New Articles for Geosphere Posted Online in July and August

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Locations and topics studied include St. Lawrence Island, the Kodiak Islands, and Yakutat Bay, Alaska; Lake City caldera, Colorado; the Canadian Cordillera; and crushed fossil turtle shells. You can find these articles at .

Fluid-driven cyclic reorganization in shallow basaltic fault zones
Bob Bamberg; Richard Walker; Marc Reichow; Audrey Ougier-Simonin
Abstract: Faults represent a critical heterogeneity in basaltic sequences, yet few studies have focused on their architectural and hydromechanical evolution. We present a detailed, multi-scale characterization of passively exhumed fault zones from the layered basalts of the Faroe Islands, which reveals cyclic stages of fault evolution. Outcrop-scale structures and fault rock distribution within the fault zones were mapped in the field and in 3-D virtual outcrop models, with detailed characterization of fault rock microstructure obtained from optical and scanning electron microscopy. The fault zones record deformation localization from decameter-wide Riedel shear zones into meter-wide fault cores that contain multiple cataclastic shear bands and low-strain lenses organized around a central slip zone. Shear bands and the slip zone consist of (ultra-) cataclasites with a zeolite-smectite assemblage replacing the original plagioclase-pyroxene host rock composition. Low-strain lenses are breccias of weakly altered host rock or reworked fault rocks. Slip zone-proximal zones show significant late-stage dilatation in the form of hydrothermal breccias or tabular veins with up to decimeter apertures. We interpret these structures as evolving from alternating shear-compaction and dilation through hydrofracture. The fault core preserves slip zone reworking, which is interpreted to indicate repeated shear zone locking and migration. The alternating deformation styles of shear-compaction and dilatation suggest episodic changes in deformation mechanisms driven by transient overpressure and release. The fault zone mechanical properties are thus governed by the combined effects of permanent chemical weakening and transient fluid-mediated mechanical weakening, alternating with cementation and healing. We suggest that the model presented for fault evolution should apply widely to shallow, basalt-hosted fault zones.
View article:

Middle Miocene faulting and basin evolution during central Basin and Range extension: A detailed record from the upper Horse Spring Formation and red sandstone unit, Lake Mead region, Nevada, USA
Melissa A. Lamb; Thomas A. Hickson; Paul J. Umhoefer; Zachary W. Anderson; Crystal Pomerleau ...
Abstract: Miocene basins of the Lake Mead region (southwestern United States) contain a well-exposed record of rifting and the evolving paleogeography of the eastern central Basin and Range. The middle Miocene Horse Spring Formation and red sandstone unit allow for detailed stratigraphic, chronostratigraphic, and structural analysis for better understanding the geologic history of extension in this region. We present new data from the White Basin and Lovell Wash areas (Nevada) to interpret the evolution of faulting, basin fill, and paleogeography. We conclude that tectonics strongly influenced sedimentation and hypothesize that climate may have played a secondary but important role in creating stratigraphic variations. Deposited from 14.5 to 13.86 Ma, the microbialitic Bitter Ridge Limestone Member of the Horse Spring Formation, the stratigraphically lowest unit in this study, records a widespread shallow and uniform lake which had moderate and steady sedimentation rates, both of which were controlled by a few faults. The persistent lake was broken up by fault reorganization followed by deposition of the highly variable fluvial-lacustrine facies of the Lovell Wash Member from 13.86 to 12.7 Ma. During this time, faulting shifted from the northeast-trending, oblique normal left-lateral White Basin fault to the northwest-trending, normal Muddy Peak fault and other smaller northwest-trending faults. The lower and middle portions of the red sandstone unit, 12.7–11.4 Ma, record an increase in the sedimentation rate of basin fill near the Muddy Peak fault as well as the return to widespread lacustrine conditions. Sedimentation and faulting slowed during deposition of the uppermost red sandstone unit, but some deformation occurred post–11.4 Ma. This study records basin-fill evolution including variations in depositional environments laterally and vertically, documents changes in the location and magnitude of faulting, supports earlier work that hypothesized faulting proceeded in discrete westward steps across the Lake Mead area, and helps constrain the paleogeographic and tectonic evolution of the region.
View article:

Kinematics and paleogeology of the western United States and northern Mexico computed from geologic and paleomagnetic data: 0 to 48 Ma
Peter Bird; Raymond V. Ingersoll
Abstract: Fault traces and offsets, cross-section length changes, paleomagnetic inclination and declination anomalies, and stress-direction indicators with ages back to 90 Ma are collected from the geologic literature on the western United States and northern Mexico. Finite-element program Restore simulates paleokinematics by weighted least squares and integrates displacements, strains, and rotations back in time, producing paleogeologic maps, as well as maps of velocity, heave rate, strain rate, and stress direction at 6 m.y. intervals. After calibrating three program parameters against neotectonic velocities from geodesy, all classes of data except inclination anomalies are fit reasonably well. The kink in the San Andreas fault near San Gorgonio Pass has been gradually restored by slip on adjacent faults and automated smoothing. Piercing-point pairs successfully restored along the San Andreas–Gulf of California plate boundary include the Pelona and Orocopia Schists at 6 Ma, the Pinnacles and Neenach Volcanics at 21 Ma, and the Jolla Vieja and Poway conglomerates adjacent to their Sonoran source at 48–42 Ma. During 18–6 Ma, rapid extension on the Oceanside detachment fault system was restored, placing present San Nicolas Island adjacent to present Rosarito, Baja California, at 18 Ma. Since ca. 18 Ma, the western Transverse Ranges have rotated 70° clockwise, restoration of which implies that sinistral faults in this province originated with NNE trends. The first contact between the Pacific and North America plates at ca. 28 Ma was not associated with any dramatic increase in dextral faulting on land; instead, the primary result was extension in the Plush Ranch–Vasquez-Diligencia basins and Colorado River corridor, probably driven by an unstable triple-junction and accelerated by heating and uplift of North America above enlarging slab windows.
View article:

Thermal history modeling techniques and interpretation strategies: Applications using HeFTy
Kendra E. Murray; Andrea L. Stevens Goddard; Alyssa L. Abbey; Mark Wildman
Abstract: Advances in low-temperature thermochronology, and the wide range of geologic problems that it is used to investigate, have prompted the routine use of thermal history (time-temperature, tT) models to quantitatively explore and evaluate rock cooling ages. As a result, studies that investigate topics ranging from Proterozoic tectonics to Pleistocene erosion now commonly require a substantial numerical modeling effort that combines the empirical understanding of chronometer thermochemical behavior (kinetics) with independent knowledge or hypotheses about a study area’s geologic history (geologic constraints). Although relatively user-friendly programs, such as HeFTy and QTQt, are available to facilitate thermal history modeling, there is a critical need to provide the geoscience community with more accessible entry points for using these tools. This contribution addresses this need by offering an explicit discussion of modeling strategies in the program HeFTy. Using both synthetic data and real examples, we illustrate the opportunities and limitations of thermal history modeling. We highlight the importance of testing the sensitivity of model results to model design choices and describe a strategy for classifying model results that we call the Path Family Approach. More broadly, we demonstrate how HeFTy can be used to build an intuitive understanding of the thermochronologic data types and model design strategies that are capable of discriminating among geologic hypotheses.
View article:

Crushed turtle shells: Proxies for lithification and burial-depth histories
Holger Petermann; Tyler R. Lyson; Ian M. Miller; James W. Hagadorn
Abstract: We propose a new proxy that employs assemblages of fossil turtle shells to estimate the timing and depth at which fossilization and lithification occur in shallowly buried terrestrial strata. This proxy, the Turtle Compaction Index (TCI), leverages the mechanical failure properties of extant turtle shells and the material properties of sediments that encase fossil turtle shells to estimate the burial depths over which turtle shells become compacted. Because turtle shells are one of the most abundant macroscopic terrestrial fossils in late Mesozoic and younger strata, the compactional attributes of a suite of turtle shells can be paired with geochronologic and stratigraphic data to constrain burial histories of continental settings—a knowledge gap unfilled by traditional burial-depth proxies, most of which are more sensitive to deeper burial depths. Pilot TCI studies of suites of shallowly buried turtle shells from the Denver and Williston basins suggest that such assemblages are sensitive indicators of the depths (~10–500 m) at which fossils and their encasing sediment become sufficiently lithified to inhibit further shell compaction, which is when taphonomic processes correspondingly wane. This work also confirms previously hypothesized shallow Cenozoic burial histories for each of these basins. TCI from mudstone-encased turtle shells can be paired with thicknesses and ages of overlying strata to create geohistorical burial curves that indicate when such post-burial processes were active.
View article:

Volcano-pluton connections at the Lake City magmatic center (Colorado, USA)
Ayla S. Pamukçu; Blair Schoene; Chad D. Deering; C. Brenhin Keller; Michael P. Eddy
Abstract: Exposed at the Lake City caldera (Colorado, USA) is the ca. 23 Ma reversely stratified (rhyolite to trachyte) Sunshine Peak Tuff and post-collapse syenite and monzonite resurgent intrusions. Existing models for this system suggest that the rhyolites are related to the trachyte and resurgent syenite through fractional crystallization, separation, and remobilization (crystal mush model), and that multiple magma batches were involved in the system (Hon, 1987; Kennedy et al., 2016; Lubbers et al., 2020). We use U-Pb zircon CA-ID-TIMS-TEA and zircon trace-element modeling to further probe age and geochemical relationships between the extrusive and intrusive units. Zircon ages and compositions from the erupted units and the syenite overlap, suggesting these magmas were related and may have mixed prior to eruption. Results from the monzonite suggest it was a contemporaneous but distinct magma batch that mixed with parts of the larger system. Trends in zircon geochemistry are decoupled from time, reflecting a complex history of accessory mineral saturation and mixing of magma batches, and a distinct high-Hf population of zircon grains hints at the existence of an additional, independent batch of rhyolitic magma in the system. The new ages we present shorten the lifetime of the Lake City magmatic system from 80 to 300 k.y. (Bove et al., 2001) to 60 to 220 k.y. and suggest the high-silica rhyolite magma crystallized over a minimum of ~160 k.y. This latter timescale likely reflects a protracted history that includes differentiation of a parent melt prior to extraction of eruptible high-silica rhyolite magma.
View article:

Development of the Whitehorse trough as a strike-slip basin during Early to Middle Jurassic arc-continent collision in the Canadian Cordillera
Leigh H. van Drecht; Luke P. Beranek; Maurice Colpron; Adam C. Wiest
Abstract: The Whitehorse trough is a synorogenic basin in the northern Cordillera that resulted from arc-collision processes along the northwestern margin of North America, but its filling history and tectonic significance remain uncertain. New detrital zircon U-Pb-Hf isotope analyses of 12 rock samples, including six basal sandstones that sit unconformably on Triassic rocks of Stikinia, were combined with published detrital zircon and fossil data to establish the depositional ages of synorogenic Laberge Group strata in Yukon and test proposed links between Intermontane terrane exhumation and basin-filling events. Laberge Group strata yielded 205–170 Ma and 390–252 Ma detrital zircon populations that indicate derivation from local Late Triassic to Middle Jurassic arc and syncollisional plutons and metamorphosed Paleozoic basement rocks of the Stikinia and Yukon-Tanana terranes. Basal sandstone units have Early Jurassic depositional ages that show the Whitehorse trough filled during early Sinemurian, late Sinemurian to Pliensbachian, and Toarcian subsidence events. Late Triassic to Early Jurassic detrital zircon grains confirm that syn-collisional plutons near the northern trough were exhumed at 0.5–7.5 mm/yr and replicate their excursion to subchondritic Hf isotope compositions as a result of increasing crustal contributions from Rhaetian to Sinemurian time. The new detrital zircon data, combined with recent constraints for Triassic– Jurassic metamorphism and magmatism in Yukon, require modification of published forearc to syncollisional basin models for the Whitehorse trough. We reinterpret Jurassic subsidence patterns and architecture of the Whitehorse trough to reflect sinistral transtension within a transform fault system that resulted from the reorganization of subduction after end-on arc collision.
View article:

Detrital zircon ages from upper Paleozoic–Triassic clastic strata on St. Lawrence Island, Alaska: An enigmatic component of the Arctic Alaska–Chukotka microplate
Jeffrey M. Amato; Julie A. Dumoulin; Eric S. Gottlieb; Thomas E. Moore
Abstract: New lithologic and detrital zircon (DZ) U-Pb data from Devonian–Triassic strata on St. Lawrence Island in the Bering Sea and from the western Brooks Range of Alaska suggest affinities between these two areas. The Brooks Range constitutes part of the Arctic Alaska–Chukotka microplate, but the tectonic and paleogeographic affinities of St. Lawrence Island are unknown or at best speculative. Strata on St. Lawrence Island form a Devonian–Triassic carbonate succession and a Mississippian(?)–Triassic clastic succession that are subdivided according to three distinctive DZ age distributions. The Devonian–Triassic carbonate succession has Mississippian-age quartz arenite beds with Silurian, Cambrian, Neoproterozoic, and Mesoproterozoic DZ age modes, and it exhibits similar age distributions and lithologic and biostratigraphic characteristics as Mississippian-age Utukok Formation strata in the Kelly River allochthon of the western Brooks Range. Consistent late Neoproterozoic, Cambrian, and Silurian ages in each of the Mississippian-age units suggest efficient mixing of the DZ prior to deposition, and derivation from strata exposed by the pre-Mississippian unconformity and/or Endicott Group strata that postdate the unconformity. The Mississippian(?)–Triassic clastic succession is subdivided into feldspathic and graywacke subunits. The feldspathic subunit has a unimodal DZ age mode at 2.06 Ga, identical to Nuka Formation strata in the Nuka Ridge allochthon of the western Brooks Range, and it records a distinctive depositional episode related to late Paleozoic juxtaposition of a Paleoproterozoic terrane along the most distal parts of the Arctic Alaska–Chukotka microplate. The graywacke subunit has Triassic maximum depositional ages and abundant late Paleozoic grains, likely sourced from fringing arcs and/or continent-scale paleorivers draining Eurasia, and it has similar age distributions to Triassic strata from the Lisburne Peninsula (northwestern Alaska), Chukotka and Wrangel Island (eastern Russia), and the northern Sverdrup Basin (Canadian Arctic), but, unlike the Devonian–Triassic carbonate succession and feldspathic subunit of the Mississippian(?)–Triassic clastic succession, it has no obvious analogue in the western Brooks Range allochthon stack. These correlations establish St. Lawrence Island as conclusively belonging to the Arctic Alaska–Chukotka microplate, thus enhancing our understanding of the circum-Arctic region in late Paleozoic–Triassic time.
View article:

Upper-plate structure and tsunamigenic faults near the Kodiak Islands, Alaska, USA
Marlon D. Ramos; Lee M. Liberty; Peter J. Haeussler; Robert Humphreys
Abstract: The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modeling and geophysical data analysis. Using seismic and bathymetric data, we characterize a regionally extensive seafloor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but we suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper-plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short wave travel paths and little tsunami warning time for nearby communities.
View article:

Revisiting the 1899 earthquake series using integrative geophysical analysis in Yakutat Bay, Alaska
Maureen A.L. Walton; Sean P.S. Gulick; Peter J. Haeussler
Abstract: A series of large earthquakes in 1899 affected southeastern Alaska near Yakutat and Disenchantment Bays. The largest of the series, a M W 8.2 event on 10 September 1899, generated an ~12-m-high tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the largest coseismic uplift ever measured. Several complex fault systems in the area are associated with the Yakutat terrane collision with North America and the termination of the Fairweather strike-slip system, but because faults local to Yakutat Bay have been incompletely or poorly mapped, it is unclear which fault system(s) ruptured during the 10 September 1899 event. Using marine geophysical data collected in August 2012, we provide an improved tectonic framework for the Yakutat area, which advances our understanding of earthquake hazards. We combined 153 line km of 2012 high-resolution multichannel seismic (MCS) reflection data with compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS data, 2018 seafloor bathymetry, published geodetic models and thermochronology data, and previous measurements of coseismic uplift to better constrain fault geometry and subsurface structure in the Yakutat Bay area. We did not observe any active or concealed faults crossing Yakutat Bay in our high-resolution data, requiring faults to be located entirely onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to be associated with the transpressional termination of the Fairweather fault system, forming a series of splay faults that exhibit a horsetail geometry. Thrust and reverse faults on the west side of the bay are related to Yakutat terrane underthrusting and collision with North America. Our results include an updated fault map, structural model of Yakutat Bay, and quantitative assessment of uncertainties for legacy geologic coseismic uplift measurements. Additionally, our results indicate the 10 September 1899 rupture was possibly related to stress loading from the earlier Yakutat terrane underthrusting event of 4 September 1899, with the majority of 10 September coseismic slip occurring on the Esker Creek system on the northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip associated with the 1899 events occurred on faults located east of Yakutat Bay.
View article:

GEOSPHERE articles are available at . 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,

# # #

For Immediate Release
6 Sept. 2022
GSA Release No. 22-48

Kea Giles