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The southern Cascadia forearc undergoes a three-stage tectonic evolution, each stage involving different combinations of tectonic drivers, that produce differences in the upper-plate deformation style. These drivers include subduction, the northward migration of the Mendocino triple junction and associated thickening and thinning related to the Mendocino Crustal Conveyor (MCC) effect, and the NNW translation of the Sierra Nevada-Great Valley (SNGV) block. We combine geodetic data, plate reconstructions, seismic tomography and topographic observations to determine how the southern Cascadia upper plate is deforming in response to the combined effects of subduction and NNW-directed (MCC- and SNGV-related) tectonic processes. The location of the terrane boundaries between the relatively weak Franciscan complex and the stronger Klamath Mountain province (KMP) and SNGV block has been a key control on the style of upper-plate deformation in the southern Cascadia forearc since the mid-Miocene. At ∼15 Ma, present-day southern Cascadia was in central Cascadia and deformation there was principally controlled by subduction processes. Since ∼5 Ma, this region of the Cascadia upper plate, where the KMP lies inboard of the Franciscan complex, has been deforming in response to both subduction and MCC- and SNGV-related effects. GPS data show that the KMP is currently moving to the NNW at ∼8–12 mm/yr with little internal deformation, largely in response to the northward push of the SNGV block at its southern boundary. In contrast, the Franciscan complex is accommodating high NNW-directed and NE-directed shortening strain produced by MCC-related shortening and subduction coupling respectively. This composite tectonic regime can explain the style of faulting within and west of the KMP. Associated with this Mendocino Crustal Conveyor crustal thickening, seismic tomography imagery shows a region of low velocity material that we interpret to represent crustal flow and injection of Franciscan crust into the KMP at intracrustal levels. We suggest that this MCC-related crustal flow and injection of material into the KMP is a relatively young feature (post ∼5 Ma) and is driving a rejuvenated period of rock uplift within the KMP. This scenario provides a potential explanation for steep channels and high relief, suggestive of rapid erosion rates within the interior of the KMP.
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Crustal Structure of Southern California from Velocity and Attenuation Tomography
Recent studies of Southern California have employed velocity and attenuation tomography to elucidate two aspects of crustal structure. Low-frequency velocity tomography reveals large-scale heterogeneity that can be approximated by a discrete set (𝐾 ≤ 10) of tectonic regions, each characterized by an isostatically balanced lithospheric column reflecting the composition and tectonic history of the region. The boundaries between tectonic regions are typically localized structures expressed at the surface by major faults, topographic fronts, and geochemical transitions. The efficacy with which tomographic regionalization (TR) represents the subsurface geography of major tectonic units in Southern California indicates that the TR idealization works surprisingly well in this part of the Pacific‐North America plate boundary. High‐frequency attenuation tomography in Southern California supports the hypothesis that the decay of P and S waves propagating through the upper and middle crust is dominated by elastic scattering from small-scale heterogeneities of high fractal dimension, rather than by anelastic dissipation. The amplitude of the scattering varies systematically among the tectonic regions and correlates with the degree of recent faulting and deformation within a region. These results motivate speculation that small-scale crustal heterogeneities responsible for high-frequency attenuation are generated within the seismogenic zone through a dynamic interplay between distributed deformation and localized fracturing.
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- Award ID(s):
- 2225216
- PAR ID:
- 10610287
- Publisher / Repository:
- Annals of Geophysics
- Date Published:
- Journal Name:
- Annals of Geophysics
- Volume:
- 67
- ISSN:
- 1593-5213
- Page Range / eLocation ID:
- S429
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.4401/ag-9149
