- Publication Date:
- NSF-PAR ID:
- 10218108
- Journal Name:
- Bulletin of the Seismological Society of America
- ISSN:
- 0037-1106
- Sponsoring Org:
- National Science Foundation
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Booth, A.M. (Ed.)The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out themore »
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SUMMARY 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-sizedmore »
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Abstract Although the Greater Caucasus Mountains have played a central role in absorbing late Cenozoic convergence between the Arabian and Eurasian plates, the orogenic architecture and the ways in which it accommodates modern shortening remain debated. Here, we addressed this problem using geologic mapping along two transects across the southern half of the western Greater Caucasus to reveal a suite of regionally coherent stratigraphic packages that are juxtaposed across a series of thrust faults, which we call the North Georgia fault system. From south to north within this system, stratigraphically repeated ~5–10-km-thick thrust sheets show systematically increasing bedding dip angles (<30° in the south to subvertical in the core of the range). Likewise, exhumation depth increases toward the core of the range, based on low-temperature thermochronologic data and metamorphic grade of exposed rocks. In contrast, active shortening in the modern system is accommodated, at least in part, by thrust faults along the southern margin of the orogen. Facilitated by the North Georgia fault system, the western Greater Caucasus Mountains broadly behave as an in-sequence, southward-propagating imbricate thrust fan, with older faults within the range progressively abandoned and new structures forming to accommodate shortening as the thrust propagates southward. We suggestmore »
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The Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks withmore »
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Many large fault zones record multiple reactivations that can be difficult to resolve and interpret in the field. Here, we use examples from Vermont and New Zealand to illustrate how structural data combined with 40Ar/39Ar geochronology can be used to reconstruct fault reactivation histories and interpret their possible origins. In SW New Zealand, the Spey-Mica Burn fault zone parallels a transpressive boundary between the Pacific and Australian plates. Integrated structural and 40Ar/39Ar data obtained from pseudotachylyte, mylonite, and other fault rocks allow us to distinguish successive phases of faulting (i.e., reactivations) from cases where different styles of brittle and ductile deformation occurred simultaneously (or nearly so) in the fault zone. Apparent age spectra from multiple minerals show age gradients that reveal four reactivations spanning ~20 Ma. The style and timing of these events correlate well to times of increased convergence rate and collisions between oceanic ridge segments and a nearby trench. Fault zones in NW Vermont also record different styles of reactivation. The Hinesburg Thrust (HT), which juxtaposes Late Proterozoic-Early Cambrian rift clastic rocks against Ordovician carbonate rocks of the Champlain Valley belt, includes a ~30 m thick zone of mylonite that is cut by a cataclastic fault and deformedmore »