Megan L. Anderson

Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis

[1] We examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the South American subduction zone. Data is from the CHARGE network, which traversed Chile and western Argentina across two transects between 30� S and 36� S. Beneath the southern and northwestern parts of the network, fast polarization direction (j) is consistently trench-parallel, while in the northeast j is trench-normal; the transition between these two zones is gradual. We infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab. We explain our observations with a model in which eastward, Nazca-entrained asthenospheric flow is deflected by retrograde motion of the subducting Nazca plate. Resulting southward flow through this area produces N-S j observed in the south and northwest; E-W j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab. INDEX TERMS: 7203 Seismology: Body wave propagation; 7218 Seismology: Lithosphere and upper mantle; 8123 Tectonophysics: Dynamics, seismotectonics; 8150 Tectonophysics: Plate boundary—general (3040); 9360 Information Related to Geographic Region: South America. Citation: Anderson, M. L., G. Zandt, E. Triep, M. Fouch, and S. Beck (2004), Anisotropy and mantle flow in the Chile-Argentina subduction zone from shear wave splitting analysis, Geophys. Res. Lett., 31, L23608, doi:10.1029/ 2004GL020906.

Seismic evidence for orthopyroxene enrichment in the continental lithosphere

We assess the ability of predicted seismic velocities (Vp, compressional wave; Vs, shear wave; and Vp/Vs) to identify regions in the upper mantle that are enriched in orthopyroxene relative to normal melt-depleted peridotite compositions. Orthopyroxene enrichment has been found in mantle xenoliths from a number of locations, including the Colorado Plateau and the Kaapvaal craton. We find that the Vp/Vs ratio is very sensitive to orthopyroxene concentration, but it is not sensitive to depletion level. We compare these predicted velocities and Vp/Vs ratios to the high Vs, low Vp/Vs anomaly found above the central Chile–Argentina flat slab. Within error, the predicted velocities of some of the orthopyroxene-enriched xenoliths match the observed velocities from the Chile-Argentina upper mantle. Because the anomaly above the central Chile–Argentina flat slab does not conform to known terrane boundaries but does align with the downgoing Juan Fernandez ridge track, we suggest that the orthopyroxene enrichment in this area may be related to flat slab processes, which would have implications for our understanding of lithospheric silica enrichment in the western U.S. and elsewhere.

Structure of the Bighorn Mountain region, Wyoming, from teleseismic receiver function analysis: Implications for the kinematics of Laramide shortening

Basement-cored uplifts are observed globally and remain an enigmatic feature of plate tectonics due to the fact that, in many cases, they occur distant from a plate boundary. The Laramide Bighorn Arch in Wyoming is an archetypal basement-involved foreland arch and provides an excellent setting for the investigation of such structures. Previous studies proposed diverse arch formation models; each of which predicts a unique crustal geometry. We use high-resolution crustal imaging from teleseismic P wave receiver functions to test these models. We obtained our data from 239 three-component seismometers deployed as part of the Bighorns Arch Seismic Experiment as well as coeval regional Transportable Array stations. A sequential, two-layer thickness VP/VS (H-κ) stacking algorithm constrains sediment and crustal structure. Receiver function Common Conversion Point stacking results in 2-D transect images across the arch. Our results define an upwarp of the crust beneath the central and northern arch that extends into the Powder River Basin, north-northeast of the arch. The lack of Moho-cutting faults or a Moho geometry mirroring the arch rules out most shortening models except a crustal detachment model where shortening was accomplished by fault-propagation folding on a thrust splay ramping off a midcrustal detachment fault. The mismatch between gentle, symmetric Moho and asymmetric Laramide arch geometries and their trends suggests a pre-Laramide origin for at least a part of the Moho high. This high, perhaps in combination with a lesser degree of Laramide lithospheric buckling, may have caused emergent Laramide thrusting and thus nucleated the Bighorn Arch. Our results suggest that midcrustal detachment can form basement-involved foreland arches and suggest the hypothesis that preexisting undulations in the Moho may have nucleated individual arches.

Saddle Mountain fault deformation zone, Olympic Peninsula, Washington: Western boundary of the Seattle uplift

The Saddle Mountain fault, first recognized in the early 1970s, is now well mapped in the Hoodsport area, southeastern Olympic Peninsula (northwestern United States), on the basis of light detection and ranging (LIDAR) surveys, aerial photography, and trench excavations. Drowned trees and trench excavations demonstrate that the Saddle Mountain fault produced a M W 6.5–7.0 earthquake 1000–1300 yr ago, likely contemporaneous with the M W 7.5 Seattle fault earthquake 1100 yr ago and with a variety of other fault and landslide activity over a wide region of the Olympic Peninsula and Puget Lowland. This near synchroneity suggests that the Saddle Mountain and Seattle fault may be kinematically linked. Aeromagnetic anomalies and LIDAR topographic scarps define an en echelon sequence of faults along the southeastern Olympic Peninsula of Washington, all active in Holocene time. A detailed analysis of aeromagnetic data suggests that the Saddle Mountain fault extends at least 35 km, from 6 km southwest of Lake Cushman northward to the latitude of the Seattle fault. A magnetic survey over Price Lake using a nonmagnetic canoe illuminated two east-dipping reverse faults with 20 m of vertical offset at 30 m depth associated with 2–4 m of vertical displacement at the topographic surface. Analysis of regional aeromagnetic data indicates that the Seattle fault may extend westward across Hood Canal and into the Olympic Mountains, where it terminates near the northward terminus of the Saddle Mountain fault. The en echelon alignment of the Saddle Mountain and nearby Frigid Creek and Canyon River faults, all active in late Holocene time, reflects a >45-km-long zone of deformation that may accommodate the northward shortening of Puget Lowland crust inboard of the Olympic massif. In this view, the Seattle fault and Saddle Mountain deformation zone form the boundaries of the northward-advancing Seattle uplift.

Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

NSF [EAR-0738935, EAR-0739001, EAR-1565475]; Colorado College Patricia Buster Scholarship Fund; National Science Foundation through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation [EAR-1261681]

Assessment of Regional-Distance Location Calibration Using a Multiple-Event Location Algorithm

Abstract We test the use of a multiple-event seismic location method to improve epicenter accuracy estimates. Regional arrival-time observations of 74 Nevada Test Site explosions with known locations comprise the test data set. We investigate epicenter accuracy as a function of the number of events in the multiple-event system that are constrained at the known hypocenter (calibration), the effect of distance between calibration and unconstrained events, and the use of velocity models with varying travel-time prediction accuracy. Further, we test the utility of using a posteriori travel-time residuals to assess location and travel-time prediction accuracy. We find that constraining one event at the known hypocenter reduces epicenter error for all other events by 58% on average compared to locations produced without constraining events. The incremental improvement in epicenter accuracy rapidly diminishes as more hypocenters are constrained, and incremental location improvement is minimal when the number of constrained hypocenters exceeds 10. Events closest to a constrained event exhibit small location bias. Distinct epicenter bias occurs when the distance between the calibration event and the relocated event is greater than a few tens of kilometers. Last, we confirm that metrics based on a posteriori travel-time residuals are poor indicators of both epicenter accuracy and velocity model-based travel-time prediction accuracy.