Basin and range crustal and upper mantle structure, northwest to

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Apr 10, 1991 - Solutions to the above NMO ... derived from our seismic model, accounts for many of the variations in the gravity ..... crustal (7.35 km/s) to the west under the town of Fallon,. Nevada ..... Allmendinger, R. W., T.A. Hauge, E. C. Hauser, C. J. Potter, .... Zoback, M. L., State of stress and modern deformation of the.
JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL. 96, NO. B4, PAGES 6247-6267, APRIL 10, 1991

Basin and Range Crustal and Upper Mantle Structure, Northwest

to Central

R. D. CATCHINGS

Nevada

AND W. D. MOONEY

U.S. Geological Survey, Menlo Park, California

We presentan interpretationof the crustaland uppermostmantle structureof the Basin and Range of northwesternNevada based on seismicrefraction/wide-anglereflection, near-vertical reflection, and gravity data. In comparisonto most previous estimates,we find that the crust is somewhat thicker (32-36 km versus22-30 km), and the uppermostmantle velocity is somewhathigher (8.0 km/s versus 7.3-7.9 km/s). Along our transects,the crust is thinnest(32 km) in the Carson Sink-Buena Vista Valley region and increasesby 2-4 km to the west and east, respectively.There is considerablecomplexity throughoutthe crustwhere velocitiesrangefrom of 2.5 km/s at the surfaceto 7.4 km/s in the lowermost crust. Variations in velocity and structure of the upper crustal layers reveal apparent basement velocity depressions(areas of lower velocitiesextendingup to 10 km in depth) that underlie some surfacerangesas well as the basins. The middle crust rises from about 20 km beneath central Nevada to within 12 km of the surface beneath the area of thinnest crust and is characterized by a modest (-0.1 km/s) changein velocity and low-velocity gradients.These midcrustallayers mark the onset of high crustal reflectivity and the apparent limiting depth to which Basin and Range faults can be traced in near-vertical reflection profiles, suggestingthat these midcrustal layers represent the transition between the brittle and ductile zones of the crust. The lower crust is more structurally complex, with layers thickeningand thinningin a systematicmannerwith the upper crustallayers; generally, where there are velocity depressionsin the upper crust, the lower crust is thickest and shallowest. The geometry of these lower crustal layers (derived from refraction modeling) coincideswith changesin the crustal reflectivity, determined from the Consortium of Continental Reflection Profiling reflection data. The lower crustal layer is unusually high in velocity (7.4 km/s) and is likely the layer identified as mantle in somepreviousstudies.We do not identify the 7.4 km/s layer as mantle because(1) there is an underlying layer with a velocity (8.0 km/s) that is more consistentwith the worldwide average velocity for the uppermantle, and (2) the 7.4 km/s layer doesnot correspondto the "reflection" Moho. Gravity modeling and comparison to existing seismic models show a general consensusin many aspectswith respectto crustal structure. This new model forms the basisfor speculationon some of the processesassociatedwith rifting of the Basin and RangeProvince. One suchprocess,lithospheric magmatism,is inferred from the strongattenuationof transmittedseismicwaves, which occursat the sameinterface at which high-amplitude,bright spot reflectionsoriginate. Unlike previous models, the overall structure and velocity of the crust and uppermost mantle of our new model are similar to other regions worldwide which have undergone high degrees of extension.

INTRODUCTION

The processof lithosphericextensionhasbeen the subjectof intensive geologicand geophysicalinvestigationsfor the past decade. It has been increasinglyrecognizedthat new methods of seismicdata acquisitionwhich utilize a broad spectrum of seismictechniqueshold the greatestpromisefor contributingto these investigations.Such a seismicmethod was implemented in northwestern

to central Nevada

in June 1986 which utilized

The northwestern to central Nevada region was chosen as an investigative site because it is a structurally complex region of active extension where other geophysical studies have been undertaken, some of which suggest conflicting results. One geophysical observation of particular interest to our program was an apparent discrepancy in crustal structure that

the velocity information of refraction seismology(which, with other constraints,limits compositionalmodels), the structural resolution of reflection seismology,and the tectonic information of earthquake seismology.This 1986 Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) seismicprogram complementedexisting seismic and potential fields data in an effort to investigatethe processesassociated with active extension of the Basin and Range Province. The field work was conducted by a consortium of academic, governmental,and private organizationsand was sponsoredby PASSCAL, the U.S. Geological Survey (USGS), and the Air Force GeophysicalLaboratory. A list of participatinginstitutions and a description of the seismicprogram and its objectives have been discussedby Catchingset al. [1988].

obtained

seismic

from

studies.

the crustal

recent

Older

reflection

seismic

thickness

and

refraction

in northwestern

older

refraction

studies Nevada

indicated was

as

little as 20-25 km [Eaton, 1963; Priestley et al., 1982; Stauber and Boore, 1978], but more recent seismic reflection studies along the 40øN corridor suggestthe crustal thickness to be a minimum of 30-32 km [Klemperer et al., 1986; Allmendinger et al., 1987; Hauge et al., 1987; Potter et al., 1987]. Furthermore, some of the earlier refraction studies indicated that the crust thickened by up to 12 km from northwestern to central Nevada, whereas the later reflection studies indicate thickening of no more than about 7 km. Such discrepancieslead to suggestionsthat seismicreflection and seismic refraction methods sampled different horizons within the Earth and were not compatible. As there are

This paper is not subject to U.S. copyright. Published in 1991 by the American Geophysical Union. Paper number 91JB00194. 6247

few

coincident

seismic

reflection

and

refraction

studies

available, this study represents a rare opportunity to compare directly the results of the two techniques since both types of data were acquired from the same sources. Two intersecting seismicprofiles were acquired during the

6248

CATCHINGS

AND MOONEY:

BASIN AND RANGE CRUSTAL AND UPPER MANTLE

1986 PASSCAL seismic program, both of which intersected existing seismic refraction profiles and one of which was approximately coincident with some of the profiles of the Consortium for Continental Reflection Profiling (COCORP) 40øN transect (CC1, CC2, CC3, and CC7; Figure 1). Both PASSCAL transects contained, at least in part, coincident seismicrefraction and seismicreflection arrays. In this paper we present our analysis of the PASSCAL wide-angle reflection/refraction data and the resulting lithospheric models from those data. We then compare that model with COCORP and PASSCAL seismic reflection data and present a gravity model derived from our seismic model. Since previous refraction surveys have been conducted in this region, we compare our results with the previous studiesin order to investigate the above mentioned discrepancy between reflection and refraction studies. Our study is one of several studies simultaneously conducted on the 1986 PASSCAL data. Other investigators include Benz et al. [1990], C. Jarchow (manuscript in preparation, 1991), Hawman et al. [1990], and Holbrook [1990].

long NW-SE profile (Figure 1). These profiles were recorded using a variety of seismicinstrument types and sources. The two

seismic

Much

of the western

AND GEOPHYSICAL United

SETTING

States from the latitudes

of

central Oregon to northern Mexico and from the longitudes of California to Utah is covered by a topographically striking series of mountain ranges and sediment-filled basins, commonly referred to as the Basin and Range Province (Figure 1). Although the Basin and Range varies in elevation between 1 and 3 km above sea level, at about 40øN latitude this expansive area can be generally characterized as a topographically lower-lying region between the topographically higher and tectonically stable Sierra Nevada and Colorado Plateau. At this latitude, the Basin and Range is also characterized by two extensive topographic depressions near its western and eastern edges, the Lahonton and Bonneville depressions[Russell, 1885], and by a topographically higher region in the center (Figure 1). Tectonism along the 40øN corridor is apparently characterized by WNW to ESE extension over the past 10 m.y. [Zoback et al., 1981; Zoback, 1989], which has given rise to NE striking basins and ranges (Figure lb). Present-day extension, as indicated by historical seismicity patterns, may be more acute within the region of the Bonneville and Lahonton depressions [Thompson and Burke, 1974; Smith, 1978; Vetter and Ryall, 1983], and the associated volcanism is primarily bimodal to basaltic [Christiansen and McKee, 1978]. Geophysical characterizations of these depressions include high Bouguer gravity values [Eaton et al., 1978], anomalous velocities of the lower crust/upper mantle (7.3-7.8 km/s) [Pakiser, 1963; Eaton, 1963; Priestley et al., 1982; Stauber and Boore, 1978], high heat flow [Lachenbruch and Sass, 1978], moderately active seismicity [Ryall and Vetter, 1982], relatively thin crust [Eaton, 1963; Prodehl, 1979; Priestley et al., 1982; Stauber, 1980; Stauber and Boore, 1978], and a relatively thin lithosphere[Priestleyand Brune, 1978;Taylor and Patton, 1986]. Many of these characterizations suggest that these depressionsare presentlyrifting at a somewhathigherrate than the adjacentregionsof the Basin and Range Province. EXPERIMENT

The 1986 PASSCAL experiment consisted of two intersectingprofiles, a 200-km-long NE-SW profile and a 300-km-

refraction

transects

were

recorded

with

a

matched array of 120 vertical component USGS seismographs and an array of 60 three-component seismographsof various types. In addition, parts of the two transects were also recorded using a 396-channel array of conventional seismic reflection

recorders.

More

detailed

information

on

the experiment is presented by Catchings et al. [1988]. The vertical component refraction data were recorded at intervals ranging from about 900 to 1000 m over the NW-SE transect

and at a 1.5-km

interval

over the NE-SW

transect.

The seismicreflection data were recorded at group intervals ranging from 67 to 100 m along the center 25 km of the NE-SW

transect and the center 45 km of the NW-SE

transect

and are presented by C. Jarchow (manuscript in preparation, 1991). Seismic sourcesused in this study were provided by 26 chemicalexplosionsrangingin size from 225 to 2720 kg (500 to 6000 lb) and two vibroseis trucks. The chemical explosions were spaced at an average interval of about 50 km on the NE-SW

GEOLOGICAL

STRUCTURE

transect

and about 45 km on the NW-SE

transect

(except near the intersectionof the two profiles, where spacing was about 15 km). Vibration points were occupied every 0.1 km over the 396-channelreflection array. INTERPRETATIVE

METHOD

Because the data presented in this paper are also evaluated by other investigators and the reported results may differ in details, we outline our interpretative method. In this interpretation and that of Benz et al. [1990] and Holbrook [1990] a forward modeling approach was undertaken. While Benz et al. used one-dimensionalreflectivity methods, Holbrook and we used a two-dimensional ray tracing and reflectivity approach. Many of the differences in interpretation are likely due to differences in (1) correlation of seismic phases, (2) interpretational methods, and (3) methods of presenting the results (e.g., time sections versus depth sections, isovelocity models versus layered models, onedimensional versus two-dimensional models, etc). For the deeper crustal and upper mantle phases, phase correlation from shot point to shot point can be especially difficult. This is particularly troublesome in that phase correlation is the basis for our entire interpretative process. To minimize some of the difficulty in phase correlations, we enhancedphases of interest by plotting the data in multiple formats, including different reduction velocities, varying gain settings (automatic gain control, true amplitude, and trace normalized), normal move-out corrected, and varying filter settings. Finally, using these varying data formats, we picked first and secondaryphases to be modeled and estimate the picks to be within 0.05 s of the actual arrivals. Our initial one-dimensional P wave model was derived by fitting first and secondary arrivals with an interactive onedimensional ray-tracing method described by Luetgert [1988a]. An initial two-dimensional P wave model was then derived by combining the one-dimensional models from each shot point and interpolating between the shot points. Where layers dipped, the apparent velocity for each onedimensional model was used to determine its true velocity. For the shallow crust (