New Articles for Geosphere Posted Online in August

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Locations and topics studied this month include Death Valley, the San Andreas fault; and Haleakala volcano’s crater and great valleys. You can find these articles at https://geosphere.geoscienceworld.org/content/early/recent .

Landslide hypothesis for the origin of Haleakala volcano’s crater and great valleys, Hawaii
Kim M. Bishop
Abstract: Active Haleakala volcano on the island of Maui is the second largest volcano in the Hawaiian Island chain. Prominently incised in Haleakala's slopes are four large (great) valleys. Haleakala Crater, a prominent summit depression, formed by coalescence of two of the great valleys. The great valleys and summit crater have long been attributed solely to fluvial erosion, but two significant enigmas exist in the theory. First, the great valleys of upper Keanae/Koolau Gap, Haleakala Crater, and Kaupo Gap are located in areas of relatively low annual rainfall. Second, the axes of some valley segments are oblique for long distances across the volcanic slopes. This study tested the prevailing erosional theory by reconstructing the volcano's topography just prior to valley incision. The reconstruction produces a belt along the volcano's east rift zone with a morphology that is inconsistent with volcanic aggradation alone, but it is readily explained if it is assumed the surface was displaced along scarps formed by a giant landslide on Haleakala's northeastern flank. Although the landslide head location is well defined, topographic evidence is lacking for the toe and lateral margins. Consequently, the slope failure is interpreted as a sackung-style landslide with a zone of deep-seated distributed shear and broad surface warping downslope of the failure head. Maximum downslope displacement was likely in the range of 400–800 m. Capture of runoff at the headscarps formed atypically large streams that carved Haleakala's great valleys and explains their existence in low-rainfall areas and their slope-oblique orientations. Sackung-style landslides may be more prevalent on Hawaiian volcanoes than previously recognized.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02215.1/607272/Landslide-hypothesis-for-the-origin-of-Haleakala

Detrital zircon provenance and depositional links of Mesozoic Sierra Nevada intra-arc strata
Snir Attia; Scott R. Paterson; Jason Saleeby; Wenrong Cao
Abstract: A compilation of new and published detrital zircon U-Pb age data from Permo-Triassic to Cretaceous intra-arc strata of the Sierra Nevada (eastern California, USA) reveals consistent sedimentary provenance and depositional trends across the entire Sierra Nevada arc. Detrital zircon age distributions of Sierra Nevada intra-arc strata are dominated by Mesozoic age peaks corresponding to coeval or just preceding arc activity. Many samples display a spread of pre-300 Ma ages that is indistinguishable from the detrital age distributions of pre-Mesozoic prebatholithic framework strata and southwestern Laurentian continental margin deposits. Synthesis of detrital zircon age data with tectonostratigraphic constraints indicates that a marine to subaerial arc was established in Triassic time, giving way to widespread shallow- to deep-marine deposition in latest Triassic to Early Jurassic time that continued until the emergence of the arc surface in the Early Cretaceous. No data presented herein require the existence of Mesozoic exotic terranes and/or outboard arcs that were previously hypothesized to have been accreted to the Sierra Nevada. We conclude that Sierra Nevada intra-arc strata formed within a coherent depositional network that was intimately linked to the southwestern United States Cordilleran margin throughout the span of Mesozoic arc activity.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02296.1/607274/Detrital-zircon-provenance-and-depositional-links

Superposition of two kinematically distinct extensional phases in southern Death Valley: Implications for extensional tectonics
Z.D. Fleming; T.L. Pavlis; S. Canalda
Abstract: Geologic mapping in southern Death Valley, California, demonstrates Mesozoic contractional structures overprinted by two phases of Neogene extension and contemporaneous strike-slip deformation. The Mesozoic folding is most evident in the middle unit of the Noonday Formation, and these folds are cut by a complex array of Neogene faults. The oldest identified Neogene faults primarily displace Neoproterozoic units as young as the Johnnie Formation. However, in the northernmost portion of the map area, they displace rocks as young as the Stirling Quartzite. Such faults are seen in the northern Ibex Hills and con­sist of currently low- to moderate-angle, E-NE– dipping normal faults, which are folded about a SW-NE–trending axis. We interpret these low-angle faults as the product of an early, NE-SW extension related to kinematically similar deformation recognized to the south of the study area. The folding of the faults postdates at least some of the extension, indicating a component of syn-exten­sional shortening that is probably strike-slip related. Approximately EW-striking sinistral faults are mapped in the northern Saddlepeak Hills. However, these faults are kinematically incompatible with the folding of the low-angle faults, suggesting that folding is related to the younger, NW-SE extension seen in the Death Valley region. Other faults in the map area include NW- and NE-striking, high-angle normal faults that crosscut the currently low-angle faults. Also, a major N-S–striking, oblique-slip fault bounds the eastern flank of the Ibex Hills with slickenlines showing rakes of <30°, which together with the map pattern, suggests dextral-oblique movement along the east front of the range. The exact timing of the normal faulting in the map area is hampered by the lack of geochronology in the region. However, based on the map relationships, we find that the older extensional phase predates an angular unconformity between a volcanic and/or sedimentary succession assumed to be 12–14 Ma based on correlations to dated rocks in the Owlshead Mountains and overlying rock-avalanche deposits with associated sedimentary rocks that we correlate to deposits in the Amargosa Chaos to the north, dated at 11–10 Ma. The mechanism behind the folding of the northern Ibex Hills, including the low- angle faults, is not entirely clear. However, transcurrent systems have been proposed to explain extension-parallel folding in many extensional terranes, and the geometry of the Ibex Hills is consistent with these models. Collectively, the field data support an old hypothesis by Troxel et al. (1992) that an early period of SW-NE extension is prominent in the southern Death Valley region. The younger NW-SE extension has been well documented just to the north in the Black Mountains, but the potential role of this earlier extension is unknown given the complexity of the younger deformation. In any case, the recognition of earlier SW-NE extension in the up-dip position of the Black Mountains detachment system indicates important questions remain on how that system should be reconstructed. Collectively, our observations provide insight into the stratigraphy of the Ibex Pass basin and its relationship to the extensional history of the region. It also highlights the role of transcurrent deformation in an area that has transitioned from extension to transtension.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02354.1/607022/Superposition-of-two-kinematically-distinct

Latest Quaternary slip rates of the San Bernardino strand of the San Andreas fault, southern California, from Cajon Creek to Badger Canyon
Sally F. McGill; Lewis A. Owen; Ray J. Weldon; Katherine J. Kendrick; Reed J. Burgette
Abstract: Four new latest Pleistocene slip rates from two sites along the northwestern half of the San Bernardino strand of the San Andreas fault suggest the slip rate decreases southeastward as slip transfers from the Mojave section of the San Andreas fault onto the northern San Jacinto fault zone. At Badger Canyon, offsets coupled with radiocarbon and optically stimulated luminescence (OSL) ages provide three independent slip rates (with 95% confidence intervals): (1) the apex of the oldest dated alluvial fan (ca. 30–28 ka) is right-laterally offset ~300–400 m yielding a slip rate of 13.5 ^+2.2/_−2.5 mm/yr; (2) a terrace riser incised into the northwestern side of this alluvial fan is offset ~280–290 m and was abandoned ca. 23 ka, yielding a slip rate of 11.9 ^+0.9 /_−1.2 mm/yr; and (3) a younger alluvial fan (13–15 ka) has been offset 120–200 m from the same source canyon, yielding a slip rate of 11.8 ^+4.2/_−3.5 mm/yr. These rates are all consistent and result in a preferred, time-averaged rate for the past ~28 k.y. of 12.8 ^+5.3/_−4.7 mm/yr (95% confidence interval), with an 84% confidence interval of 10–16 mm/yr. At Matthews Ranch, in Pitman Canyon, ~13 km northwest of Badger Canyon, a landslide offset ~650 m with a ¹⁰Be age of ca. 47 ka yields a slip rate of 14.5 ^+9.9 /_−6.2 mm/yr (95% confidence interval). All of these slip rates for the San Bernardino strand are significantly slower than a previously published rate of 24.5 ± 3.5 mm/yr at the southern end of the Mojave section of the San Andreas fault (Weldon and Sieh, 1985), suggesting that ~12 mm/yr of slip transfers from the Mojave section of the San Andreas fault to the northern San Jacinto fault zone (and other faults) between Lone Pine Canyon and Badger Canyon, with most (if not all) of this slip transfer happening near Cajon Creek. This has been a consistent behavior of the fault for at least the past ~47 k.y.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02231.1/607021/Latest-Quaternary-slip-rates-of-the-San-Bernardino

Geological and seismic evidence for the tectonic volution of the NE Oman continental margin and Gulf of Oman
Bruce Levell; Michael Searle; Adrian White; Lauren Kedar; Henk Droste ...
Abstract: Late Cretaceous obduction of the Semail ophiolite and underlying thrust sheets of Neo-Tethyan oceanic sediments onto the submerged continental margin of Oman involved thin-skinned SW-vergent thrusting above a thick Guadalupian–Cenomanian shelf-carbonate sequence. A flexural foreland basin (Muti and Aruma Basin) developed due to the thrust loading. Newly available seismic reflection data, tied to wells in the Gulf of Oman, suggest indirectly that the trailing edge of the Semail Ophiolite is not rooted in the Gulf of Oman crust but is truncated by an ENE-dipping extensional fault parallel to the coastline. This fault is inferred to separate the Semail ophiolite to the SW from in situ oceanic Gulf of Oman crust to the NE. It forms the basin margin to a “hinterland” basin formed atop the Gulf of Oman crust, in which 5 km of Late Cretaceous deep-water mudstones accumulated together with 4 km of Miocene and younger deep-water mudstones and sandstones. Syndepositional folding included Paleocene–Eocene folds on N-S axes, and Paleocene to Oligocene growth faults with roll-over anticlines, along the basin flank. Pliocene compression formed, or tightened, box folds whose axes parallel the modern coast with local south-vergent thrusts and reversal of the growth faults. This Pliocene compression resulted in large-scale buckling of the Cenozoic section, truncated above by an intra-Pliocene unconformity. A spectacular 60-km-long, Eocene(?) to Recent, low-angle, extensional, gravitational fault, down-throws the upper basin fill to the north. The inferred basement of the hinterland basin is in situ Late Cretaceous oceanic lithosphere that is subducting northwards beneath the Makran accretionary prism.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02376.1/607023/Geological-and-seismic-evidence-for-the-tectonic

Geometrical Breakdown Approach to interpretation of depositional sequences
Masoud Aali; Bill Richards; Mladen R. Nedimović; Vittorio Maselli; Martin R. Gibling
Abstract: Seismic and sequence stratigraphic analyses are important methodologies for interpreting coastal and shallow-marine deposits. Though both methods are based on objective criteria, terminology for reflection/stratal stacking is widely linked to eustatic cycles, which does not adequately incorporate factors such as differential subsidence, sediment supply, and autogenic effects. To reduce reliance on model-driven interpretations, we developed a Geometrical Breakdown Approach (GBA) that facilitates interpretation of horizon-bound reflection packages by systematically identifying upward-downward and landward-seaward trajectories of clinoform inflection points and stratal ter­minations, respectively. This approach enables a rigorous characterization of stratal surfaces and depositional units. The results are captured in three-letter acronyms that provide an efficient way of recognizing repetitive stacking pat­terns through discriminating reflection packages objectively to the maximum level of resolution provided by the data. Comparison of GBA with selected sequence stratigraphic models that include three and four systems tracts and the accommodation succession approach shows that the GBA allows a greater level of detail to be extracted, identifying key surfaces with more precision and utilizing more effectively the fine-scale resolution provided by the input seismic data. We tested this approach using a synthetic analogue model and field data from the New Jersey margin. The results demonstrate that the geometric criteria constitute a reliable tool for identifying systems tracts and provide an objective and straightforward method for practitioners at all levels of experience.
View article: https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02371.1/607024/Geometrical-Breakdown-Approach-to-interpretation

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, gsaservice@geosociety.org.

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