Low-angle normal faults and seismicity: A review

Although large, low-angle normal faults in the continental crust are widely recognized, doubts persist that they either initiate or slip at shallow dips (<30°), because (1) global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30° and (2) Andersonian fault mechanics predict that normal faults dipping less than 30° cannot slip. Geological reconstructions, thermochronology, paleomagnetic studies, and seismic reflection profiles, mainly published in the last 5 years, reinforce the view that active low-angle normal faulting in the brittle crust is widespread, underscoring the paradox of the seismicity data. For dip-slip faults large enough to break the entire brittle layer during earthquakes (M_w ∼ 6.5), consideration of their surface area and efficiency in accommodating extension as a function of dip θ suggests average recurrence intervals of earthquakes R' ∝ tan θ, assuming stress drop, rigidity modulus, and thickness of the seismogenic layer do not vary systematically with dip. If the global distribution of fault dip, normalized to total fault length, is uniform, the global recurrence of earthquakes as a function of dip is shown to be R ∝ tan θ sin θ. This relationship predicts that the frequency of earthquakes with nodal planes dipping between 30° and 60° will exceed those with planes shallower than 30° by a factor of 10, in good agreement with continental seismicity, assuming major normal faults dipping more than 60° are relatively uncommon. Revision of Andersonian fault mechanics to include rotation of the stress axes with depth, perhaps as a result of deep crustal shear against the brittle layer, would explain both the common occurrence of low-angle faults and the lack of large faults dipping more than 60°. If correct, this resolution of the paradox may indicate significant seismic hazard from large, low-angle normal faults

Electrical conductivity images of Quaternary faults and Tertiary detachments in the California Basin and Range

Comparison of an electrical resistivity section derived from magnetotelluric (MT) data to a geologic section extending eastward from the Sierra Nevada near latitude 36°20′N shows that the crust is dominated by steeply dipping conductive features that correlate with active strike-slip faults. While there is a subhorizontal conductor at a depth ∼20 km beneath some of the profile, it is broken by vertical structures associated with the active strike-slip faults. The continuous subhorizontal anomalies in the lower crust typically observed in extensional regions are therefore absent in the resistivity section. The present-day strike-slip tectonic regime as indicated by geodetic data in this part of the Basin and Range is not producing features that could be inferred to indicate subhorizontal shear zones resulting from lateral crustal flow during extension. Because the Miocene tectonic regime resulted in the formation of metamorphic core complexes and thus was accompanied by such flow, the present regime appears to represent a fundamental transition in the mode of crustal deformation in the region. A serendipitous result of our study was the identification on resistivity sections of carbonate aquifers in the upper crust. Comparison of resistivities from the MT section to measured fluid resistivities from springs and boreholes suggests that the aquifers must be heterogeneous, with more saline brines occupying the deepest portions of the carbonates

Subsidence history of the Ediacaran Johnnie Formation and related strata of southwest Laurentia: Implications for the age and duration of the Shuram isotopic excursion and animal evolution

The Johnnie Formation and associated Ediacaran strata in southwest Laurentia are ~3000 m thick, with a Marinoan cap carbonate sequence at the bottom, and a transition from Ediacaran to Cambrian fauna at the top. About halfway through the sequence, the Shuram negative carbon isotopic excursion occurs within the Rainstorm Member near the top of the Johnnie Formation, followed by a remarkable valley incision event. At its type locality in the northwest Spring Mountains, Nevada, the Johnnie lithostratigraphy consists of three distinctive sand-rich intervals alternating with four siltstone/carbonate-rich intervals, which appear correlative with other regional ­Johnnie Formation outcrops. Carbon isotope ratios in the sub–Rainstorm Member part of the Johnnie Formation are uniformly positive for at least 400 m below the Shuram excursion and compare well with sub–Shuram excursion profiles from the ­Khufai Formation in Oman. There is historical consensus that the Johnnie and overlying formations were deposited on a thermally subsiding passive margin. Following previous authors, we used Paleozoic horizons of known biostratigraphic age to define a time-dependent exponential sub­sidence model, and extrapolated the model back in time to estimate the ages of the Shuram excursion and other prominent Ediacaran horizons. The model suggests that the Shuram excursion occurred from 585 to 579 Ma, and that incision of the Rainstorm Member shelf occurred during the 579 Ma Gaskiers glaciation. It further suggests that the base of the Johnnie Formation is ca. 630 Ma, consistent with the underlying Noonday Formation representing a Marinoan cap carbonate sequence. Our results contrast with suggestions by previous workers that the Shuram excursion followed the Gaskiers event by some 20–30 m.y. We suggest instead that the Shuram and Gaskiers events were contemporaneous with the biostratigraphic transition from acantho­morphic to leiospherid acritarchs, and with the first appearance of widespread macroscopic animal life, 38 m.y. prior to the Ediacaran-Cambrian boundary

Detecting Large-scale Intracontinental Slow-slip Events (SSEs) Using Geodograms

Since the advent in the 1980s of GPS networks to monitor crustal velocity fields, interpretations of geodetic data have generally been based on maps of Earth's surface showing average horizontal site velocity over a specified period of time and plots showing velocity gradients as a function of a position coordinate (e.g., Donnellan et al. 1993; Bennett et al. 1999). For continuous networks, these plots are typically supplemented by time series of position in order to assess the importance of time-dependent or transient behavior (Bock et al. 1993; Hudnut et al. 2002). Thus far, regional transient motions have been revealed by plotting position time series from multiple sites on a common time axis. These plots have been effective in demonstrating the existence of slow-slip events (SSEs) on subduction megathrust interfaces around the globe (e.g., Miller et al. 2002; Melbourne et al. 2005; Schwartz and Rokosky 2007). A large-scale intraplate SSE in the northern Basin and Range Province that occurred between 1999 and 2005 was initially identified by plotting continuous time series with a vertical time axis, arranged according to a spatial position coordinate for each site (Davis et al. 2006, their Figure 3A). Because transient motions are by definition changes in velocity, however, the spatial coherence and magnitude of velocity changes are most directly addressed by plotting the time dependence of velocity rather than position. Here, we describe a method for calculating velocity time series, and then we use these to construct a "geodogram" from raw continuous GPS time series. The new time series reveal additional transient motions from 2005 to 2007 that are interpreted to reflect the onset of a new SSE beginning in late 2006

The Neotethyan Sanandaj-Sirjan zone of Iran as an archetype for passive margin-arc transitions

The Sanandaj-Sirjan zone of Iran is a northwest trending orogenic belt immediately north of the Zagros suture, which represents the former position of the Neotethys Ocean. The zone contains the most extensive, best preserved record of key events in the formation and evolution of the Neotethys, from its birth in Late Paleozoic time through its demise during the mid-Tertiary collision of Arabia with Eurasia. The record includes rifting of continental fragments off of the northern margin of Gondwanaland, formation of facing passive continental margins, initiation of subduction along the northern margin, and progressive development of a continental magmatic arc. The latter two of these events are critical phases of the Wilson Cycle that, elsewhere in the world, are poorly preserved in the geologic record because of superimposed events. Our new synthesis reaffirms the similarity between this zone and various terranes to the north in Central Iran. Late Paleozoic rifting, preserved as A-type granites and accelerated subsidence, was followed by a phase of pronounced subsidence and shallow marine sedimentation in Permian through Triassic time, marking the formation and evolution of passive margins on both sides of the suture. Subduction and arc magmatism began in latest Triassic/Early Jurassic time, culminating at ~170 Ma. The extinction of arc magmatism in this zone, and its shift northeastward to form the subparallel Urumieh-Dokhtar arc, occurred diachronously along strike, in Late Cretaceous or Paleogene time. Post-Cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid-Tertiary time

Results of the Basin and Range Geoscientific Experiment (BARGE): A marine-style seismic reflection survey across the eastern boundary of the central Basin and Range Province

Approximately 120 km of marine-style deep seismic reflection data were shot during a survey on the waters of Lake Mead in southeastern Nevada. The survey extends from near the abrupt eastern edge of the Basin and Range Province (BRP) to a point ~80 km into the extended domain. Data quality throughout the survey ranged from fair to poor; the recorded data include significant towing noise and occasionally problematic diffractions and sideswipe from canyon walls. The upper 2–4 s of the data shows well-defined reflections from sedimentary fill, but below that point, reflectivity is weak. Lower crustal reflectivity is generally absent under the eastern part of the survey, with a slight increase in reflectivity to the west. The reflection Moho appears as a series of weakly defined, discontinuous reflections, most of which occur at 10–11 s. A particularly interesting feature of the data set is the relative lack of reflectivity from the lower crust, which is a region of strong reflectivity on other seismic reflection data sets from the BRP

Comment on "Neotethyan subduction ignited the Iran arc and back-arc differently" by Shafaii Moghadam et al. (2020)

Shafaii Moghadam et al. (2020) contribute important new data on Late Cretaceous-Tertiary subduction- related magmatism in Iran, but their plate convergence model, wherein Neotethyan subduction begins in mid-Cretaceous time (c. 100 Ma), overlooks well established facts relating to the tectonic history of Neotethys, in regard to global plate reconstructions, paleolatitude data, the regional stratigraphy, geochronology and geochemistry, and metamorphic history. Based on their model, Neotethys subduction beneath Eurasia began at ~100 Ma, meaning that the Neotethys was spreading and bounded by opposing passive margins during Jurassic and Early Cretaceous time, for ~100 Ma prior to their proposed onset of Neotethyan convergence. Consequently, their subduction model contradicts (1) the Indian Ocean spreading history derived from magnetic anomalies; (2) continental paleolatitude data from paleomagnetism; (3) sedimentary and igneous evolution of the Mesozoic continental margins in Arabia and southern Asia, (4) the age and geochemistry of Jurassic igneous rocks in southernmost Eurasia; and (5) the preservation of Early to Middle Jurassic eclogite metamorphism and exhumation on the northern side of the Arabia-Eurasia suture. Reconciliation of each of these omissions and contradictions of their model would be welcome, and perhaps an advisory that readers may wish to evaluate their concept of Cretaceous subduction initiation with due circumspection

Comment on "Neotethyan subduction ignited the Iran arc and back-arc differently" by Shafaii Moghadam et al. (2020)

Shafaii Moghadam et al. (2020) contribute important new data on Late Cretaceous-Tertiary subduction- related magmatism in Iran, but their plate convergence model, wherein Neotethyan subduction begins in mid-Cretaceous time (c. 100 Ma), overlooks well established facts relating to the tectonic history of Neotethys, in regard to global plate reconstructions, paleolatitude data, the regional stratigraphy, geochronology and geochemistry, and metamorphic history. Based on their model, Neotethys subduction beneath Eurasia began at ~100 Ma, meaning that the Neotethys was spreading and bounded by opposing passive margins during Jurassic and Early Cretaceous time, for ~100 Ma prior to their proposed onset of Neotethyan convergence. Consequently, their subduction model contradicts (1) the Indian Ocean spreading history derived from magnetic anomalies; (2) continental paleolatitude data from paleomagnetism; (3) sedimentary and igneous evolution of the Mesozoic continental margins in Arabia and southern Asia, (4) the age and geochemistry of Jurassic igneous rocks in southernmost Eurasia; and (5) the preservation of Early to Middle Jurassic eclogite metamorphism and exhumation on the northern side of the Arabia-Eurasia suture. Reconciliation of each of these omissions and contradictions of their model would be welcome, and perhaps an advisory that readers may wish to evaluate their concept of Cretaceous subduction initiation with due circumspection

Tectonic Evolution of the Death Valley Region

Progress in understanding the evolution of continents hinges on seamlessly applying techniques of modern structural geology to the largest possible regions of the crust. In most areas, meaningful practice of regional structural geology is limited by a lack of correspondence between highly strained crust and well-defined regional strain markers, that is, large-scale geologic features whose initial geometry can be reasonably inferred, and their kinematic evolution constrained, through structural, stratigraphic, isotopic, paleomagnetic, and geodetic study. A ~100,000-km^2 segment of the U.S. Cordilleran orogen, encompassing the celebrated landscapes of Death Valley National Park and five nearby parks that are among the most visited in the U.S., was severely deformed in late Cenozoic time. In addition to spectacular geologic exposures, the region harbors a rare endowment of regional structural markers, developed before and during late Cenozoic deformation. The markers are defined by isopachs and facies boundaries in the west-thickening Neoproterozoic-Paleozoic Cordilleran miogeocline, by pre-Cenozoic thrust faults that disrupt the miogeoclinal wedge, and by proximal Tertiary terrigenous detrital strata and their source regions. The region is still tectonically active, providing an opportunity to compare deformation patterns of the last decade, constrained by geodetic studies, with late Cenozoic deformation patterns spanning 15-20 m.y. These scientific assets have attracted the attention of significant numbers of structural geologists over the last three decades, and distinguished the region as the birthplace of, and testing ground for, an impressive number of fundamental tectonic ideas. Oroclinal bending of mountain ranges, continental transform faulting and "pull-apart" basins, low-angle normal faulting, the influence of plate motions on intracontinental deformation, the "rolling hinge" model of progressive extensional deformation, the fluid crustal layer or "crustal asthenosphere" concept, and Pratt isostatic compensation of mountain ranges were all originally discovered or have their best known expressions in the region. This remarkable history of geologic investigation and innovation continues unabated as growing numbers of scientists recognize it as a unique place on Earth to ponder the nature and origin of large-scale continental deformation