James H. Luetgert

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 architecture of the cascadia forearc.

Seismic profiling data indicate that the thickness of an accreted oceanic terrane of Paleocene and early Eocene age, which forms the basement of much of the forearc beneath western Oregon and Washington, varies by approximately a factor of 4 along the strike of the Cascadia subduction zone. Beneath the Oregon Coast Range, the accreted terrane is 25 to 35 kilometers thick, whereas offshore Vancouver Island it is about 6 kilometers thick. These variations are correlated with variations in arc magmatism, forearc seismicity, and long-term forearc deformation. It is suggested that the strength of the forearc crust increases as the thickness of the accreted terrane increases and that the geometry of the seaward edge of this terrane influences deformation within the subduction complex and controls the amount of sediment that is deeply subducted.

Composition of the crust in the Grenville and Appalachian Provinces of North America inferred from V P /V S ratios

We use the ratios between P and S wave velocities (VP/VS), derived from seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crust exhibits VP/VS increasing with depth from 1.64 to 1.84; there is a clear distinction between the Grenville Province (average VP/VS=1.81) and the Appalachian Province (average VP/VS=1.73) which persists at all depths. The boundary between these provinces is east dipping extending for 100 km east of the Champlain thrust. In the Appalachian Province the increase in VP/VS ratios with depth from 1.67 to 1.74±0.02 may reflect a normal decrease of silica content in the continental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous VP/VS ratio of 1.82±0.02 is observed extending to a depth of 10 km; this correlates with the abundance of Ca-plagioclase in the Marcy Anorthosite. At greater depth (15–20 km), where seismic lamination and high electrical conductivity is observed, VP/VS is 1.84±0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25-km-thick lower crust of the Grenville Province the VP/VS is 1.84±0.02 and P-velocity is 7.0±0.1 km/s, which are typical for plagioclase-bearing rocks (gabbro-norite). The high VP/VS ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% of the worlds known anorthosites, high VP/VS ratio is related to high plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid-Proterozoic.

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.

Transition from slab to slabless: Results from the 1993 Mendocino triple junction seismic experiment

Three seismic refraction-reflection profiles, part of the Mendocino triple junction seismicexperiment,allowustocompareandcontrastcrustanduppermantleoftheNorth American margin before and after it is modified by passage of the Mendocino triple junction. Upper crustal velocity models reveal an asymmetric Great Valley basin overlying Sierran or ophiolitic rocks at the latitude of Fort Bragg, California, and overlying Sierran or Klamath rocks near Redding, California. In addition, the upper crustal velocity structure indicates that Franciscan rocks underlie the Klamath terrane east of Eureka, California.TheFranciscancomplexis,onaverage,laterallyhomogeneousandisthickestinthe triple junction region. North of the triple junction, the Gorda slab can be traced 150 km inboardfromtheCascadiasubductionzone.Southofthetriplejunction,strongprecritical reflections indicate partial melt and/or metamorphicfluids at the base of the crust or in theuppermantle.BreaksinthesereflectionsarecorrelatedwiththeMaacamaandBartlett Springs faults, suggesting that these faults extend at least to the mantle. We interpret our datatoindicatetectonicthickeningoftheFranciscancomplexinresponsetopassageofthe Mendocino triple junction and an associated thinning of these rocks south of the triple junction due to assimilation into melt triggered by upwelling asthenosphere. The region of thickenedFranciscancomplexoverliesazoneofincreasedscattering,intrinsicattenuation, or both, resulting from mechanical mixing of lithologies and/or partial melt beneath the onshore projection of the Mendocino fracture zone. Our data reveal that we have crossed thesouthernedgeoftheGordaslabandthatthisedgeand/ortheoverlyingNorthAmerican crust may have fragmented because of the change in stress presented by the edge.

Crustal structure of the southeastern Grenville Province, northern New York State and eastern Ontario

The Grenville province exposes an oblique cross section through mid-lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the Grenvillian orogeny (1.1 Ga). The southeastern Grenville province is divided into two subterranes by the Carthage-Colton mylonite zone, a 110-km-long lineament characterized by intense ductile shear and igneous intrusion, which separates the amphibolite facies metasediments of the Central Metasedimentary Belt to the west from the granulite facies metaplutonics of the Central Granulite Terrane to the east. The recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the Grenville province raises questions concerning the deep structure of these subterranes and, in particular, the means by which the mid-lower crustal rocks exposed in the Grenville province were emplaced. Seismic refraction/wide-angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern Grenville province. A travel time inversion for velocity and interface depth was applied to the seismic data, together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern Grenville province. In the Central Metasedimentary Belt the upper crust is characterized by velocities in the range 6.3–6.4 km/s and a Poissons ratio of 0.26 ± 0.01 which are attributed to quartzofeldspathic rocks. Farther east in the Central Granulite Terrane, upper crustal velocities of 6.55 km/s and a Poissons ratio of 0.28 ± 0.01 are associated with the Marcy Anorthosite. The seismic homogeneity of the upper crust in the region of the Carthage-Colton mylonite zone suggests that this boundary is a shallow feature, limited to the upper 2–3 km of the crust. The deep crustal structure of the southeastern Grenville province is characterized by two discrete and laterally discontinuous seismic interfaces. In the Central Metasedimentary Belt the top of the lower crust is delineated by an eastward dipping interface at 24–28 km depth. In the Central Granulite Terrane, prominent en echelon reflections, referred to as the Tahawus complex, form a gently arched dome at 17–22 km depth. Interpretation of the Tahawus complex as a zone of layered mafic cumulates is supported by its high velocity (7.1 km/s) and Poissons ratio (0.27 ± 0.02). The lower crust is characterized by a velocity of 7.0–7.2 km/s and an anomalously high Poissons ratio of 0.30 ± 0.02, which are representative of pyroxene-garnet granulites. In contrast, velocities of 6.8–7.0 km/s are modeled beneath the Central Granulite Terrane and appear to signify a lateral change in composition. The Moho lies at 44–45 km depth and is characterized by pronounced en echelon reflection segments, suggesting compositional interlayering around the crust-mantle boundary. The velocity of the upper mantle is 8.0–8.2 km/s. An anomalous upper mantle layer with a reversed velocity of 8.6 km/s dips eastward from 50 to 60 km depth beneath the southeastern Grenville province. Our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the Grenvillian orogeny are preserved in the mid-lower crust as a layered cumulate body (Tahawus complex) and in the upper mantle as an eclogitic lens, possibly delaminated from the overthickened crust during uplift of the southeastern Grenville province.

Crustal structure of the western New England Appalachians and the Adirondack Mountains

We present an interpretation of the crustal velocity structure of the New England Appalachians and the Adirondack Mountains based on a seismic refraction/wide-angle reflection experiment in eastern North America extending from the Adirondacks in New York State through the northern Appalachians in Vermont and New Hampshire to central Maine. Modeling of the eastern portion of the profile within the New England Appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5.5 to 6.2 km/s, a midcrustal velocity of 6.4 km/s, and a lower crustal velocity of approximately 6.8 km/s. Crustal thickness increases from 36 km beneath Maine to 40 km in Vermont. Little evidence is seen for structures at depth directly related to the White Mountains or the Green Mountains. A major lateral velocity change in the upper and mid crust occurs between the Appalachians and the Adirondacks. This boundary, projecting to the surface beneath the Champlain Valley, dips to the east beneath the Green Mountains and extends to a depth of ∼25 km below the eastern edge of the Connecticut Valley Synclinorium in Vermont. The Tahawus Complex, a series of strong horizontal reflections at 18–24 km depth beneath the Adirondack Highlands, is seen to dip eastward beneath Vermont. Upper crustal rocks in the Adirondack Mountains have Poissons ratios of 0.28±0.01 that can be correlated with the Marcy Anorthosite. Pois sons ratios of 0.24±0.01 calculated for rocks of the Connecticut Valley Synclinorium indicate a siliceous upper crust in Vermont. The lower crust is considered to be best represented by intermediate to mafic granulites; a high Poissons ratio (0.26–0.27) tends to support a mafic lower crust in the New England Appalachians. This seismic refraction/wide-angle reflection experiment provides further evidence for the obduction of the allochthonous western Appalachian units onto Grenvillian crust above a zone of detachment that penetrates at least to midcrustal depths and was the locus of successive Paleozoic thrusting.

Strong-motion observations of the M 7.8 Gorkha, Nepal, earthquake sequence and development of the N-shake strong-motion network

We present and describe strong-motion data observations from the 2015 M 7.8 Gorkha, Nepal, earthquake sequence collected using existing and new Quake-Catcher Network (QCN) and U.S. Geological Survey NetQuakes sensors located in the Kathmandu Valley. A comparison of QCN data with waveforms recorded by a conventional strong-motion (NetQuakes) instrument validates the QCN data. We present preliminary analysis of spectral accelerations, and peak ground acceleration and velocity for earthquakes up to M 7.3 from the QCN stations, as well as preliminary analysis of the mainshock recording from the NetQuakes station. We show that mainshock peak accelerations were lower than expected and conclude the Kathmandu Valley experienced a pervasively nonlinear response during the mainshock. Phase picks from the QCN and NetQuakes data are also used to improve aftershock locations. This study confirms the utility of QCN instruments to contribute to ground-motion investigations and aftershock response in regions where conventional instrumentation and open-access seismic data are limited. Initial pilot installations of QCN instruments in 2014 are now being expanded to create the Nepal–Shaking Hazard Assessment for Kathmandu and its Environment (N-SHAKE) network. Online Material: Figures of Pg arrivals, earthquake locations, epicenter change vectors, and travel-time misfit vector residuals, and tables of QCN and NetQuake stations and relocated hypocenter timing, location, and magnitude.

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.

Lithoprobe east onshore-offshore seismic refraction survey: constraints on interpretation of reflection data in the Newfoundland Appalachians

Abstract Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections.