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

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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 dockingand 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 -121 -120 38 . Stoc!+n Nevada Los A •CC-2 \ 37 \ •,0 I 20Ikm Fresno.a/ •.•,, UpperCenozoic sediments _• Mesozoicgraniticrocks 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 38 ø 0 30 6o 9o 12o 15o 0-2 ..• , ! i ! 2-4 4-6 .; Event 6-8 8- 10 ......• Distribution 10-12 12 ~ 14 14-16 16-18 18 - 2O .....•. •..•.•_ .<.:..•.> 20 - 22 22 - 24 24 - 26 26 - 28 ::)8 o 30 30 - 32 32 - 34 34 - 36 36 - 38 38 - 40 >40 37 ø o -120 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
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