Kate C. Miller

Origin of High Mountains in the Continents: The Southern Sierra Nevada

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

Crustal and uppermost mantle structure along the Deep Probe seismic profile

The Rocky Mountain region has undergone a complex tectonic history that includes Proterozoic accretion to form the North American craton, late Paleozoic deformation, Cretaceous to early Tertiary shortening, and Oligocene to Recent extension. Understanding the effects of these events on lithospheric structure was the primary goal of the Deep Probe seismic experiment. This is a lithospheric-scale study of the Rocky Mountain region that attempted to image crust and upper mantle structures up to 500 km depth to provide insights on the effect of various tectonic events on todays continental structure. To accomplish this goal, instruments were deployed along a 2400-km-long transect from New Mexico to Canada to record explosions 10 times more powerful than those employed in conventional crustal studies. The Deep Probe results provide new constraints on the location and geometry of the Archean–Proterozoic boundary near the Colorado–Wyoming border, as well as new information on crustal thickness, and uppermost mantle velocities along the profile. Geophysical modeling of the profile used well log and geologic data to evaluate the composition and structure of the uppermost crust. Seismic refraction and reflection, gravity, and receiver function studies were employed to constrain properties of the lower crust and upper mantle structure. The final model shows that seismic velocities along the Deep Probe profile range from 3.5 km/s in the basins to over 8.2 km/s in the upper mantle. At the southern end of the profile, the model indicates a crustal thickness of about 35 km beneath the Basin and Range province. The crust gradually thickens to about 40 to 45 km going north along the profile into the Colorado Plateau. An area of 50 km-thick crust under northwestern Colorado may reflect Proterozoic tectonism related to the suture zone between the Archean and Proterozoic terranes. Northwestward thinning of the crust to about 40 km under southern Wyoming is interpreted as evidence for a relict (2.0 Ga) passive continental margin. The crust in the Archean Wyoming province thickens to over 50 km going north, and then thins again under southern Canada. This thickening is due to a lowermost crustal layer that is about 20 km thick and is confined to the Archean Wyoming province. This lower crustal layer has velocities ranging from 7.05 to 7.3 km/s, which corresponds to a mafic composition. Thus, this layer is interpreted as mafic material that was probably underplated during the Archean. The uppermost mantle of the Archean Wyoming province has lower velocities (∼8.1 km/s) on average than typical cratonal areas, which is consistent with it being located in and adjacent to the North American Cordillera, which has undergone significant recent tectonism.

A NEW VIEW INTO THE CASCADIA SUBDUCTION ZONE AND VOLCANIC ARC : IMPLICATIONS FOR EARTHQUAKE HAZARDS ALONG THE WASHINGTON MARGIN

In light of suggestions that the Cascadia subduction margin may pose a significant seismic hazard for the highly populated Pacific Northwest region of the United States, the U.S. Geological Survey (USGS), the Research Center for Marine Geosciences (GEOMAR), and university collaborators collected and interpreted a 530-km-long wide-angle onshore-offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation. We observed (1) an increase in the dip of the Juan de Fuca slab from 2°–7° to 12° where it encounters a 20-km-thick block of the Siletz terrane or other accreted oceanic crust, (2) a distinct transition from Siletz crust into Cascade arc crust that coincides with the Mount St. Helens seismic zone, supporting the idea that the mafic Siletz block focuses seismic deformation at its edges, and (3) a crustal root (35–45 km deep) beneath the Cascade Range, with thinner crust (30–35 km) east of the volcanic arc beneath the Columbia Plateau flood basalt province. From the measured crustal structure and subduction geometry, we identify two zones that may concentrate future seismic activity: (1) a broad (because of the shallow dip), possibly locked part of the interplate contact that extends from ∼25 km depth beneath the coastline to perhaps as far west as the deformation front ∼120 km offshore and (2) a crustal zone at the eastern boundary between the Siletz terrane and the Cascade Range.

The Yavapai-Mazatzal boundary: A long-lived tectonic element in the lithosphere of southwestern North America

A seismic reflection profile crossing the Jemez lineament in north-central New Mexico images oppositely dipping zones of reflections that converge in the deep crust. We interpret these data as a Paleoproterozoic bivergent orogen, centered on the Jemez lineament, that formed during original Proterozoic crustal assembly by collision of Mazatzal island arcs with Yavapai proto–North American continent at ca. 1.68–1.65 Ga. The two major sets of reflections within the Yavapai-Mazatzal transition boundary dip at 15°–20°, and we interpret them as part of a south-dipping thrust system and as a north-dipping crustal-scale duplex that formed synchronously with the thrust system. The upper crust shows structures recording a succession of tectonic and magmatic events from the Paleoproterozoic to the Holocene. Notable among these structures is a system of nappes that formed during development of the bivergent orogen. Elements of the nappe system are exposed in Rocky Mountain uplifts and have been dated as having formed at 1.68 Ga, at depths of 10 km and at temperatures of >500 °C. We also see continuous bright reflections in the upper part of the middle crust that we associate with basaltic sills that postdate accretion. The data show that the Yavapai-Mazatzal suture is low angle (∼20°), an observation that explains why the boundary between the provinces has previously been so hard to define in the surface geology. The Jemez lineament overlies the root of this bivergent orogen that we also suggest is a Paleoproterozoic zone of weakness that has subsequently acted as a conduit for magmas and a locus of tectonism.

Amplification of Seismic Waves by the Seattle Basin, Washington State

Recordings of the 1999 M w 7.6 Chi-Chi (Taiwan) earthquake, two local earthquakes, and five blasts show seismic-wave amplification over a large sedimentary basin in the U.S. Pacific Northwest. For weak ground motions from the Chi-Chi earthquake, the Seattle basin amplified 0.2- to 0.8-Hz waves by factors of 8 to 16 relative to bedrock sites west of the basin. The amplification and peak frequency change during the Chi-Chi coda: the initial S -wave arrivals (0–30 sec) had maximum amplifications of 12 at 0.5–0.8 Hz, whereas later arrivals (35–65 sec) reached amplifications of 16 at 0.3–0.5 Hz. Analysis of local events in the 1.0- to 10.0-Hz frequency range show fourfold amplifications for 1.0-Hz weak ground motion over the Seattle basin. Amplifications decrease as frequencies increase above 1.0 Hz, with frequencies above 7 Hz showing lower amplitudes over the basin than at bedrock sites. Modeling shows that resonance in low-impedance deposits forming the upper 550 m of the basin beneath our profile could cause most of the observed amplification, and the larger amplification at later arrival times suggests surface waves also play a substantial role. These results emphasize the importance of shallow deposits in determining ground motions over large basins.

Seismic survey probes urban earthquake hazards in Pacific Northwest

A multidisciplinary seismic survey earlier this year in the Pacific Northwest is expected to reveal much new information about the earthquake threat to U.S. and Canadian urban areas there. A disastrous earthquake is a very real possibility in the region. The survey, known as the Seismic Hazards Investigation in Puget Sound (SHIPS), engendered close cooperation among geologists, biologists, environmental groups, and government agencies. It also succeeded in striking a fine balance between the need to prepare for a great earthquake and the requirement to protect a coveted marine environment while operating a large airgun array.

Shortening within underplated oceanic crust beneath the Central California Margin

Structures within a tectonically underplated layer of early Miocene age oceanic crust indicate that the lower crust of offshore central California has been deformed under compression prior to the inception of the San Andreas transform system along the continental margin. Shortening in the lower crust is inferred from the geometry of two structures, the Santa Lucia monocline and the Santa Maria antiform, delineated by an integrated interpretation of multichannel and wide-angle seismic reflection data in conjunction with regional gravity data and teleseismic receiver functions. The 700 km of multichannel seismic data and 300 km of wide-angle seismic data from the 1986 Pacific Gas and Electric/EDGE seismic experiment form the primary data base used to establish the northwesterly trend and extent of the monocline and the WNW trend of the antiform. In cross-sectional view, the Santa Lucia monocline is a 30° kink in the oceanic crust, while the Santa Maria antiform is characterized by local thickening of the oceanic crust. These structures are probably the result of convergence between the Monterey and Arguello microplates that took place as subduction along the margin waned. The multichannel seismic reflection data image two Miocene age shelf basins, the Santa Lucia and offshore Santa Maria basins that formed within a transtensional stress regime. The offshore Santa Maria basin directly overlies the Santa Maria antiform, suggesting that either the upper and lower crustal stress regimes were completely decoupled in Miocene time or that the two features formed in different places. Differential slip between the upper crust and lower crust may have been accommodated along a detachment zone located above oceanic crust. Convergence within the oceanic crust and apparent differential motion between upper and lower crust are indicative of the processes that can affect the transition from subduction to an oceanic-continental transform zone.

Integrated crustal structure across the south central California Margin: Santa Lucia escarpment to the San Andreas Fault

The continental margin of south central California has undergone different styles of deformation over the past 30 m.y. as the North America-Pacific plate boundary has evolved from subduction to transform tectonics. Results presented here identify strike-slip and compressional structures in the lower crust that probably developed during this period of tectonic transition. An integrated model of crustal structure along deep seismic line PG&E-3 crosses the entire continental margin west of the San Andreas fault near the latitude of San Luis Obispo. A compressional wave velocity model was developed using coincident marine multi-channel seismic and onshore/offshore wide-angle reflection/refraction seismic data and was tested independently with gravity modeling. A high-velocity layer (6.8–7.0 km/s, about 6 km thick) in the lower crust that extends east possibly as far as the San Andreas fault is interpreted as tectonically underplated oceanic crust. The upper and middle crust of the Patton and Sur-Obispo Franciscan terranes extends to depths of 14 to 20 km. The velocity and density modeling indicate lower average velocities and densities for the Patton terrane, supporting earlier interpretations that it is a distinct terrane within the Franciscan complex. Abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the Santa Lucia Escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 Ma following the subduction of the Pacific-Arguello ridge. An imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust. Convergence between the Monterey and Arguello microplates during the final stages of Farallon subduction (about 24 to 16 Ma) probably caused the imbrication.

Crustal structure along the west flank of the Cascades, western Washington

Knowledge of the crustal structure of the Washington Cascades and adjacent Puget Lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region. We present a model for crustal velocity structure derived from analysis of seismic refraction/wide-angle reflection data collected in 1991 in western Washington. The 280-km-long north-south transect skirts the west flank of the Cascades as it crosses three tectonic provinces including the Northwest Cascades Thrust System (NWCS), the Puget Lowland, and the volcanic arc of the southern Cascades. Within the NWCS, upper crustal velocities range from 4.2 to 5.7 km s−1 and are consistent with the presence of a diverse suite of Mesozoic and Paleozoic metasediments and metavolcanics. In the upper 2–3 km of the Puget Lowland velocities drop to 1.7–3.5 km s−1 and reflect the occurrence of Oligocene to recent sediments within the basin. In the southern Washington Cascades, upper crustal velocities range from 4.0 to 5.5 km s−1 and are consistent with a large volume of Tertiary sediments and volcanics. A sharp change in velocity gradient at 5–10 km marks the division between the upper and middle crust. From approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from ∼6.0 to 7.2 km s−1. These high velocities do not support the presence of marine sedimentary rocks at depths of 10–20 km beneath the Cascades as previously proposed on the basis of magnetotelluric data. Crustal thickness ranges from 42 to 47 km along the profile. The lowermost crust consists of a 2 to 8-km-thick transitional layer with velocities of 7.3–7.4 km s−1. The upper mantle velocity appears to be an unusually low 7.6–7.8 km s−1. When compared to velocity models from other regions, this model most closely resembles those found in active continental arcs. Distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect. In the NWCS and Puget Lowland, most of the seismicity occurs below the base of the upper crust as defined by a seismic boundary at 5–10 km depth and continues to 20–30 km depth. The region of transition between the NWCS and the Puget Lowland appears as a gap in seismicity with notably less seismic activity north of the boundary between the two. Earthquakes within the Cascades are generally shallower (0–20 km) and are dominated by events associated with the Rainier Seismic Zone.

Using expert knowledge in solving the seismic inverse problem

In this talk, we analyze how expert knowledge can be used in solving the seismic inverse problem. To determine the geophysical structure of a region, we measure seismic travel times and reconstruct velocities at different depths from this data. There are several algorithms for solving this inverse problem.