A 3D Crust and Upper-Mantle Velocity Model for the Gulf of California Region from Surface Waves F. Di Luccio1, R. W. Clayton2 and P. Persaud2 Submitted to J. Goephys. Res. March 1, 2006 1Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143, Rome, Italy. Phone 0039-06-51860561, [email protected]. 2California Institute of Technology, Seismological Laboratory, MS 252-21, Pasadena, CA 91125. Phone 001-626-395-6909, [email protected] and [email protected]. 1 Abstract We use Rayleigh wave group velocity dispersion measurements from 30 local earthquakes to determine the crust and upper-mantle shear-wave velocity for the Gulf of California region. The 3D model has its best resolution in the Gulf itself and deteriorates toward the edges due to a lack of station coverage. The results show shallow slow anomalies associated with the well-developed rifting in the southern and central part of the Gulf (Alarcon and Pescadero basins, respectively), and slow anomalies associated with the sedimentary basins. There is a significant mid-Gulf fast velocity that is spatially coincident with the active volcanic region on the Baja peninsula. The results show that the diffuse rifting in the northern gulf extends into the upper mantle, in contrast to the southern Gulf where both the rifting and velocity anomalies tend to be in narrow zones. Consequently, the rifting process is interpreted as done by flow across a broad zone in the lower crust. A thinning in the crustal thickness is found from north to south beneath the Gulf, with a shallower Moho in the eastern side underneath the Sonora and Sinaloa regions. intrusions into young sediments [Lonsdale, Introduction 1989]. The amount of volcanism along the transform faults is poorly determined due to the The Gulf of California is one of only a lack of samples. The basins become deep from few easily-accessible, newly forming ocean north to the south, with the northern ones basins where seafloor spreading is still not fully showing the thicker sedimentary blanket established. At the southern margin of the [Lonsdale, 1989]. Pacific-North America plate boundary, the Gulf Seismological studies confirm the of California extends ~1400 km from southern complexity in the pattern of spreading because California to the spreading mid-ocean ridge of strong lateral variations in the crustal and system of the East Pacific Rise. East-west upper-mantle structure beneath the Gulf extension in the Gulf of California appears to [Thatcher and Brune, 1973]. Crustal have started approximately 12 Ma when the deformation beneath the Gulf ranges from subduction of the lithosphere along the western classic ridge-transform structures in the south to margin of Baja California ceased [Lonsdale, diffuse deformation in the north, where a large 1989] and a series of basins and long transform number of shallow small-offset normal faults faults were produced. About 6Ma the Pacific- exist [Persaud et al.,2003]. North America plate margin was then shifted Various schools of thought exist on about 250 km inland roughly parallel to the what type of crust underlies the Gulf, whether coast line [Oskin et al., 2001]. This shear zone crustal composition and thickness varies along is thus characterized by basins, orthogonal or strike and what the possible causes of these oblique to the en echelon-type transform faults, inferred differences might be. Seismic with most of the active basins located where refraction studies in the northern Gulf require a extension is predominant. Tectonic crustal structure that is similar to that found in complexities are related to the size and re- the continental borderland of Southern orientation of these basins in time. Some of the California and a crustal thickness of ~20-25 km basins especially at the mouth of the Gulf, [Phillips, 1964]. More recent work also including the Guaymas basin, also show provides large values of crustal thickness for seamounts, whereas in the central and northern the northern Gulf and supports a crustal Gulf magmatism is represented as dike or sill structure in the northern Gulf that is not typical 2 of normal oceanic crust [Gonzales et al., 2005]. through the upper mantle in the Gulf are 5-10% Along a profile at ~31°N latitude, Moho depth slower than in normal mantle [Thatcher and estimates from P-to-S converted phases Brune, 1973]. In the uppermost 200 km of increases from 33±3 km near the Pacific coast mantle beneath the Gulf, the S-wave velocity is of Baja California to 40±4 km beneath the 6-8% slower than in the Preliminary Reference western part of the Peninsular Ranges batholith Earth Model, similar to other ridges/rifts (e.g., [Lewis et al., 2001]. The crustal thickness then the East Pacific Rise and Red Sea rift) [e.g. decreases rapidly across the eastern Peninsular Ritsema and van Heijst, 2000]. It is estimated Ranges and Main Gulf Escarpment to a fairly that the margins of the Gulf have undergone at uniform thickness of (15-18)±2 km within and least 255±10 km of horizontal separation of the on the margins of the northern Gulf [Lewis et upper crust [Oskin et al., 2001] and the mantle al., 2001]. This eastward thinning of the crust is looks seismically similar to that found at a mid- also confirmed for the entire Baja California ocean ridge. However, seafloor-spreading peninsula by a recent study by Persaud et al. magnetic lineations extend only as far north as [2005], where crustal thickness variations in the the Pescadero basin [Larson et al., 1972]. Gulf of California region are obtained from Regional constraints on crust and upper receiver functions, using teleseismic data mantle velocities are important for studies of recorded at the NARS-Baja array. They find a lithospheric structure and deformation. This is thinning of the crust from the western side of particularly true in target areas like the Gulf of the Baja California peninsula towards the Gulf California, where the scientific community as that is consistent with the compositional hopes to gain a fundamental understanding of boundary going from southern California to the lithospheric rupture. Thus far, the basic tip of Baja California and separating the lithospheric properties of the Gulf and its western and eastern Peninsular Ranges margins remain unresolved. Here we present a Batholith (PRB) [Langenheim and Jachen, surface wave dispersion study of regional 2003]. Thinner crust is also found beneath the earthquake data recorded by the NARS-Baja southernmost stations (NE76, NE77, NE79 in array, a set of 14 broadband seismic stations Figure 1) compared to the crustal thickness for surrounding the Gulf of California and the 5 the northern stations (NE71, NE72, NE73, broadband stations of the Red Sismologica de NE74 and NE75 in Figure 1) which ranges Banda Ancha (RESBAN network) (Figure 1). from 28 to 38 km. Surface wave studies are key to building 3D Furthermore, from the analysis of deep regional velocity models from a sparse seismic seismic profiles and new gravity data, network because they provide horizontal González-Fernández et al. [2005] constrain the sampling through the model with broad beams, crustal depth below the upper Delfín and the and because they utilize the dispersive Tiburón basins in the northern Gulf of characteristics of the largest amplitude and California. They find the crust progressively mostly easily recognizable waves recorded in thickens outwards east (Sonora region) and seismograms to determine the first-order west (Baja California peninsula); a crustal velocity structure along event-station paths. In thickness of 19 km has been estimated this study, we analyze Rayleigh waves from underneath the coastline, whereas the crust goes regional earthquakes to create a 2D map of the up to 14 and 17 km in the upper Delfín and the group velocities in the region, which are then Tiburón basins respectively and thickens down be used to determine the 3D structure. to 19.5 km in between the two basins Noticeable differences in the group velocities [González-Fernández et al., 2005]. are found for the northern and southern Gulf of Regional waveforms propagating California as well for the Peninsular Range and 3 adjacent basins. Due to the fact that the NARS- in Figure 2, where the frequency content of the Baja array straddles the plate boundary, we are waveform vertical components clearly depend able to make additional first-order comparisons on the propagation path. Dispersion curves of of the crust on opposite sides of the conjugate Rayleigh waves are computed in the period margins. This paper contributes to the range 5-100 s, with 75% of the measures being understanding of the Pacific-North America in the band 5-50 s. The period range of each boundary deformation by defining the crustal dispersion curve depends on the magnitude of and upper mantle structure beneath the Gulf of the earthquake and the source to receiver California and its western and eastern confining distance, with longer periods from larger events regions. being better recorded at longer distances. Group velocities of the Rayleigh wave fundamental Data analysis and model construction mode were computed using a Multiple Filter Analysis (MFT) technique [Dziewonski, 1969], We use a standard procedure [van der with a phase matched processing by MFT Lee and Federiksen, 2005] to determine the 3D [Herrin, 1977] to isolate the fundamental mode velocity model from surface waves. The first wave. We use a program by Herrmann and step is to determine the group-velocity Ammon [2002] which provides a fast and dispersion curves for the surface wave paths efficient method of analyzing multiply that cross the model. These are then converted dispersed signals. To measure group velocity to maps of group velocity by a tomographic from seismogram vertical component, the inversion, and finally, the dispersion relation at quality of the recording has first been evaluated each x-y point in the model is inverted for the in terms of signal to noise ratio, to avoid noise vertical velocity.