The Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ1, and σ3 with vertical σ2. The activation angle between the trend of σ1 and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M6+) earthquakes.
Synorogenic extension and extrusion in southern Taiwan
Paleostress inversion of 141 outcrop-scale faults across the eastern flank of the southern Central Range of Taiwan, where leveling and GPS data suggest a steep gradient in rock uplift rates yields two main kinematic phases of deformation. Phase 1 consists of 93 normal faults that generally dip moderately northeast, whereas phase 2 consists of 48 strike-slip faults that generally dip steeply west-northwest. Both phases record NE-trending subhorizontal extension but different orientations of principal shortening; in phase 1, the principal shortening axis is nearly vertical, whereas in phase 2, it plunges gently to moderately southeast. The northeast extension is consistent with extension directions obtained from GPS and earthquake focal mechanisms in the central part of the southern Central Range. However, these indicators of contemporary deformation also reveal more complicated states of stress along the eastern and western flanks of the range and in the deep crust southwest of the range. We interpret these more complicated stress states as reflecting the “forceful extrusion” of the southern Central Range, where the lower crust is being pinched between more rigid crustal blocks represented by the Peikang High and the Luzon Arc. In this context, the temporal progress from strike-slip to normal faulting observed in outcrops may reflect the advection of the rocks from lower to higher structural levels. The northeast extension normal faults can be interpreted as accommodating the lateral and vertical movement of the crust in the southern Central Range. Based on thermochronological data and the onset of extrusion in southwest Taiwan in the late Pleistocene, we infer that this SW extrusion process may be younger than 0.5 Ma.
more »
« less
- Award ID(s):
- 1650162
- NSF-PAR ID:
- 10491832
- Publisher / Repository:
- Elsevier B.V.
- Date Published:
- Journal Name:
- Tectonophysics
- Volume:
- 840
- Issue:
- C
- ISSN:
- 0040-1951
- Page Range / eLocation ID:
- 229562
- Subject(s) / Keyword(s):
- ["Taiwan","mountain building","extension"]
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
SUMMARY -
Oblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (<160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults.more » « less
-
null (Ed.)ABSTRACT New paleoseismic trenching indicates late Quaternary oblique right-lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south-to-north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary oblique right-lateral slip on west-northwest-striking forearc faults north of the Olympic Mountains and in the southern Strait of Georgia are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike-slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right-lateral slip and the published regional SHmax directions. Vertical separation and the formation of scarp-derived colluvium along one fault also indicate a dip-slip component. Radiocarbon charcoal dating within offset glacial till and scarp-derived colluvium suggest a single surface rupturing earthquake at 9.4±3.4 ka. The oblique right-lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west-northwest-striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting.more » « less
-
Abstract The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel 40Ar/39Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ∼1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ∼100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration.more » « less
-
more » « less
Abstract Observations over the last few decades from a number of orogenic systems have highlighted the possible importance of tectonic exhumation, i.e., ductile thinning and normal faulting, in exhuming rocks once buried in high-pressure conditions. Taiwan is one of the few active orogens in the world where rocks that once experienced high-pressure metamorphism (> 50 km) are exposed at the Earth’s surface, providing a natural laboratory for advancing our understanding of exhumation processes. We integrate previously published studies of the Taiwan orogen with new structural, geochronological, and fluid inclusion microthermometry data to argue that tectonic extrusion and structural thinning played a critical role in exhuming the metamorphic core of the orogen until very recently, ca. 0.7 Ma. We propose a two-stage process for exhuming the high-pressure metamorphic rocks of the Yuli Belt: an initial stage where exhumation is driven primarily by pressure gradients in a subduction channel and a second stage that is initiated as an orogen-parallel regional-scale strike-slip zone, the Tailuko shear zone, is offset by an orogen-normal strike-slip zone. The offset generates an extensional bend that is filled with extruding high-pressure rocks as the upper crust is structurally thinned. Evidence for tectonic thinning comes primarily from a low-angle penetrative foliation that records significant vertical shortening and a suite of sub-vertical late-stage, mineral-filled veins. Isotopic dating indicates that the second stage started ca. 2.4–3.1 Ma and ended at ca. 0.7 Ma when the northern Backbone Range orocline started to form. We propose the low-angle foliation formed in the footwall of a regional-scale extensional shear zone that rooted to the east, beneath the forearc. Combined tectonic and erosional processes may have limited the topographic growth of the orogen from ~3.0 to < 1.0 Ma.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.tecto.2022.229562
