Abstract Differing interpretations of geophysical and geologic data have led to debate regarding continent-scale plate configuration, subduction polarity, and timing of collisional events on the western North American plate margin in pre–mid-Cretaceous time. One set of models involves collision and accretion of far-traveled “exotic” terranes against the continental margin along a west-dipping subduction zone, whereas a second set of models involves long-lived, east-dipping subduction under the continental margin and a fringing or “endemic” origin for many Mesozoic terranes on the western North American plate margin. Here, we present new detrital zircon U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath terranes in the Klamath Mountains of northern California and southern Oregon that provide a test of these contrasting models. Our data show that portions of the Rattlesnake Creek terrane cover sequence (Salt Creek assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic) and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone samples of the Galice Formation are no older than ca. 158–153 Ma (middle Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based on a comparison of our data to published magmatic and detrital ages representing provenance scenarios predicted by the exotic and endemic models (a crucial geologic test), we show that our samples were likely sourced from the previously accreted, older terranes of the Klamath Mountains and Sierra Nevada, as well as active-arc sources, with some degree of contribution from recycled sources in the continental interior. Our observations are inconsistent with paleogeographic reconstructions that are based on exotic, intra-oceanic arcs formed far offshore of North America. In contrast, the incorporation of recycled detritus from older terranes of the Klamath Mountains and Sierra Nevada, as well as North America, into the Rattlesnake Creek and Western Klamath terranes prior to Late Jurassic deformation adds substantial support to endemic models. Our results suggest that during long-lived, east-dipping subduction, the opening and subsequent closing of the marginal Galice/Josephine basin occurred as a result of in situ extension and subsequent contraction. Our results show that tectonic models invoking exotic, intra-oceanic archipelagos composed of Cordilleran arc terranes fail a crucial geologic test of the terranes’ proposed exotic origin and support the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the North American continental margin.
more »
« less
Middle Jurassic to Early Cretaceous tectonic evolution of the western Klamath Mountains and outboard Franciscan assemblages, northern California–southern Oregon, USA
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 the possibility that such materials were removed via subduction and/or strike
slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice
Formation–filled basin demonstrably occurred adjacent to North America; and (3)
the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench
assemblages underplated >100 km inboard of the subduction margin, presumably
during a previously unrecognized phase of shallow-angle subduction. In aggregate,
these relations suggest that the Klamath Mountains and adjacent Franciscan complex
represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic
subduction zone, wherein the downgoing oceanic plate took a variety of trajectories
into the mantle. We speculate that the downgoing plate contained alternating tracts of
smooth and dense versus rough and buoyant lithosphere—the former gliding into the
mantle (facilitating slab rollback and upper plate extension) and the latter enhancing
basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific
Ocean, with subduction of the Australian plate beneath New Guinea and adjacent
island groups providing perhaps the best analog.
more »
« less
- Award ID(s):
- 1846811
- NSF-PAR ID:
- 10335543
- Editor(s):
- Booth, A.M.
- Date Published:
- Journal Name:
- GSA Field Guide
- Volume:
- 62
- Page Range / eLocation ID:
- 73–130
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract The Klamath Mountains in northern California and southern Oregon are thought to record 200+ m.y. of subduction and terrane accretion, whereas the outboard Franciscan Complex records ocean‐continent subduction along the North American margin. Unraveling the Klamath Mountains' Late Jurassic history could help constrain this transition in subduction style. Key is the Mesozoic Condrey Mountain Schist (CMS), comprising, in part, a subduction complex that occupies a structural window through older, overlying central Klamath thrust sheets but with otherwise uncertain relationships to more outboard Klamath or Franciscan terranes. The CMS consists of two units (upper and lower), which could be correlated with (a) other Klamath terranes, (b) the Franciscan, or (c) neither based on regional structures and limited extant age data. Upper CMS protolith and metamorphic dates overlap with other Klamath terranes, but the lower CMS remains enigmatic. We used multiple geochronometers to constrain the timing of lower CMS deposition and metamorphism. Maximum depositional ages (MDAs) derived from detrital zircon geochronology of metasedimentary rocks are 153–135 Ma. Metamorphic ages from white mica K‐Ar and Rb‐Sr multi‐mineral isochrons from intercalated and coherently deformed metamafic lenses are 133–116 Ma. Lower CMS MDAs (<153 Ma) predominantly postdate other Klamath terrane ages, but subduction metamorphism appears to start before the earliest coherent Franciscan underplating (ca. 123 Ma). The lower CMS thus occupies a spatial and temporal position between the Klamath Mountains and Franciscan and preserves a non‐retrogressed record of the Franciscan Complex's early history (>123 Ma), otherwise only partially preserved in retrogressed Franciscan high grade blocks.
-
null (Ed.)Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America.more » « less
-
more » « less
Abstract We use traveltimes from a temporary seismic deployment of 30 broadband seismometers and a national catalog of arrival times to construct a finite‐frequency teleseismic
P ‐wave tomographic model of the upper mantle beneath eastern Indonesia, where subduction of the Indo‐Australian plate beneath the Banda Arc transitions to arc‐continent collision. The change in tectonics is due to a change from oceanic to continental lithosphere in the lower plate as inferred from geological mapping and geophysical, geochemical, and geodetic measurements. At this inferred transition, we seismically image the subducted continent‐ocean boundary at upper mantle depths that links volcanism on Flores to amagmatic orogenesis on Timor. Our tomographic images reveal a relatively high‐velocity feature within the upper mantle, which we interpret as the subducted Indo‐Australian slab. The slab appears continuous yet deformed as a result of the change in buoyancy due to the composition of the incoming continental lithosphere. Accordingly, there is a difference in dip angle between the oceanic and continental sections of the slab albeit not a gap or discontinuity. We suggest the slab has deformed without tearing to accommodate structural and kinematic changes across the continent‐ocean boundary as the two sections of the slab diverge. These results suggest that deformation in tectonic collisions can be localized along a continent‐ocean boundary, even at depth. We propose that future slab tearing may develop where we observe slab deformation in our study region and that a similar process may take place in collisions generally. -
Abstract The magmatic response above subducting ocean lithosphere can range from weak to vigorous and from a narrow zone to widely distributed. The small and young Cascade Arc, riding on the margin of the tectonically active North American plate, has expressed nearly this entire range of volcanic activity. This allows an unusually good examination of arc initiation and early growth. We review the tectonic controls of Cascade-related magmatism from its inception to the present, with new considerations on the influences of tectonic stress and strain on volcanic activity. The Cascade Arc was created after accretion of the Siletzia oceanic plateau at ~ 50 Ma ended a period of flat-slab subduction. This (1) initiated dipping-slab subduction beneath most of the northern arc (beneath Washington and Oregon) and (2) enabled the more southerly subducting flat slab (beneath Nevada) to roll back toward California. As the abandoned flat slab fragmented and foundered beneath Oregon and Washington, vigorous extension and volcanism ensued throughout the northwest USA; in Nevada the subducting flat slab rolled back toward California. Early signs of the Cascade Arc were evident by ~ 45 Ma and the ancestral Cascade Arc was well established by ~ 35 Ma. Thus, from ~ 55–35 Ma subduction-related magmatism evolved from nearly amagmatic to regional flare-up to a clearly established volcanic arc in two different tectonic settings. The modern Cascades structure initiated ~ 7 Ma when a change in Pacific plate motion caused partial entrainment of the Sierra Nevada/Klamath block. This block pushes north and west on the Oregon Coast Ranges block, breaking the arc into three segments: a southern extensional arc, a central transitional arc, and a northern compressional arc. Extension enhances mafic volcanism in the southern arc, promoting basalt decompression melts from depleted mantle (low-K tholeiites) that are subequal in volume to subduction fluxed calcalkaline basalts. Compression restricts volcanic activity in the north; volcanism is dominantly silicic and intra-plate-like basalts cluster close to the main arc volcanoes. The transitional central arc accommodates dextral shear deformation, resulting in a wide volcanic arc with distributed basaltic vents of diverse affinities and no clear arc axis.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/2021.0062(04)
