U-Pb ages from sediment in major rivers of south- central Alaska archive several major episodes of magmatism associated with the tectonic growth of this convergent margin. Analysis of detrital zircons from major trunk rivers of the Tanana, Matanuska-Susitna, and Copper River watersheds (N = 40, n = 4870) documents major <250 Ma age populations that are characteristic of the main phases of Mesozoic and Paleogene magmatism in the region as documented from limited U-Pb ages of igneous rocks. Key points from our detrital record include: (1) Major magmatic episodes occurred at 170, 150, 118, 95, 72, 58, and 36 Ma. The overall pattern of these ages suggests that felsic magmatism was episodic with periodicity ranging between ~14 and 32 m.y. with an average of ~22 m.y. (2) Magmatism in south-central Alaska shows similar age trends with both the Coast Mountains batholith and the along-strike Alaska Peninsula forearc basin strata, demonstrating a spatial and temporal relationship of felsic magmatism along the entire northern Cordilleran margin. (3) Topography and zircon fertility appear to influence the presence and/or absence of detrital zircon populations in individual watersheds. Results from this study indicate that regionally integrated detrital zircon populations from modern trunk rivers are faithful recorders of Mesozoic and Paleogene magmatic events along a convergent margin, but there appears to be a lag time for major rivers to record Neogene and ongoing magmatic events.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02270.1/600742/Detrital-zircon-geochronology-of-modern-river
Low-temperature thermochronology constraints on the evolution of the Eastern Kunlun Range, northern Tibetan Plateau
Chen Wu; Jie Li; Lin Ding
Abstract: Signals of uplift and deformation across the Tibetan Plateau associated with the Cenozoic India-Asia collision can be used to test debated deformation mechanism(s) and the growth history of the plateau. The spatio-temporal evolution of the Eastern Kunlun Range in northern Tibet provides a window for understanding the intracontinental tectonic evolution of the region. The Eastern Kunlun Range exposes the Cenozoic Kunlun left-slip fault and kinematically linked thrust belts. In this contribution, integrated field observations and apatite fission-track thermochronology were conducted to constrain the initiation ages of localized thrust faults and the exhumation history of the Eastern Kunlun Range. Our analyses reveal four stages of cooling of the Eastern Kunlun Range. We relate these four stages to the following interpreted tectonic evolution: (1) an initial period of early Cretaceous cooling and slow exhumation over the early Cenozoic, which is associated with the formation of a regional unconformity observed between Cretaceous strata and early Cenozoic sediments; (2) rapid Oligocene cooling that occurred at the eastern domain of the Eastern Kunlun Range related to the southern Qaidam thrusts; (3) extensive rapid cooling since the early-middle Miocene in most of the eastern-central domains and significant uplift of the entire range; and (4) a final pulse of rapid late Miocene-to-present cooling associated with the initiation of the Kunlun left-slip fault and dip-slip shortening at the western and eastern termination of the left-slip fault. Early Cenozoic deformation was distributed along the northern extent of the Tibetan Plateau, and overprinting out-of-sequence deformation migrated back to the south with the initiation of Miocene-to-present deformation in the Eastern Kunlun Range.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02358.1/600735/Low-temperature-thermochronology-constraints-on
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GEOSPHERE articles are available at https://geosphere.geoscienceworld.org/content/early/recent. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please refer to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, email@example.com.
2 August 2021
The Geological Society of America
Release no. 21-46
Contact: Kea Giles
For Immediate Release