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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.
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Successive accretions of future allochthonous terranes and multiple subduction zone jumps: Implications for Tethyan evolution
The accretion of future allochthonous terranes (e.g., microcontinents or oceanic plateaus) onto the southern margin of Asia occurred repeatedly during the evolution and closure of the Tethyan oceanic realm, but the specific geodynamic processes of this protracted convergence, successive accretion, and subduction zone initiation remain largely unknown. Here, we use numerical models to better understand the dynamics that govern multiple terrane accretions and the polarity of new subduction zone initiation. Our results show that the sediments surrounding the future terranes and the structural complexity of the overriding plate are important factors that affect accretion of multiple plates and guide subduction polarity. Wide (≥400 km) and buoyant terranes with sediments behind them and fast continental plate motions are favorable for multiple unidirectional subduction zone jumps, which are also referred to as subduction zone transference, and successive terrane-accretion events. The jumping times (∼3−20+ m.y.) are mainly determined by the convergence rates and rheology of the overriding complex plate with preceding terrane collisions, which increase with slower convergence rates and/or a greater number of preceding terrane collisions. Our work provides new insights into the key geodynamic conditions governing multiple subduction zone jumps induced by successive accretion and discusses Tethyan evolution at a macro level. More than 50 m.y. after India-Asia collision, subduction has yet to initiate along the southern Indian plate, which may be the joint result of slower plate convergence and partitioned deformation across southern Asia.
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- PAR ID:
- 10488157
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic‐Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion‐related strain, we use geologic mapping, kinematic inversion of fault‐plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF.P‐ andT‐axes from kinematic inversions indicate predominantly left‐lateral slip. Left‐lateral brittle faulting cross‐cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non‐conformably overlain by a >300 m‐thick sequence of clastic marine shelf and slope sediments that are not left‐laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left‐lateral slip to the Eocene. Eocene left‐lateral slip is temporally and kinematically consistent with regional southwest‐northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes.more » « less
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Interpretations of pre-collisional configurations of Greater India are highly controversial and predict distinct processes during the India-Asia collision. To better determine the possible pre-collisional configuration(s) of Greater India, we conduct a mass-balance analysis combined with previously published geologic, paleomagnetic, and geodynamic evidence. The mass-balance analysis determines the magnitude of northern Greater India (NGI) width needed to provide sufficient crustal accretion to form the Tethyan-Greater Himalaya orogenic wedge in Cenozoic time. Applying endmember crustal thicknesses of 10–40 km to a mass-balance equation yields a broad range of plausible pre-collisional NGI widths of ∼3016±1000 km and ∼956±283 km, respectively, which we further assess considering contrasting models/evidence. The integrated evidence requires a thin NGI continental crust to form 1) continuous Tethyan-Greater Himalayan crustal thickening, 2) a narrow foredeep width of Himalayan foreland basin, 3) continuous Gangdese arc magmatism with oceanic-subduction-style mantle wedge, and 4) low-magnitude exhumation in the North Himalaya and Gangdese arc-forearc from ∼60-30 Ma. Adding the structurally restored ∼740 km wide southern Greater India, the synthesized analyses yield two possible configurations: 1) an ∼1350±440 km wide and ∼23-30 km thick NGI indicating an ∼2080±450 km wide Greater India with ∼500-1000 km wide oceanic basin systems in both Asia and NGI; and 2) a ≥1815±630 km wide and ∼10-23 km thick Zealandia-type NGI indicating a ≥2550±640 km wide pre-collisional Greater India without or with limited ∼500-1000 km Xigaze back-arc oceanic basin. The former is conditionally consistent with the integrated evidence by assuming no Cenozoic oceanic subduction initiation within NGI and predicts multi-stage collision since ∼60 Ma. The latter is consistent with the integrated evidence and predicts an approximate-single-stage collision at ∼60 Ma. Both configurations predict significant post-collisional NGI crustal shortening that may have been accommodated by the Eocene-Oligocene Greater Himalayan structural discontinuities.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/B37263.1
