Closure of the Pacific Ocean basin by the convergence of its surrounding plates, some of which have deep continental roots, implies that there is net mass flux out of the mantle under the Pacific. Here we report on a shear-wave splitting study designed to test the prediction that there should be flow around its southern margin. Our results show no evidence for present-day flow around the tip of southern South America. Instead, the results suggest present-day flow directions in the southern Atlantic that parallel the South American absolute plate motion direction, even under Antarctica. The results also provide evidence for absolute plate motion driven by the basal drag of ocean basin-scale mantle flow, and suggest that ∼200 km thick flow boundary layers exist under South America and Antarctica, and also demonstrate that mantle flow directions cannot be reliably inferred from present-day plate morphology.
Geodetic elevation changes record the broad‐scale deformation associated with the M = 7.0 October 28, 1983, Borah Peak, Idaho, earthquake on the Lost River fault. The crest of the Lost River Range rose 0.2 m, and adjacent Thousand Springs Valley subsided 1.0 m, in relation to reference points 45 km from the main shock epicenter. The deformation was modeled by dislocations in an elastic half‐space. A planar fault with uniform dip slip of 2.05±0.10 m, dipping 47°±2°SW and extending to a vertical depth of 13.3±1.2 km, fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution. The geodetic moment is 2.6±0.5×10 19 N m (2.6±0.5×10 26 dyn cm), and the estimated static stress drop is 2.9±0.4 MPa (29±4 bars). Tests for coseismic slip on listric faults (which flatten with depth) and on detachments (horizontal faults or shear zones) showed fits to the geodetic data that are inferior to those for planar high‐angle faults. No detectable coseismic slip occurred on the Mesozoic White Knob thrust fault, although the low‐angle thrust sheet intersects the south end of Lost River fault near the 1983 mainshock epicenter. If the high‐angle Lost River fault abuts a flat‐lying detachment fault or shear zone, such a structure must lie at depths of >12 km, near the brittle‐ductile transition, where stick‐slip behavior gives way to creep. The depth and geometry of faulting at Borah Peak is similar to that inferred from seismic and geodetic evidence for the 1954 M = 7.2 Fairview Peak, Nevada, and the 1959 M = 7.3 Hebgen Lake, Montana, events, suggesting that if detachments are active at these localities, they are deep and most likely slip by creep.
Abstract Indications of transient crustal displacement associated with the 3 March 1985, M, = 7.8, Central Chile earthquake are evidenced by various observational devices. Almost half a meter of coastal uplift at localities close to the epicentral region was detected by repeated leveling lines. A tide gauge at Valparaiso revealed minor coseismic coastal subsidence (∼ 10 cm) continuously developing after the earthquake. Two limnigraphs, 27 km apart, that were situated at the extremes of Rapel Lake to the south of the leveling line, have recorded continuously the equipotential lake level for more than 10 years, providing a permanent very‐long‐base tiltmeter. The water level difference at the two limnigraphs as a function of time resembles a ramp function, beginning approximately at the time of the earthquake occurrence and gradually developing over a period of 10 months with a maximum amplitude of 120 mm or 4.4 p radians in tilt. The shape of the time‐dependent tilt is mimicked by the sea level signal recorded at Valparaiso, ∼ 100 km away from Rapel Lake, showing a maximum coastal subsidence of 0.6 m. Comparisons of sea level changes produced by the 1971, M, = 7.5, earthquake indicates that they represent rupture in different portions along the seismogenetic region as well as a different rupture mode. Gravity surveys carried out in three different pre‐ and post‐seismic epochs, along the segment of the leveling line which shows major coseismic uplift, indicate that the whole region has subsided, post‐seismically, 10 em in 5 years. These observations are interpreted in terms of a variable slip dislocation model. Inversions show that it is the more than 2 m of fault displacement in 10 months of post‐seismic movement along the contact between the Nazca and South America plates, which is interpreted to be responsible for the time‐dependent elevation changes.
The gridfiles here contain velocity and viscosity models presented in the following paper:
- Mark, H.F., D.A. Wiens, E.R. Ivins, A. Richter, W. Ben Mansour, M.B. Magnani, E. Marderwald, R. Adaros, & S. Barrientos. Lithospheric erosion in the Patagonian slab window, and implications for glacial isostasy. 2022. Geophysical Research Letters, DOI: 10.1029/2021GL096863. * * * The paper contains detailed information on how the velocities and viscosities were calculated. For information on the tomography methods and viscosity calculation used, see the Methods section and references therein, including:
- Barmin, M. P., Ritzwoller, M. H., & Levshin, A. L. (2001). A Fast and Reliable Method for Surface Wave Tomography. Pure Appl. Geophys., 158, 25. - Bensen, G. D., Ritzwoller, M. H., Barmin, M. P., Levshin, A. L., Lin, F., Moschetti, M. P., et al. (2007). Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements. Geophysical Journal International, 169(3), 1239–1260. https://doi.org/10.1111/j.1365-246X.2007.03374.x - Jin, G., & Gaherty, J. B. (2015). Surface wave phase-velocity tomography based on multichannel cross-correlation. Geophysical Journal International, 16. - Lin, F.-C., & Ritzwoller, M. H. (2011). Helmholtz surface wave tomography for isotropic and azimuthally anisotropic structure. Geophysical Journal International, 186(3), 1104–1120. https://doi.org/10.1111/j.1365-246X.2011.05070.x - Lin, F.-C., Ritzwoller, M. H., & Snieder, R. (2009). Eikonal tomography: surface wave tomography by phase front tracking across a regional broad-band seismic array. Geophysical Journal International, 177(3), 1091–1110. https://doi.org/10.1111/j.1365-246X.2009.04105.x - Shen, W., Ritzwoller, M. H., Schulte-Pelkum, V., & Lin, F.-C. (2013). Joint inversion of surface wave dispersion and receiver functions: a Bayesian Monte-Carlo approach. Geophysical Journal International, 192(2), 807–836. https://doi.org/10.1093/gji/ggs050 - Ivins, E. R., Wal, W. van der, Wiens, D. A., Lloyd, A. J., & Caron, L. (2021). Antarctic Upper Mantle Rheology. Geological Society, London, Memoirs, 56. https://doi.org/10.1144/M56-2020-19 - Wu, P., Wang, H., & Steffen, H. (2013). The role of thermal effect on mantle seismic anomalies under Laurentia and Fennoscandia from observations of Glacial Isostatic Adjustment. Geophysical Journal International, 192(1), 7–17. https://doi.org/10.1093/gji/ggs009 * * * The files contain: ### patagonia_vels.grd: Vsv at points throughout the study area, spaced 0.3x0.3 degrees laterally and at 500m depth intervals from 500m to 200 km. Points where the velocity is not constrained are filled with -1. ### patagonia_visc.grd: log10 of viscosity in Pa s at points throughout the study area, spaced 0.3x0.3 degrees laterally and at 500m depth intervals from 500m to 200 km. Points where the viscosity is not constrained are filled with -1. ### patagonia_sed_thickness.grd: Sediment thickness in km at points throughout the study area, spaced 0.3x0.3 degrees laterally. Points where the sediment thickness is not constrained are filled with -1. ### patagonia_moho_depth.grd: Moho depth in km at points throughout the study area, spaced 0.3x0.3 degrees laterally. Points where the Moho depth is not constrained are filled with -1.
Abstract Geological processes in Southern Patagonia are affected by the Patagonian slab window, formed by the subduction of the Chile Ridge and subsequent northward migration of the Chile Triple Junction. Using shear wave splitting analysis, we observe strong splitting of up to 2.5 s with an E‐W fast direction just south of the triple junction and the edge of the subducting Nazca slab. This region of strong anisotropy is coincident with low uppermost mantle shear velocities and an absence of mantle lithosphere, indicating that the mantle flow occurs in a warm, low‐viscosity, 200–300 km wide shallow mantle channel just to the south of the Nazca slab. The region of flow corresponds to a volcanic gap caused by depleted mantle compositions and absence of slab‐derived water. In most of Patagonia to the south of this channel, splitting fast directions trend NE‐SW consistent with large‐scale asthenospheric flow.
Abstract The Patagonian slab window has been proposed to enhance the solid Earth response to ice mass load changes in the overlying Northern and Southern Patagonian Icefields (NPI and SPI, respectively). Here, we present the first regional seismic velocity model covering the entire north‐south extent of the slab window. A slow velocity anomaly in the uppermost mantle indicates warm mantle temperature, low viscosity, and possibly partial melt. Low velocities just below the Moho suggest that the lithospheric mantle has been thermally eroded over the youngest part of the slab window. The slowest part of the anomaly is north of 49°S, implying that the NPI and the northern SPI overlie lower viscosity mantle than the southern SPI. This comprehensive seismic mapping of the slab window provides key evidence supporting the previously hypothesized connection between post‐Little Ice Age anthropogenic ice mass loss and rapid geodetically observed glacial isostatic uplift (≥4 cm/yr).
Patagonia is one of the key places to study the interaction of plate tectonics and mantle flow patterns with geological processes. This part of the continent is shaped by the northward migration of the Chile Triple Junction, currently marked by subduction of the Chilean spreading ridge at latitude 46oS, opening a slab window beneath Southern Patagonia. The idea of slab window was hypothesized to explain the volcanic gap between north Patagonia and the southern part of the peninsula. The analysis of volcanic rock composition shows the transition between a domain with the signature of slab melt (metasomatized MORB) and a domain with no slab signature (OIB source mantle). Along the Pacific coast, other slab windows were suggested in Central America, California and North Cordillera. The analysis of uplifted terranes and seismic imaging tried to constrain the geometry of these slab windows and map the mantle flow pattern that controls the present-day surface expression (topography, volcanism distribution). From a limited seismic coverage, early studies mapped the Patagonian slab window from body wave tomography and shear wave splitting. The recent deployment of a temporary seismic array from 2018 to 2021 and the Chilean seismic networks fills the data gap between the seismically active northern part of Patagonia and the more poorly studied southern part. This presentation will show the results of our recent seismic studies in Patagonia and help constrain the geodynamical processes associated with the slab window. From the analysis of SKS and similar core phases, we determine the pattern of azimuthal seismic anisotropy resulting from the mantle flow pattern beneath South America. Fast splitting directions are generally NE-SW throughout most of Southern Patagonia, similar to the pattern of large-scale azimuthal seismic anisotropy from global and regional surface wave models. However, between 46oS - 48oS, we observe large splitting values and an E-W direction showing the effect of the slab edge. This is consistent with models of rapid upper mantle flow from the Pacific around the southern edge of the Nazca slab. Seismic imaging using receiver functions and Rayleigh waves from earthquakes and ambient noise show very low upper mantle velocities and an absence of mantle lithosphere in this region, suggesting the lithosphere has been thermally eroded by the dynamics of the slab window. We will also show and discuss preliminary results of a body wave tomographic analysis of the same seismic station dataset.
'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(72)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(-72)'/%3e%3cuse xlink:href='%23c' transform='rotate(144)'/%3e%3c/svg%3e)
