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Nature of the Crust in the Northern Gulf of California and Salton Trough GEOSPHERE, V

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Nature of the Crust in the Northern Gulf of California and Salton Trough GEOSPHERE, V Research Paper THEMED ISSUE: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms GEOSPHERE Nature of the crust in the northern Gulf of California and Salton Trough GEOSPHERE, v. 15, no. 5 Jolante W. van Wijk1, Samuel P. Heyman1,2, Gary J. Axen1, and Patricia Persaud3 1Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA https://doi.org/10.1130/GES02082.1 2SM Energy, 1775 Sherman Street, Suite 1200, Denver, Colorado 80203, USA 3Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell-Kniffen, Baton Rouge, Louisiana 70803, USA 9 figures; 2 tables CORRESPONDENCE: [email protected] ■ ABSTRACT elevation 34°N (m) 1 Salton Sea CITATION: van Wijk, J.W., Heyman, S.P., Axen, G.J., 2400 and Persaud, P., 2019, Nature of the crust in the north- In the southern Gulf of California, the generation of new oceanic crust 1600 2 3 800 ern Gulf of California and Salton Trough: Geosphere, has resulted in linear magnetic anomalies and seafloor bathymetry that are Cerro Prieto v. 15, no. 5, p. 1598–1616, https://doi.org/10.1130 32°N 0 characteristic of active seafloor-spreading systems. In the northern Gulf of ˗800 AZ NM /GES02082.1. Mexico California and the onshore (southeastern California, USA) Salton Trough region, ˗1600 Wagner Basin ˗2400 a thick sedimentary package overlies the crystalline crust, masking its nature, 4 Science Editor: Shanaka de Silva 30°N ˗3200 North American plate Guest Associate Editor: Carolina Pagli and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or con- GP Gulf of California BF Received 6 November 2018 tinental) and develop a conceptual model for rifting. Gravity anomalies in the 28°N Revision received 14 May 2019 northern Gulf of California and Salton Trough are best fit with crustal densities Baja California Accepted 18 July 2019 that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are 26°N Pacific plate C Published online 14 August 2019 MP assumed. Because extensive mafic underplated bodies would produce gravity N anomalies that are not in agreement with observed gravity data, we propose, 24°N following earlier work, that the anomalies might be due to serpentinized per- ↑ idotite bodies such as found at magma-poor rifted margins. The density and 200 km seismic velocities of such serpentinized peridotite bodies are in agreement 22°N RP with observed gravity and seismic velocities. Our conceptual model for the 118°W 114°W 110°W 106°W Salton Trough and northern Gulf of California shows that net crustal thinning Figure 1. Map of the main plate boundary features of the Gulf of Califor- here is limited because new crust is formed rapidly from sediment deposition. nia extensional province showing elevation and bathymetry (modified As a result, continental breakup may be delayed. from Umhoefer, 2011). The two large black arrows indicate relative plate motion between the North American and Pacific plates. Yellow dashed lines trace extinct spreading ridges and associated transform faults. White dashed lines trace active known spreading segments and ■ INTRODUCTION associated transform faults. Orange lines outline the −3% shear wave velocity anomaly contour from Wang et al. (2009). GP—Guadalupe mi- croplate; MP—Magdalena microplate; RP—Rivera plate; BF—Ballenas The Gulf of California and southern Salton Trough (northwestern Mexico transform fault; C—Carmen spreading center; AZ—Arizona; NM—New and southeastern California, USA) form the oblique Gulf extensional province Mexico. Transects 1–4 are used in the potential-field models. (Gastil et al., 1979) of generally right-lateral transform and strike-slip faults, short seafloor-spreading segments, and pull-apart and extensional basins, localizing into dextral transtension by ca. 8 Ma (Atwater and Stock, 1998, 2013; extending northwest from the East Pacific Rise spreading ridge to the San Bennett, 2009; Seiler, 2009; Seiler et al., 2010; Bennett et al., 2013, 2016; Bennett Andreas fault in the continental U.S. (Figs. 1, 2; e.g., Fuis and Kohler, 1984; and Oskin, 2014; Darin et al., 2016). There is general agreement that exten- Lonsdale, 1989, 1991; Stock and Hodges, 1989; Lizarralde et al., 2007; Umhoefer, sion in the gulf commenced sometime after the cessation of subduction of 2011; Bennett and Oskin, 2014; Abera et al., 2016; van Wijk et al., 2017; Umhoefer the Guadalupe and Magdalena microplates (fragments of the Farallon plate; This paper is published under the terms of the et al., 2018). Extension in the northern Gulf extensional province may have e.g., Lonsdale, 1989, 1991; Stock and Lee, 1994; Umhoefer, 2011; Bennett and CC-BY-NC license. started between ca. 19–17 Ma and 12.2 Ma (Bennett et al., 2016), progressively Oskin, 2014). Magnetic anomalies indicate that seafloor spreading started © 2019 The Authors GEOSPHERE | Volume 15 | Number 5 van Wijk et al. | Salton Trough and northern Gulf of California Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/5/1598/4831648/1598.pdf 1598 by guest on 30 September 2021 Research Paper in the southern Gulf of California by ca. 3.5 Ma during the Pliocene (Larson, 1972; Klitgord et al., 1974; Lonsdale, 1989; DeMets, 1995), but north of ~28° 116°W 115°W FAULTS latitude, these anomalies are absent and the nature of the crust (continental, S Dextral “transitional” [sometimes defined as thinned, with igneous intrusions, and A Normal F 2 underplating], or oceanic) is debated (Fuis and Kohler, 1984; Lachenbruch e High-angle: Lin et al., 1985; Axen, 2008; Schmitt et al., 2013; Persaud et al., 2016; Han et al., Low-angle: 2016). The absence of seafloor-spreading magnetic anomalies in the northern 114°W 113°W Gulf may be explained either by the nature of the crust (if it is continental, no S 2-14 3 J Line seafloor-spreading magnetic anomalies are expected), or by temperatures F W 33°N L that are close to or above the Curie temperature so that underlying igneous I i F ne (oceanic) crust would not be magnetized. We explore both ideas in this study EF 1 with potential-field modeling and a one-dimensional geotherm analysis. Potential-field data may discriminate between oceanic and continental WSD U crust based on differences in composition and thus density and magnetic P .S.A susceptibility. Potential-field models are, however, nonunique and are there- e M n C exi i co fore constrained in this study by seismic reflection and refraction profiles, n P 32°N su 32°N receiver functions, and tomographic images (Fuis and Kohler, 1984; Parsons F l a and McCarthy, 1996; Lewis et al., 2001; González-Escobar et al., 2009, 2014; r Barak et al., 2015; Persaud et al., 2016; Han et al., 2016). Burial history (geo- Li history) and thermal models were developed for the Salton Trough to obtain ne 4 F the geotherm, the depth to the Curie isotherm, and the antigorite breakdown C depth. We have developed gravity models for four transects: one across the W P WB a CSF F Wagner and Consag Basins (here referred to as the Wagner basin transect) ci f 31°N and three in the onshore Salton Trough region (Figs. 1, 2). We selected these N i R c W a P transects because of the availability of geophysical data sets to constrain the O n F S ce g F potential-field models. e CB a s The nature of the crust in the northern Gulf of California and Salton Trough n has been a topic of research for decades. The crust is covered with a thick Gulf of layer of sediments that mask its nature, and the nonuniqueness of seismic U 116°W D California velocities and gravity data, which have been used to understand the crustal S B 0 50 100 structure in the northern Gulf extensional province, makes inferences on its 115°W 114°W 30°N nature difficult. Fuis and Kohler (1984) developed a gravity anomaly model kilometers across the Salton Trough with layers of sediments overlying a metasedimentary Figure 2. Main structural features of the Salton Trough and northern Gulf of California and loca- 3 layer that, in turn, overlies a layer with a density of 3100 kg/m . They obtained tions of Salton Trough model lines 1–3 and the Wagner Basin line (line 4). CB—Consag Basin; a reasonable fit between model and data, with an error between ~3 and 8 WB—Wagner Basin; UDB—upper Delfin Basin. Dextral faults (blue): CPF—Cerro Prieto fault; mGal in the Salton Trough. From this model, they inferred the lower crust to EF—Ensenada fault; IF—Imperial fault; SAF—San Andreas fault; SJF—San Jacinto fault. Normal faults (red): CF—Consag fault; CSF—Consag Sur fault; PF—Percebo fault; WF—Wagner fault; be oceanic (mafic igneous) in nature. In their model, layer thicknesses and WSD—West Salton detachment fault; WSF—Wagner Sur fault. Locations of wells State 2-14 (2- Moho depth were constrained only in a few locations. Han et al. (2016) noted 14) and Chevron Wilson No. 1 (W) are shown by yellow dots. that an upper crust composed of either stretched continental crust or a high- grade metamorphosed sedimentary layer would be in agreement with seismic velocities, and proposed that a gabbroic lower crust may be present below composition similar to that of the East Pacific Rise. They proposed a leaky the Salton Trough region. Persaud et al. (2016) found that the seismic velocity transform model for emplacement of these basalts and rhyolites.
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