New Articles for Geosphere Posted Early Online

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Topics this month include the Colorado River extensional corridor; the central Azores volcanic islands; the Belvidere Mountain Complex, northern Appalachians; and Cretaceous seamount chains near the northwestern Hawaiian Ridge. You can find these articles at .

Late Triassic tectonic stress field of the southwestern Ordos Basin and its tectonic implications: Insights from finite-element numerical simulations
Li-Jun Song; Zeng-Zhen Wang
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.

Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA
Stephen M. Rowland; Slava Korolev; James W. Hagadorn; Kaushik Ghosh
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.

Progressive Miocene unroofing of the Big Maria and Riverside Mountains (southeastern California, USA) along the southwestern margin of the Colorado River extensional corridor
Megan E. Flansburg; Daniel F. Stockli
The Colorado River extensional corridor (CREC) consists of Miocene meta-morphic core complexes exhumed along top-to-the-NE low-angle detachment faults. The Big Maria and Riverside Mountains of southeastern California (USA) are located on the southwestern margin of the CREC, where little is known about the nature and timing of large-magnitude extension. We present the first detailed (U-Th)/He thermochronometric data from these ranges, elucidating the geometry and timing of upper-crustal extensional unroofing and exhumation. The Riverside Mountains yielded ca. 72–50 Ma zircon (U-Th)/He (ZHe) ages in the hanging wall of the Riverside detachment fault, and the corrugated footwall yielded ca. 50–18 Ma ZHe ages, indicating the preservation of an exhumed ZHe partial retention zone. Apatite (U-Th)/He data further indicate a potential secondary Miocene breakaway in the northeastern end of the range. Although the Big Maria Mountains have been thought to lie outside of the CREC, our new zircon and apatite (U-Th)/He data show that the entirety of the Big Maria Mountains was tectonically exhumed in the footwall of a detachment fault and cooled from >6 km depth between 22 and 15 Ma. ZHe data from both ranges suggest the Big Maria Mountains are part of the CREC and were exhumed from underneath the Riverside Mountains by a contemporaneous but structurally lower detachment—the Big Maria detachment—that is regionally correlative with the breakaway zone that delimits the western CREC margin. This detachment is temporally coeval with the structurally higher detachment system that forms the Whipple-Buckskin-Rawhide-Harcuvar-Harquahala metamorphic core complex belt to the northeast.

Exhumed fluvial landforms reveal evolution of late Eocene–Pliocene rivers on the Central and Northern Great Plains, US A
Jesse T. Korus; R.M. Joeckel
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.

Emplacement history of volcaniclastic turbidites around the central Azores volcanic islands: Frequencies of slope landslides and eruptions
Yu-Chun Chang; Neil C. Mitchell; Julie C. Schindlbeck-Belo; Thor H. Hansteen; Armin Freundt ...
Volcanic islands export clastic material to their surrounding oceans by explosive eruptions, lava emissions, biogenic production on their shelves, and failure of their slopes, amongst other processes. This raises the question of whether geological events (in particular, eruptions and landslides) can be detected offshore and dated, and whether any relationships (for example, with climate changes) can be revealed using sediment cores. The volcanically active central Azorean islands (Faial, Pico, São Jorge, and Terceira), with their neighboring submarine basins, are potentially good candidates for such an analysis. Here, chronostratigraphies of four gravity cores collected amongst the islands are constructed based on twelve radiocarbon dates and two dates derived by geochemically correlating primary volcaniclastic turbidites with ignimbrites on Faial and Terceira Islands. Age-depth models are built from the hemipelagic intervals to estimate individual turbidite dates. Volumes of turbidites are modeled by multiplying basin areas with bed thickness, allowing for various turbidite thinning rates and directions. The volumes of landslide-generated turbidites are only comparable with the largest volumes of their adjacent upper-slope submarine landslide valleys; therefore, such turbidites in the cores likely derive from these largest landslides. Emplacement intervals between turbidites originating from both landslides and pyroclastic density currents are found to be mostly a few thousand years. Frequencies of landslide-generated turbidites and hemipelagic sedimentation rates were both highest in the past 8 k.y. compared to preceding periods up to 50 k.y. High hemipelagic sedimentation rates are interpreted to be related to sea-level rise, allowing more shelf bioproduction and release of particles by coastal erosion. The coincident increased frequencies of submarine landslides may also be associated with the increased sediment supply from the islands, resulting in a more rapid build-up of unstable sediments on submarine slopes. Notably, the emplacement frequencies of turbidites of pyroclastic density current origins do not suggest the decreased eruption frequency toward the Holocene that has been found elsewhere.

Evolution of slip partitioning in a major continental margin strike-slip fault system during a transition to oblique plate-margin tectonics: Insight into the evolution of the Garlock fault zone, California (USA)
Joseph E. Andrew; J. Douglas Walker; William M. Rittase
The Walker Lane belt and Eastern California shear zone of California, USA, are active, plate boundary–related dextral systems with transtensional and transpressional deformation, respectively. They are separated by the sinistral Garlock fault, creating a complex system without an overall integrated formation and evolution model. We examine the deformation within the eastern segment of the Garlock fault zone over geologic timescales by determining the slip history of faults. We assess the progression of faulting and associated deformation along the WSW-striking Garlock fault zone and how it applies to the overall NNW-directed dextral system. Previous studies found that large synthetic fault strands take up 30% of the slip of the Garlock fault zone and have proposed multiple mechanisms to explore how to accommodate regional NNW-directed shear across the Garlock fault without cutting its trace. We analyze an unstudied section of faulting in one of the more complex areas of regional deformation via compiled and reinterpreted published geologic data for an analysis of total and incremental slip on the main faults of the eastern Garlock fault zone. We identify geologic offset features to interpret total slip, timing, and deformation evolution. We find that 30% of the total slip of the Garlock zone occurs on strands other than the Garlock fault sensu stricto, with the locus of main slip sidestepping during the evolution of accommodation of through-going, regional dextral shear. Our results support ideas of the creation and evolution of the regional dextral system via stress concentration on a sub-Garlock lithospheric anisotropy with a resulting lowering of the plastic yield stress. Our results also show an eastward increase in fault system complexity, which may imply an underappreciated seismic hazard of the eastern Garlock fault zone.

Newly recognized blueschist-facies metamorphism (glaucophane-omphacite-garnet), Belvidere Mountain Complex, northern Appalachians
Ian W. Honsberger
An occurrence of blueschist-facies metamorphism in the Appalachian orogen is newly recognized in northwestern New England, United States. Inclusions of glaucophane and omphacite occur in a relict garnet core from a retrogressed garnet-barroisite amphibolite of the Belvidere Mountain Complex in Vermont. Pressure-temperature pseudosection and mineral composition isopleth calculations demonstrate that the Belvidere Mountain Complex blueschist-facies mineral assemblage of glaucophane–magnesio-hornblende–omphacite–chlorite–rutile–quartz–clinozoisite–garnet was stable at ~1.65–2.0 GPa and ~450–480 °C. Garnet-absent amphibolite with barroisite and chlorite inclusions in clinozoisite records high-pressure epidote-amphibolite–facies metamorphism at ~1.0–1.4 GPa and ~515–550 °C. These new findings quantify deep subduction of the Belvidere Mountain Complex during the Cambrian to Ordovician Taconic orogenic cycle and suggest that more blueschist-facies mineral assemblages could be revealed in the Appalachians with detailed analysis of retrogressed rocks.

Syntectonic sediment loading and fold-thrust belt structural architecture: An example from the central Appalachians (USA)
Mark A. Evans
Fluid inclusion microthermometry of synkinematic veins is used to estimate the maximum syntectonic load that was deposited on the wedge top in the central Appalachians (northeastern United States) during the Alleghanian orogeny. The restored loads indicate two major depocenters during the Alleghanian orogeny: one above Broadtop synclinorium, with as much as 7 km of Pennsylvanian–Permian load probably sourced by the erosion of rocks uplifted by the growing Blue Ridge massif and emplacement of the North Mountain thrust sheet; the other above the Anthracite belt, with as much as 16 km of syntectonic load likely sourced by the erosion of rocks uplifted by the growing Reading Prong massif. The loads were generally <3 km in the intervening Juniata culmination. In areas of high load, the structural architecture of the basin is that of widely spaced thrusts (~17–22 km) with large leading-edge anticlines in the Cambrian–Ordovician lithotectonic unit, while in areas of low load, thrusts are more closely spaced (~15 km) and deformed into an imbricate stack. The relationship between observed syntectonic loads, thrust spacing, and structural style reflect modeled relationships.

New insights into the age and origin of two small Cretaceous seamount chains proximal to the Northwestern Hawaiian Ridge
Arturo Sotomayor; Andrea Balbas; Kevin Konrad; Anthony A.P. Koppers; Jasper G. Konter ...
The Northwestern Hawaiian Ridge is an age-progressive volcanic chain sourced from the Hawaiian mantle plume. Proximal to the Northwestern Hawaiian Ridge are several clusters of smaller seamounts and ridges with limited age constraints and unknown geodynamic origins. This study presents new bathymetric data and 40Ar/39Ar age determinations from lava flow samples recovered by remotely operated vehicle (ROV) from two east–west-trending chains of seamounts that lie north of the Pūhāhonu and Mokumanamana volcanoes. The previously unexplored Naifeh Chain (28°48′N,167°48′W) and Plumeria Chain (25°36′N, 164°35′W) contain five volcanic structures each, including three guyots in the Naifeh Chain. New 40Ar/39Ar age determinations indicate that the Naifeh Chain formed ca. 88 Ma and the Plumeria Chain ca. 85 Ma. The Cretaceous ages, coupled with a perpendicular orientation of the seamounts relative to absolute Pacific plate motion at that time, eliminate either a Miocene Hawaiian volcanic arch or Cretaceous mantle-plume origin. The seamounts lie on oceanic crust that is modeled to be 10–15 Ma older than the corresponding seamounts. Here, two models are put forth to explain the origin of these enigmatic seamount chains as well as the similar nearby Mendelssohn Seamounts. (1) Diffuse lithospheric extension results in the formation of these seamounts until the initiation of the Kula-Pacific spreading center in the north at 84–79 Ma, which alleviates the tension. (2) Shear-driven upwelling of enriched mantle material beneath young oceanic lithosphere results in an age-progressive seamount track that is approximately perpendicular to the spreading ridge. Here we show that all sampled seamounts proximal to the Northwestern Hawaiian Ridge are intraplate in nature, but their formations can be attributed to both plume and plate processes.

GEOSPHERE articles are available at . Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Justin Samuel 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,

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For Immediate Release
28 March 2023
GSA Release No. 23-18

Justin Samuel