Crustal Structure and Composition of the Southern Foothills Metamorphic Belt, Sierra Nevada, California, from Seismic Data

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B4, PAGES 6865-6880, APRIL 10, 1994

Crustal structure and composition of the southern Foothills Metamorphic Belt, Sierra Nevada, California, from seismic data

Kate C. Miller Departmentof GeologicalSciences, The Universityof Texasat E1 Paso

Walter D. Mooney U.S. GeologicalSurvey, Menlo Park,California

Abstract. The FoothillsMetamorphic Belt is an accretedterrane consisting of Paleozoicand Mesozoicmetamorphic rocks that separates the Great Valley from the SierraNevada batholith in northernand central California. Until recently,the only availablegeophysical data for thisarea werereconnaissance refraction surveys, and gravity and magnetic data. New insightsinto the structureof thedeep crust are provided by theinterpretation of a seismicreflection profile (CC- 2), acquiredin 1984by theU.S. GeologicalSurvey at the southernend of the Foothills MetamorphicBelt. Our interpretation isconstrained bya newseismic velocity model derived from coincidentmicroearthquake data. Earthquakehypocenters that occur at unusuallygreat depthsof 12 to 30 km makethe dataset particularly useful for obtainingdeep crustal velocity information.The velocity model shows velocities of 5.2 to 6.3 km s'• for theupper 12 km of thecrust, and 6.7 to 6.8 km s'1 from 12 km to an estimatedMoho at 32 km. The uppercrustal velocitiescorrespond to metamorphicrocks and serpentinites of theFoothills Metamorphic Belt as well asto dioritesand granodioritesof the SierraNevada batholith, while the lower crustal velocitiesare interpretedto representintermediate to mafic granulites.The majorityof the earthquakehypocenters aswell as a 6.7 km s'• layerin thevelocity model corresponds in depth to thick zonesof westdipping midcrustal reflections that may representmajor shearzones formedduring the late Jurassic Nevadan orogeny or synbatholithicductile shear zones that accommodated crustal extension associated with batholith intrusion. These reflections are truncatedupdip by an inferredsubvertical contact that coincides with the westernedge of the SierraNevada batholith and the southwardtrace of the BearMountains fault zone. The updip truncationof midcrustalshear zones and high lowercrustal velocities indicate that strike-slip faultingand magmatic underplating can be importantprocesses during the docking and welding of an accreted terrane.

Introduction compositionof the continentalcrust changesdramatically, is poorly understood. Outcrop geometry [Bailey et al., 1970] The FoothillsMetamorphic Belt (Figure 1) is the long band and limited drill hole and geophysicaldata indicate that the of metamorphicrocks of Mesozoic and Paleozoicage that Cretaceousage sedimentsof the Great Valley are underlain by separatesthe Great Valley forearcbasin from the SierraNevada mafic igneous rock derived from oceanic crust or island arcs batholith in northern and central California. The belt is [Cady, 1975; Saleebyet al., 1986]. The "oceanicaffinity" of structurallycomplex and has been alternativelyinterpreted as the Great Valley crust is further supported by seismic a Jurassiccollisional zone [Schweickert, 1981] or an intra-arc refractiondata [Holbrook and Mooney, 1987], which suggests to forearc transtensional-transpressional shear system that both the middle and lower crust are gabbroic. In contrast, [Saleeby, 1981]. Since the Foothillsmetamorphic rocks are gravity modeling, limited refraction data [Bateman and Eaton, the countryrock into which the Sierra Nevada batholithhas 1967; Oliver, 1977], and xenolith compositions[e.g., Dodge intruded[e.g., Bateman, 1981], late Mesozoic plutonsare also et al., 1988] indicate that the Sierranupper crust (0-22 km) is importantelements of the crustalstructure at the southernend granitic, while the lower crust (22-45 kin) is intermediate to of the Foothills MetamorphicBelt (Figure 1). The deep crustal mafic in composition. A large gradient in the magnetic field structureand compositionof the Foothills MetamorphicBelt along the east edge of the valley (Figure 1) is frequently have not been extensively studied, but are important to a interpreted as the location of the present-daytransition from completeunderstanding of the nature of the deep crust in more mafic crust of oceanic affinity to that of more silicic northern and central California. crust of continentalaffinity [Cady, 1975]. The interposition A complextectonic history combined with a lack of crustal of highly deformed Foothills metamorphicrocks between the geophysicaldata has meant that the deep crustal structureof Great Valley and the Sierra Nevada domains suggeststhat this the Foothills Metamorphic Belt, a zone across which the zone is both structurally and compositionallycomplex. The presenceof a microearthquakenetwork and a seismic Copyright1994 by the AmericanGeophysical Union. reflection profile at the southern end of the Foothills Belt Paper number93JB02755. (Figure 2) permit the first real opportunity to simultaneously 0148-0227/94/93JB-02755505.00 interpret the compositionand structureof the crust within this

6865 6866 MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT

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Mesozoic metamorphic rocks and MesozoicGreatValleysequence Paleozoic ophioliticrocks MesozoicFranciscanComplex Paleozoic metamorphicrocks

Figure 1. Geologic map of study area. Chief rock units after Jennings[1977]. Abbreviations: MFZ, MelonesFault Zone; BMFZ, Bear MountainsFault Zone; SAF, San AndreasFault; GIC, GuadalupeIgneous Complex;WRP, White Rock Pluton;and BLT, Bass Lake Tonalite. Solid trianglesare earthquakestation locations. Thick solid line is seismicreflection profile CC-2. Dashedline in the Great Valley is the axis of the steepestgradient on the northeastside of the Great Valley magnetichigh. Dot-dashedlines are refraction profiles: B&E, Batemanand Eaton; C&M, Colburnand Mooney; and H&M, Holbrookand Mooney.

terrane. Earthquakehypocenters occur at depthsas great as 8 velocity model by comparing observed velocities to to 40 km [Wong and Savage, 1983] which makes the data set laboratorystudies of rock velocities. The new velocity model particularly useful for obtaining deep crustal velocity containsa 6.7 km s '• layerat thesame depth that a prominent informationfrom seismicinversion techniques. In this study set of west dipping reflections occur on the seismic reflection a five-layer velocity model is determined from profile, CC-2 (Figure 1). This velocity constrainsthe material microearthquaketravel time data. In addition, a set of station at the depths of these reflectors to an intermediate to mafic correctionsthat are a by-product of the modeling process, composition;that is, somewhatless mafic than the gabbroic lends insight into the lateral heterogeneityof the velocity composition inferred for the Great Valley but significantly field. The crustal composition is then inferred from the more mafic than the granitic compositioninferred at the same MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT 6867

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Figure 2. Isostatic residual gravity map [Jachens and Griscom, 1985] with the distribution of epicenters (open circles) for all events recorded by the Woodward-Clyde and U.S.G.S. networks from 1977 to 1989 superimposed. Gravity values range from -55 mGal (darkest areas) to 40 mGal (lightest areas). Black lines indicate contactsin Figure 1. Thick solid line is location of seismic line CC-2. Triangles indicate network stations. Line CC-2 is locatedat the north edge of a large gravity low. Also note diffuse spatial distributionof earthquakeepicenters. Inset: histogramshowing depth distributionfor all eventsrecorded by the Woodward- Clyde and USGS networksfrom 1977 to 1989. Hypocentersare unusuallydeep for crustal earthquakesand provide a good sourceof deep crustal velocity information. depth for the Sierra Nevada. Two equally viable the microearthquake data offers the best opportunity for interpretations of the dipping reflectors are that they are obtaining better velocity control in the deep crust of this remnants of Nevadan shear zones that have been truncated by region. Some lateral averaging is an inevitable result of strike-slip faults or intrusion of the Sierra Nevada batholith, parameterizing a heterogeneousEarth in terms of a flat-layer or they are synbatholithicshear zones which accommodated velocity model. Under thesecircumstances by-products of the the crustal extension associated with batholithic intrusion. inversion procedure such as ray path plots, resolution values, Factors that complicate the structural interpretation are (1) a and station corrections form a basis for evaluating the large gravity low, atypical of the Foothills, in general, that averaging process. occurs within the study area, and (2) an intrusion that cuts across a portion of the Foothills rocks at the latitude of the seismic line. Method

Microearthquake Data Analysis An iterative damped least squares inversion technique [Crosson, 1976] is used to derive the flat-layer velocity The analysis of microearthquake data consists of using model, a set of station corrections,and hypocenterlocations arrival time data to derive a one-dimensionalvelocity model from microearthquakearrival time data. Travel time residuals and accompanying station corrections for the central Sierra for each event are obtainedby ray tracing through an initial Nevada Foothills. Since this area lacks deep velocity velocity model. The arrival times were collected from a 22- information from explosion sourceexperiments, inversion of station seismographnetwork deployedin the study area for a

MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT 6869

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Figure 4. A map view of ray paths for eventsused in the inversion. This illustratesspatial coveragefor the velocity model. Select stationnames are annotated.

Results individual runs. The resolution matrix is a measure of the reliability of model parametersestimated by the inversion. A The preliminary flat-layer velocity model input to the diagonal value near 1.0 indicates that the estimatedmodel inversionwas derived from the three-layerrefraction model of parameteris a good estimateof the "true" model parameter. Batemanand Eaton [1967] (Figure 5) which crossesthe study Lower valuesmean that a poor estimateof the true model has area near Fresno. The Bateman and Eaton [1967] model was been made. Representativemodel-to-model variations in the generalized from Eaton [1963]. The model consistsof two diagonal values of the resolution matrix, correspondingto layersabove a mantlehalf-space: a 6.0km s -1 layer from 0 individual velocity layers, are found in Table 1. Velocity to 15km; a 6.9km s-1 layer from 15 km to Mohoand a 7.9km layers in the preferred model have resolution values greater s-1 layer below Moho. The early refraction results indicate that than 0.9 except for the layer below Moho. the Moho is at about 32 km depth beneaththe study area, and The preferred model (Figure 5, Table 1) consistsof five dips eastward. In different inversion runs this initial model layers: a shallowlayer, 4 km thick, with a velocity of 5.24 was modified by increasingthe number of layers in the model km s'l; an upper-crustallayer, 8 km thick,with a velocityof to five and six layers and by perturbingthe depth to the top of 6.3km s-l; a midcrustallayer of 6.7km s -I anda thicknessof 6 the6.9 km s -I layer. km;a lowercrustal layer of 6.8 km s-I anda thicknessof 14 Individual runs all resulted in a similar reduction of data km;and an upper mantle of 8.0km s -1. Diagonalvalues of the varianceand travel time residuals. Data variancewas typically resolution matrix for this model (Table 1) as well as the reduced from 0.3 to 0.01 s, while the rms travel time residual histogram of earthquakedistribution with depth (Figure 3) decreased from 0.5 to 0.08 s. (The initial variance was indicatethe reliability of the layer velocities. The velocity of actuallymuch higher but was reducedto approximately0.3 s in the shallow5.2 km s-I layer is controlledentirely by shot a preliminary inversion). The resultant velocity models had data, since no earthquakesoccur at depthsless than 6.0 km. similar characteristics. Velocities for the upper 4 km of the As a result the velocity of the layer is well constrained(as crustwere consistently near 5.2 km s'l. Fordepths between 4 indicatedby a high resolutionvalue), but the depth to the base and 15 km, velocitiesof 6.3 to 6.4 km s-1 were calculated. of the layer is not. Hypocenter distribution in the upper, From a depth of 15 km to the Moho, velocities of 6.7 to 7.0 middle, and lower crustal layers is good and constrainsthese km s-1 were obtained. layers well. The diagonal values in the resolution matrix provided the Neither the depth nor the velocity of the upper mantle is guidelinesfor choosinga final velocity model from amongthe well constrainedas indicatedby a resolutionvalue of only 0.4.

MILLER AND MOONEY: SoIYrI-I]ERN FOOTHlI.I.S METAMORPHIC BELT 6871

Table1. Rep•ntati,veModel Results lower velocities associated with a high degree of Depth,kin VelocitltZm s -• Resolution serpentinizationof the original ultramafic rock. For example, an ultramafic rock that is 50% serpentinizedcan drop to -6.2 PreferredModel Results 0.0 5.24 0.906 km s'• in velocitycompared to 7.6 km s'• or morefor 4.0 6.31 0.900 unserpentinized dunite or harzburgite [Christensen, 1978]. Metamorphic rocks with measured velocities consistentwith 12.0 6.66 0.906 those observed in the upper two layers of the model include 18.0 6.80 0.922 variably serpentinizedultramafic rocks, meta-graywackes,and 32.0 7.98 0.411 schists(Figures 8a and 8b). Diorites and granodioritesof the Sierra Nevada batholith (Figures 8c and 8d) may also Other Model Results contributeto the 6.3 km s-1 layer. Likely petrologic 0.0 5.24 0.906 equivalentsof the6.7 and6.8 km s'l middleand lower crustal 4.0 6.31 0.900 layers are intermediateto mafic granulites(Figure 8e-f). 12.0 6.66 0.960 Comparison of this one-dimensional velocity model to 18.0 6.77 0.892 other refraction velocity models along strike in the northern 28.0 7.45 0.731 Sierra Nevada Foothills [Spieth et al., 1981], in the western Great Valley [Colburn and Mooney, 1986; Holbrook and Mooney, 1987], and in the Sierra Nevada [Bateman and Eaton, 0.0 5.23 O.9O8 1967] indicates significant differencesin crustal composition 4.0 6.42 0.967 among these tectonic provinces. A graph (Figure 5) illustrates 15.0 6.68 0.924 the relationship between typical velocity-depth functions 21.0 6.84 0.689 from the two-dimensionalrefraction models and this study. 25.0 6.99 0.546 The velocity-depth functions from the western Great Valley 32.0 7.94 0.377 showvelocities of lessthan 4.0 km s-l in the predominantly sedimentary rocks of the upper 4 to 6 km of the crust, 0.0 5.24 O.9O6 overlying an -6 km thick upper crustallayer with velocitiesof 4.0 6.29 0.894 6.0 to 6.4 km s-1, a midcrustallayer with velocitiesnear 6.7 12.0 6.67 0.962 km s-1 anda 7.0 to 7.2 km s'l layerat thebase of thecrust (ModelA, Figure5). A discontinuous7.0 km s'l layer 18.0 6.81 0.924 encountered at a depth of 12 km near the north end of the 36.0 8.10 0.151 Holbrook and Mooney [1987] transect is interpreted to represent part of an ophiolite that lies directly beneath the 0.0 5.24 O.9O6 Great Valley Sequence [Holbrook and Mooney, 1987]. 4.0 6.20 0.767 Significantly, this layer is missing from southern end of the 9.0 6.61 0.972 Holbrook and Mooney transect, and is not encountered on 21.0 6.90 0.780 either the Colburn and Mooney [1986] transect15 km to the 25.0 7.02 0.568 west or in the results from this study. Lithological candidates 32.0 7.96 0.373 forthe midcrustal and basal crustal layers (6.7 to 7.2 km s-1) of the Great Valley include mafic rocks such as gabbro, perhaps at amphibolite to granulite facies metamorphic grade 0.0 5.23 0.908 [Holbrook and Mooney, 1987]. The average velocity in the 4.0 6.46 0.978 middle to lower crust of the southern Foothills (6.7 to 6.8 km 18.0 6.60 0.665 s-1) is significantlyslower than that observed in the Great 21.0 6.80 0.705 Valley, suggesting that the Foothills lower crust has a less 25.0 6.98 0.576 mafic composition. 32.0 7.91 0.381 The model from this study is significantly different from that of Spieth et al. [1981], the only model that samplesthe Foothills terrane exclusively (Figure 5). Their model was that have been metamorphosedto upper greenschistand lower derived from receivers recording aftershocks of the 1975 amphibolite facies. All have been locally intruded by Oroville earthquake and constrainscrustal structure along the batholithicrocks of the Sierra Nevada. Drill hole data [Cady, strike of the Foothills Belt, 200 km north of this study area. 1975; May and Hewitt, 1948] suggest that the intruded In theirmodel, a 5 km thick7.0 km s-1 layer occurs at a depth metamorphicsequence extends westward beneath the eastern of 5 km and has been interpreteda sheet of ultramafic rocks edgeof the Great Valley. associatedwith an ophiolite. No such distinct layer occurs in These geologicstudies provide a basisfor interpretingthe the upper crust at the southernend of the Foothills Belt despite crustalvelocity model. Figure 8 comparesthe velocity model geologic evidence for the presence of ophiolitic basement from this study to laboratory measurements of P wave rock [Saleeby, 1982]. One explanationfor this differencemay velocities on a number of rock types found in the area. The be that mafic basementrocks in the upper 10 km of the crust of laboratory results have been temperature corrected for the the southern Foothills are more highly serpentinized than to geothermal gradient in the Sierra Nevada [Lachenbruch and the north. A secondmajor difference between the two models Sass, 1978]. Sources for the laboratory measurementsare is that Spieth et al. inferred a very slow midcrustal layer with shown in Table 2. Important to this study is the observation velocitiesof 6.2 km s'l from10 to 30 km depth.By contrast that serpentinite velocities (Figure 8g) vary widely, with themodel from this study has velocities of 6.7 to 6.8 km s-1 6872 MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT

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Figure 6. Map showingstation corrections (in seconds)for earthquakestations. Correction pattern is probablydue to lateralheterogeneity in the velocityfield of the middleto lower crust. over the same interval. This suggests that the overall Saleeby et al., 1986]. This type of modeling has been the composition of the middle to lower crust of the southern basis for the interpretationof the Great Valley as being Foothills is predominantlyintermediate to mafic granulites, floored by high density, strongly magnetic rocks that are while the northern part of the Foothills is dominatedby derivedfrom oceaniccrust. Theserocks generallydisappear slowervelocity metamorphic rocks suchas schistsand highly eastward beneath the eastern Great Valley where a large serpentinizedultramafic rocks. gradientin the magneticfield, along with a nearly coincident Differences between the Sierra Nevada batholith and the gradientin the gravity field occurs(Figures 1 and 2). The low FoothillsMetamorphic Belt are evidentfrom a comparisonof average velocities found in the upper 12 km of the one- the Batemanand Eaton [1967] model. This two-layermodel dimensionalvelocity model are consistentwith the complete consistsof an uppercrustal layer of 6.0 km s'• thatis 15km disappearanceof the high-density (and presumably high thickand a lowercrustal layer of 6.9 km s'• thatis 15-20km velocity) layer within the Foothills. In its place, presumably, thick. The 6.0 km s'• layeris clearlyassociated with the are variably serpentinizedperidotite with infolds of schists graniticrocks of the batholithwhile the deeperlayer may be and metagraywackesof the Foothills MetamorphicBelt, and composedof intermediateto mafic metamorphicand intrusive granitesand granodioritesof the Sierra Nevada batholith. rocks. Thus the Foothills appear to have an intermediate rather than felsic midcrustand a lower crust of comparable Seismic Reflection Data composition to that beneath the Sierras. The one-dimensionalsouthern Foothills velocity model An 80-km-long seismicreflection profile, CC-2, crossesthe supplies additional constraintson the nature of the Foothills southernpart of the microearthquakenetwork (Figure 1). The MetamorphicBelt. Previousinformation about this region profile is well positioned to image the transition from the has beenobtained almost entirely from gravityand magnetic Great Valley to the Sierra Nevada batholith since it trends modeling [e.g., Cady, 1975; Griscom and Jachens, 1990; acrossthe centralpart of the valley, crossesthe Great Valley MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT 6873

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Figure 7. Map of station delays (in seconds)for shot stationsused in the inversion. Solid circles are shot locations, small squares are shot stations, triangles are earthquake station locations. Note the consistent eastwarddecrease in stationdelay values that is related to the eastwardthinning of Great Valley sediments. magnetic high, the southerntip of the Foothills Metamorphic onto the continental crust during Nevadan age deformation, Belt and into the Sierra Nevada batholith. The data, collected while the subhorizontal reflection may represent the base of in 1984, are sign-bit data acquired in a split spread geometry the batholith. In the following paragraphs,we further explore with an 800-channel array and a vibroseis source. Maximum the origin of these reflections by examining their geometry offsetsreached 12.2 km with a group interval of 30.5 m and a and reflective character in light of the one-dimensional vibration interval of 91.5 m. Basic processingsteps carried velocity model determined here, the gravity and the regional out by the contractor included cross correlation, statics, geology. norma!-moveout, bandpassfiltering, automatic gain control, The subhorizontal reflection near 3 s at the eastern end of crooked-line gathers, common-depth-point stacking and the profile is probably best interpreted as the base of the migration. The techniqueof extendedcorrelation [e.g., Okaya silicic level of the batholith. Four arguments favor this and Jarchow, 1989] was usedto extendthe recordlength from interpretation. First, subhorizontal reflections have been 10 to 15 s [Zoback and Wentworth, 1986]. previouslyidentified at the basesof plutons[e.g., Lynn et at., The most prominent featuresin the data (Figure 9) are the 1981]. Second,the reflection lies beneaththe surfaceoutcrop gently west dipping sedimentsin the upper 1 to 2 s of the of granitic rocks (Figure 1). Third, the reflection lies at a western part of the profile, the packets of more steeply depth of 9 to 10 km which is comparableto the depth at which dipping reflections (sub-Foothills reflection pair) at 2 to 8 s a jumpfrom 6.3 to 6.7km s-• is foundin theone-dimensional in the east central portion of the line and a subhorizontal velocity model. Finally, comparison of specific gravity reflection at 3 s near the easternend of the line. The gently measurementsto the residual gravity field in the batholith west dipping reflectionsin the shallow crust correspondto the indicatesthat the silicic rocks extend to an averagedepth of Jurassic-to-Cretaceousage sedimentsof the GreatValley. The 10 km [Oliver et al., 1986]. Ductile shear zones, which have origin of the other reflections in the mid-crust is more been observedto occur along steep compositionalgradients or enigmatic. In a preliminaryinterpretation of the data, Zoback at internal contacts between mafic and silicic batholithic and Wentworth[1986] and Wentworthet al. [1987] propose phases [Saleeby, 1990] may also contributeto reflectivity at that the west dipping reflectionsmay representthe remnants this level. of thrust faults along which island arc material was obducted Clues to the tectonic origin of the sub-Foothills reflection 6874 MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT

a) 8

Granodiorite

This Study

4 0 5 10 15 20 25 30 35 > 5 Depth (km)

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This Study

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0 5 10 15 20 25 30 35 Depth (km)

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8 Serpenfinite This study

Diorite

Th•s Study

4 0 5 10 15 20 25 30 35 Depth (km) 4 0 5 10 15 20 25 30 35 Depth (kin)

Figure 8. Comparisonof laboratorymeasurements of compressionalwave velocities for a number of rock types to the one-dimensionalvelocity model from this study: (a) graywacke; (b) schist; (c) diorite; (d) granodiorite;(e) intermediate granulite; (f) mafic granulite; (g) serpentinite. Sourcesfor laboratory data are given in Table 2. pair come from their subparallelism, separation and dip. an averagevelocity of 6.5 km S'l . The one-dimensional Close examination of the data shows that each reflection is velocity model for the Foothills assignsa velocity of 6.7 km actually a packet of reflectionsof 0.5 to 0.7 s duration. This s-I to depthsof 12 to 18km, suggesting that material enclosed sort of reflectivity signature can be typical of shear zones by the reflections is probably intermediate to mafic rocks. [e.g., Christensen and Szymanski, 1988; Jones and Nur, Curiously, most of the earthquake activity also occurs at the 1984], sedimentary layering, or of cumulate layering [e.g., depth of these reflections. However, the diffuse epicenter Lynn et al., 1981]. With nearly 7 to 8 km of material distributionand mixed focal mechanismsolutions [Wong and separatingthem, the reflections bracket a substantialportion Savage, 1983] make it difficult to attribute a genetic link of the mid-crust that contains few other reflections. On a between the reflections and the seismicity. migrated record (Figure 10), the sub-Foothillsreflection pair Their relatively steep dip and dominantly midcrustaldepth dips 30ø to 35ø westwardover a distanceof 25 km, assuming suggests that the sub-Foothills reflection pair is probably MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT 6875

Table 2. LaboratoryMeasurements References ! i .. Numberof Samples References Diorite 10 Christensen [1977]; Chroston and Brooks [1985]' Kanamori and Mizutani [ 1965] and Kern and Schenk[ 1985] Granodiorite 19 Bonner and Schock [1981]- Kanamori and Mizutani [ 1965]' Schock et al. [1974]' Christensen [1978]' Christensen [1977]' Chroston and Brook• [1985]; and Kern and Schenk [1988] Schist 10 Christensen [1966]' Chroston and Brooks [1985]' Fountain [ 1976]; and Kern and Schenk [1988] Intermediate Granulite 2 2 Manghnani et al. [1974]' Christensenand Fountain [1975]; and Chroston and Brooks [1985] Mafic Granulite 3 2 Christensen and Fountain [1975]; Chroston and Evans [1983]; Chroston and Brooks [1985]' Fountain [1976]; Kern and Schenk[1985]' and Manghnani et al. { 1974] Serpentinite 16 Christensen [1966]' Christensen [1972]; Christensen [1978]; Kern and Schenk[1985]' and Simmons [1964] Graywacke 1 1 Stewart and Peselnick [1977]' and Stewart and Peselnick [1978] Theseprimary references were gathered from the compilation of Holbrook [19'88]. bestviewed as two shearzones. The 30ø to 35ø dip observed well-consolidated graywackes of Figure 8a have measured on CC-2 is comparable to that observedin other major shear velocitiesof lessthan 6 km s-1. Cumulatelayering within an Zonesimaged in crustalreflection data [e.g., Matthews and intrusionmight satisfy the velocity criterion, but it is difficult Cheadle, 1986; Phinney and Roy-Chowdhury, 1989; Mooney to find a mechanismfor tilting an intrusive body of this extent and Meissner, 1992]. Sedimentary layering is not a good uniformly 30ø westward, particularly if the intrusion po..st- candidatefor a sourceof reflectivity since the one-dimensional dates Nevadan deformation. The uplift of the Sierra Nevada in velocity model contains layers with velocities much greater Cenozoictime hasresulted in a westwardtilt of only 3ø to 5ø than those measured in most sediments. For example, the [Christensen, 1966].

Bend in Merge line E I

•' •7 •.• •&• L• •-7

10 I

0 10 km I I

Figure 9. Unmigratedreflection profile CC-2. Vertical exaggerationis approximately1:1 assumingan averagecrustal velocity of 6.5km s-1. Themost prominent features on the profile are the shallow (less than 2 s) west dipping sedimentarysequence that correspondsto the Great Valley sedimentsand the more steeply dippingreflectons at 3 to 8 s in the eastcentral portion of the profile (arrowson west sideof profile). Arrows on the right mark the updiptermination of the west dippingreflections. 6876 MILLER AND MOONEY: SOUTHERNFOOTHILLS METAMORPHIC BELT

W BMFZ GV •' I • FMB • I• SNB

0 10 km I t Figure10. Migratedreflection profile CC-2. Vertical exaggeration is approximately 1'1assuming an averagecrustal velocity of 6.5km s -l. Thewest dipping midcrustal reflections aretruncated ata locationbelow thesurface contact ofmetamorphic rockswith batholithic rocks, a presumably near-vertical discontinuity.

Critical to the tectonicinterpretation of the sub-Foothills steeplyeast-dipping intra-arc fault with moderateto large reflectionpair is theobservation that they end abruptly in the amountsof east-side-upreverse-slip and a possibleearly updipdirection. The downdip terminations apparently result historyof strike-slip. Higher-grademetamorphism at the from deteriorationin the the signal-to-noiseratio (snr) southernend of theBMFZ suggeststhat the it mayhave served beneaththe Great Valley sediments [M.D. Zoback,personal as a conduitfor pluton-associatedfluids [Miller and Paterson, communication,1991]. However,the updip terminations 1991]. Theseresults, taken in conjunctionwith the observed occurin an areaof reasonablygood snr and can be relatedto reflector geometrieson CC-2 suggestthat the updip surfacegeology. The updip truncation (Figure 9) ismarked by terminationof thesub-Foothills reflector pair is relatedto slip thegradual disappearance of the long duration reflection (A, on the BMFZ and/orsubsequent batholith intrusion. Figure11) andthe complete breakup of reflectorcontinuity nearB andC (Figure11). Onthe migrated record (Figure 10), truncationoccurs directly below the surfacecontact between PossibleOrigins for WestDipping Reflections Foothillsterrane rocks and the westernedge of batholithic Two equallyviable, but distinctlydifferent tectonic models rocksat a depthof 3 to 4 km,as well as immediately west of hax/epreviously been proposed for westdipping shear zones th.•subhorizontal base-of-batholith reflection. This geometry associated with the Foothills reflector pair. In one the suggeststruncation bya near-verticalfauR and/or by batholith intrusion. reflectorsare associated with east directed thru.sting during the Nevadanorogeny [Saleeby et al., 1986;Wentworth et al., Additionalevidence for this interpretationof reflector 1987]. In the other, the reflectivity is attributedto geometrycomes from recent geologicmapping near the synbatholithicductile shear zones that accommodatedlateral GuadalupeIgneous Complex and the White Rock Pluton spreadingcaused by theinflux of a largevolume of'batholithic [Tobischet al., 1989]. Thiswork suggests that a near-verticalmaterial [Saleeby, 1990]. In the followingparagraphs, we ductileshear zone representing the southern extent of theBear examinehow well the individualmodels fit the existing Mountainsfault zone (BMFZ) forms the west flank of the geologicaland geophysicaldata. A schematicCrOss-section GuadalupeIgneous Complex and continues southward along (Figure12) illustratesgeological relationships that stem from theFoothills terrane/batholith contact, until it is truncatedby the two alternative models. the 115 m.y.-oldBass Lake tonalite,10 km northof the CC-2 The last major shorteningevent within the Foothills profile(Figure 1). Miller and Paterson[ 1991] show that the Metamorphicbelt was the Nevadan orogeny, a short-lived,but BMFZ was activefrom sometimeprior to 160 Ma until intensedeformational event in the late Jurassic(155 +/- 3 approximately123 Ma. They interpretthe fault zone as a m.y.B.P.)[Schweickert et al., 1984]. Eastdirected thrusting MILLER AND MOONEY: SOUTHERN FOOTHILLS METAMORPHIC BELT 6877

Figure 11. Enlargementof data within box in Figure 9. The updip truncationis marked by the gradual disappearanceof the long durationreflection at A and the completebreak up of reflectorcontinuity near B and C. associated with this event is well documented in the northern outcrop, suggestingthat Cretaceous-agesynbatholithic shear Foothills where ophiolitic rocks of the Smartville nappe are zones are a possible explanation for the observedreflections in thrust contact with the Calaveras accretionary prism [Saleeby, 1990]. In this interpretation, the termination of complex [Moores and Day, 1984]. By analogy, the west west dipping reflections at a subvertical contact between dipping reflection on CC-2 may represent similar thrust batholithic rocks and rocks of the Foothills Metamorphic Belt zones. To accountfor the updip terminationof the reflections would result from the truncationof extensionalshear zones by below the surfacecontact between the Foothills metamorphic the Bass Lake thermal-magmaticwelt as it ascendedthrough rocks and the batholith, we would suggest that the the crust. hypothesizedNevadan-age shear zones have beentruncated by The fact that the B MFZ is cut out at the latitude of the one of three mechanisms: strike-slip faulting or reverse seismic line and the occurrenceof a large gravity low in the motion along a near-verticalshear zone that correspondsto same region make it difficult to make a definitive choice the BMFZ or subsequentintrusion of batholithicrocks (Figure between these two tectonic models. Some evidence for shear 12). Of these mechanisms,termination by intrusion of the zones on the east side of the B MFZ is necessaryto establish -115 Ma Bass Lake tonalite [Stem et al., 1981] is most age relationships. The absence of the BMFZ and any clearly inferred from the surfacegeology where the batholith Foothills rocks east of it makes it impossibleto judge whether is now in direct contactwith the Foothills metamorphicrocks remnants of the shear zones occur on the east side of the fault. Alternatively, the west dipping shear zones may have been If they are of Nevadan age, then there should be remnants of truncatedby an earlier period of reverseor strike-slipfaulting the shearzones on the east side of the fault, possiblyoffset by on the BMFZ. The BMFZ can still be tracedsouthward along strike-slip motion. If they are Cretaceous in age, then they the west flank of the batholithicrocks to a positiononly 10 would probably show a cross-cutting relationship with the km north of the CC-2 profile (Figure 2) suggestingthat the fault. The presenceof an unusually large gravity low (Figure Bear Mountainsfault zone hasprobably been cut out by pluton 2) on the flank of the Sierra at this lattitude suggeststhat there intrusion at the latitude of the seismic profile. The west is something anomalous about the Foothills crust in this dipping shear zones could easily have been offset by slip region, and makes it difficult to generalize the observed along the fault zone as it was active from before 160 Ma until reflections to the whole length of the Foothills. Wells have 123 Ma [Miller and Paterson, 1991]. encountered granite here [Cady, 1975; Oliver, 1977, and Studies of exposures of midcrustal tonalitic rocks in the references therein], suggesting that an intrusion may be southernSierra Nevada have documentedthe developmentof responsible for the anomaly, but the age of the material is extensionalshear zones that accommodatedlateral spreading unknown. If the intrusion is of Cretaceousage then the west causedby the influx of a large volume of batholithic material dipping reflectionsmay indeed be synbatholithicshear zones [Saleeby, 1990]. There the baseof the Bear Valley Springs as Nevadan-age shear zones should have been cut out by batholith has been subjectedto low-dipping ductile shear at intrusion. The two tectonic models can be reconciled by solidus to hot-subsolidusconditions [Saleeby, 1990]. The consideringthe possibilitythat Nevadan-ageshear zones were west dipping reflectionson CC-2 occur at depthscomparable zones of weakness that later accommodated extension to the depth of formation of the shear zones now observedin associatedwith Cretaceousmagmatism. SierraNevada Earthquake Foothills BMFZ Batholith Distribution Great Valley MetamorphicBelt ß _ / BassLake -- Tonalite • ' 6.-'1-- I \ / I I / 10 / t/ • , • 10 II \\ \\\x

•, 20 II xx // E + -I- -I- + + ..p + -I- + -I-

30 -A •' '•.8- •" L/ - A 30

0 10 km Mantle

40 I I 40

c• c• o o o

Great Valley and Foothills Sierra Nevada MetamorphicBelt (Intrusive Rocks)

"• CretaceousGreat Valleyand Sequenceyounger andPaleozoic serpentinizedmetamorphicultramafic rocks rocks 'L• Granite/granodiodte/silicictonalite

[• Gmnite/granodiorite(AmIntermediate phibolite/granulite) composition rocks \• Tonalite/Intermediaterocks(Granulite) composition

L• Mesozoicmetamorphic rocks • Mafic(Granulite)composition rocks

Figure 12. A schematiccross section along the CC-2 transect. The west dippingreflections are interpreted as ductile shear zones formed either during the late JurassicNevadan orogeny or as a result of extension associatedwith Cretaceousmagmatism. Truncationof the shearzones was probablycaused by one of three mechanisms:(1) intrusionof batholithicrocks; (2) strike-slipfaulting or (3) reversemotion along the near- verticalshear zone that correspondsto the Bear Mountainsfault zone(BMFZ). At the surface,the BMFZ is actually cut out by the Bass Lake Tonalitc, resultingin an intrusivecontact between the batholithand Foothillsrocks. Note that earthquakeepicenters occur at depthscomparable to the shearzones.

Conclusions accretion. Finally, the similarity of lower crustal velocities beneath the Foothills belt and the western Sierra Nevada, and the smoothly east dipping crust-mantle boundary suggests In the past fifteen years, the concept of crustal growth that the lower crust of the Foothills terrane has been through the accretion of terranes has found widespread magmatically welded and homogenized during the application to studies of the western Cordillera of North developmentof the CretaceousSierra Nevadan arc. America [Joneset al., 1977; Monger et al., 1985]. More than 200 terranes have been identified from Alaska to Mexico Acknowledgements. The geophysicaldata usedin this studywere [Coneyet al., 1980; Silberling et al., 1992], and geophysical madeavailable to the authorsby a numberof people. WoodySavage studieshave begun to reveal the three-dimensionalgeometry and Marcia McLaren of Pacific Gas and Electric Co. provided of these accreted terranes. For much of the Cordillera, terranes earthquakearrival-time data from the Woodward Clyde network. have beenstacked one upon the other as nappesor giant thrust David Oppenheimerprovided earthquake arrival-time data from the U. sheets[Monger et al., 1985]. S. GeologicalSurvey network.Bob Jachenssupplied the digitalgravity Our study in the Foothills MetamorphicBelt has provided grid from the U.S. GeologicalSurvey database. The seismicreflection several important insights into the accretion, welding, and datawere processed by Interseisand obtained through Carl Wentworth, modification of this terrane. We have described evidence for Eric Geist and Mark Zoback. Donna Eberhart-Phillipsand Lynn Dietz high-angledeep crustalshear zones (Figure 12) that may have gave valuable advice on running VELEST, the one-dimensional inversionprogram. SimonKleinpeter and GeorgeThompson provided servedas primary thrustzones during terraneaccretion. These helpfulreviews of the initial manuscript.We thankJason Saleeby, Bob shear zones may also have accommodatedcrustal extension Jachens,and associateeditor A. Griscom for particularly helpful associated with the intrusion of the Sierra Nevada batholith. reviewsthat led to significantimprovements in the manuscript.This The updip truncationof theseshear zones in the vicinity of a work was initiated as part of K. C. Miller's dissertationat Stanford major subverticalshear zone (BMFZ, Figure 12) suggeststhat University. The work was completedwith help from Air ForceOffice translational motion can be a significant factor in terrane of ScientificResearch grant no. F49620-92-J-0438. MILLER AND MOONEY: SOUTI-[ERN FOOTHILLS METAMORPHIC BELT 6879

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