Electrical conductivity of the crust in central Baja California, México, based on magnetotelluric observations

Accepted Manuscript Electrical conductivity of the crust in central Baja California, México, based on magnetotelluric observations J.M. Romo, E. Gómez-Treviño, C. Flores-Luna, J. García-Abdeslem PII:

S0895-9811(16)30235-8

DOI:

10.1016/j.jsames.2017.08.024

Reference:

SAMES 1778

To appear in:

Journal of South American Earth Sciences

Received Date: 28 October 2016 Revised Date:

24 August 2017

Accepted Date: 30 August 2017

Please cite this article as: Romo, J.M., Gómez-Treviño, E., Flores-Luna, C., García-Abdeslem, J., Electrical conductivity of the crust in central Baja California, México, based on magnetotelluric observations, Journal of South American Earth Sciences (2017), doi: 10.1016/j.jsames.2017.08.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Electrical conductivity of the crust in central Baja California, México, based on magnetotelluric

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observations Romo, J.M., Gómez-Treviño E., Flores-Luna C. and García-Abdeslem J.

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Abstract

Crustal and sub-crustal structure of northwestern Mexico (peninsular California)

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resulted from major accretion episodes occurred during the long-lived subduction of the Farallon plate beneath the North American plate, since late Jurassic time. A magnetotelluric profile across central Baja California reveals several electrical conductivity anomalies probably associated to the crustal boundaries of distinct Mezosoic terranes juxtaposed in the current peninsular crust. It is known that

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electrical conductivity is significantly increased by the pervasive presence of conductive minerals generated during metamorphic processes in highly sheared zones. We interpret a striking sub-horizontal conductivity anomaly reveled in the

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model as explained by the presence of high-salinity fluids released after

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dehydration of the subducted Magdalena microplate (Farallon plate?). The presence of fluids at the base of the peninsular crust may produce a zone of weakness, which supports the idea that Baja California lithosphere has not been entirely coupled to the Pacific plate. In addition, crustal thickness is estimated in our model in about 35 km beneath the western Peninsular Ranges batholith (PRB) and 20 km beneath the eastern PRB. This crustal thickness is in good agreement with independent estimations of a thinner crust in the Gulf of California margin and a thicker crust along the axial PRB.

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1. Introduction The western continental margin of North America was a convergent margin since late Triassic to middle Miocene times, when the Farallon Plate was

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subducting under the North American Plate. It has been inferred that during the convergence, the Farallon plate carried several long oceanic island arcs that were accreted to the continental crust (Gastil et al., 1975, 1981; Gastil, 1993;

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Busby et al., 1998, 2006; Sedlock et al., 1993; Dickinson and Lawton, 2001, Busby, 2004). Off the continental margin, the subduction ceased at about 16.5

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Ma in northern Baja California and 12.5 Ma in the south (Lonsdale, 1991), when the East Pacific Rise approached the trench leaving the Guadalupe and Magdalena micro plates as remnants of the Farallon Plate (Figure 1). During this long history of convergence, several events of tectonic and

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magmatic accretion contributed to the growth of the existent continental crust, thus building the current Baja California lithosphere (Fletcher et al., 2007; Alsleben et al., 2008, 2014; Busby, 2012; Ferrari et al., 2017). The present-day

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trans-tensive tectonic regime of the Pacific/North American plate boundary, in the Gulf of California segment initiated in late Middle Miocene time (Seiler et al., However, transtension

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2010; Bennett et al., 2014; Sutherland et al., 2012).

occurred along the Baja California margin after the end of subduction of the Farallon-derived microplates (Normark et al., 1987; Spenser and Normark, 1979).

It is possible that deep marks of these major tectonic episodes remain imprinted in the rocks of the Baja California lithosphere; and consequently, their contrasting physical properties may produce geophysical anomalies observable from the surface. Hence, geophysical information may well shed light on current

ACCEPTED MANUSCRIPT geological hypotheses about the evolution of the Baja California lithosphere. In this study we investigate the electrical conductivity structure of the crust in central Baja California using natural electromagnetic waves, by mean of the

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magnetotelluric method (e.g. Chave and Jones, 2012). The resulting geophysical model provides original information about the current condition of the peninsular crust. The enhancement of electrical conductivity in the lithosphere is caused by the presence of fluids in interconnected pore spaces and/or by the occurrence of

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sufficiently extended films of conductive minerals as serpentine or graphite (Newton, 1990, Park et al., 1991; Soyer and Unsworth, 2006; Ichiki et al., 2009;

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Wannamaker et al., 2009; Matsuno et al., 2010; Worzewski et al., 2011). Both situations are geologically plausible in Baja California: 1) a relatively recent subduction zone is an efficient mechanism for the transport of fluids at considerable depths beneath the continental lithosphere (Jodike, et al., 2006;

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Brasse et al., 2009; Reynard et al., 2011), and 2) shear zones marking the sutures between old Mesozoic terranes are extensive metamorphic belts containing serpentine and/or hydrothermal graphite deposited along micro

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2002).

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fracture surfaces during tectonic shearing (Upton and Craw, 2008; Santos et al,

2. Geologic setting

Existing geological models explaining the evolution of Baja California

lithosphere consider that two parallel arcs were active during Jurassic to Early Cretaceous time: an island arc (Alisitos arc) developed on oceanic crust at some distance from the continent, and a continental volcanic arc along the cratonal margin (Todd et al., 1988; Busby et al., 1988; Walawender et al., 1991; Thomson and Girty, 1994; Wetmore et al., 2002). According to Johnson et al. (1999), the

ACCEPTED MANUSCRIPT continental arc may date from late Triassic or early Jurassic and was active until 127 Ma ago. The Alisitos oceanic arc established about 140 Ma ago toward the west, and a back-arc extensional basin developed in between both active arcs.

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The back-arc basin accumulated thick sequences of volcanic and epiclastic deposits until 115 or 108 Ma ago, when subduction of the oceanic crust juxtaposed the two arcs forming a suture zone (black dashed line in Figure 1) along the northern Baja California peninsula. From 108 to 97 Ma ago the plutonic

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activity shaping the Peninsular Range Batholith (PRB) stitched the suture zone. The PRB is composed by two distinctive terranes: (1) a more mafic western PRB

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composed of rocks from a Jurassic-Cretaceous island arc (Alisitos terrane), and its oceanic crust sutured against the eastern PBR: (2) a more silicic crustal terrane developed on continental lithosphere between 90-120 Ma (Gastil, 1975; Todd et al., 1988; Langenheim and Jachens, 2003). The compositional boundary

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in between both terranes separates magnetite-bearing plutons to the west, from ilmenite bearing plutons to the east (Gastil et al., 1990; Langenheim et al., 2014), and this lithological boundary locally coincides with a ductile shear zone

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interpreted as associated to the accretion of the Alisitos arc to the North America continental margin (Johnson et al., 1999; 2003; Alsleben et al., 2014). The oldest

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rocks of the western oceanic terrane crop out in Cedros Island and Vizcaino peninsula (Figure 2). These rocks are Late Triassic and Jurassic arc-ophiolite assemblages with blue schists metamorphosed under low-temperature highpressure conditions during the Cretaceous subduction of the Farallon plate. The blueschists rocks underlie upper-plate Mesozoic rocks including island arc basalt, ophiolite assemblages, and volcanogenetic to terrigenous rocks probably formed in the Early Cretaceous to Paleogene subduction complex and fore-arc basin (Sedlock 1996, 2003; Kimbrough, 2001). A serpentinite-matrix melange occupy

ACCEPTED MANUSCRIPT fault zones up to 500 m thick at all contacts between the blueschists and the upper-plate sequence. These two rock assemblages were probably exhumed in the Late Cretaceous to Paleogene subduction process. (Busby, 2004; Sedlock,

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2003) Subduction continued beneath the continental margin during Tertiary time, with an associated forearc basins similarly as in the Great Valley in California and and the Vizcaíno region in central Baja California (Figure 2). At ~29 Ma, the

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Pacific/Farallon spreading center made contact with the North America trench in

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central California and the relative Pacific-North America plate motion was imparted at the base of the continental crust (Mammerickx and Klitgord, 1982; Atwater and Stock, 1998; Bohannon and Parsons, 1995). Subduction continued to the south until 16.5 Ma in northern Baja California, and until 12.5 Ma in southern Baja California (Lonsdale, 1989). The plate boundary deformation was

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partitioned into deformations belts in either side of Baja California: pure extension perpendicular to the Gulf of California in the east side, and dextral strike-slip shearing along the Tosco Abreojos/San Benito fault system, parallel to the

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abandoned trench (Figure 1). This model implies that ~300 km of NNW displacement occurred along the western deformation belt in the lapse between

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12.5 and 6 Ma, when the transtensional strain in the Gulf of California became established (Bennett et al., 2014; Seiler et al., 2010; Sutherland et al., 2012). More recently, Fletcher et al. (2007) found that < 150 km of slip can be

accommodated in the western margin during the last 14.5 Ma. They come to this result by reconstructing the original position of the source of the Magdalena fan, an apparently beheaded submarine depocenter, filled ca. 15-13 Ma ago, which is overlying oceanic crust at the base of the continental slope south of Bahia

ACCEPTED MANUSCRIPT Magdalena (Figure 1). These authors conclude that faults along western Baja California can only accommodate a small fraction of the total Pacific-North American slip, which must be balanced by an increase in the shear across the

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Gulf of California region. In consequence, they propose that transtensional rifting initiated in the Gulf of California since the time that the highly fragmented Magdalena ridge was abandoned, 12.3 m.y. ago, implying that this region should contain the weakest lithosphere and/or was the site of the greatest applied

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tectonic stress.

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In a recent review paper, Ferrari et al. (2017) integrate the current knowledge about the tectonic and magmatic evolution of western Mexico. They show that the continental magmatism that occurred during the last phase of subduction of Farallon Plate beneath North America, can be linked to a progressive thinning of the upper plate and upwelling of asthenospheric mantle through a slab-free area

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(slab window) opened in the subducted Farallon plate beneath western Mexico, in Eocene time (ca. 50-40 m.y. ago). At this time extensional basins developed across the Central Mexican Plateau and the easternmost part of the Sierra

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Madre Occidental (SMO). A ~250 km wide rift zone, from eastern SMO to the site of the future Gulf of California developed by the end of Oligocene (30 Ma). This

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early crustal extension period was characterized by high-volume silicic magmatism and reduced plate convergence rates. An important consequence of the voluminous magmatism was thickening and strengthening of the crust beneath the SMO, probably an essential factor for the location of the future Gulf of California rift, along its western margin. The extension zone narrowed to an 80-100 km belt by ca. 18-19 Ma ago, and magmatism became more effusive and intermediate in composition forming the Comondú Group in Baja California, a

ACCEPTED MANUSCRIPT distinctive, ~1 km-thick volcano sedimentary succession that crops out in Baja California Sur (Figure 2). Rapidly extending, narrow tectonic depressions developed along the future site of the Gulf of California rifting zone. Thus,

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extension in the Gulf of California region seems to have initiated by 30 Ma ago, initially as a wide rift zone that ultimately narrowed along the current active transtensional rift. By ca. 12 Ma, the crust had thinned to half its original thickness along the axis of the rifting zone, and the Magdalena ridge was

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abandoned very close to the trench. Hence, Baja California micro-plate gradually acquired the Pacific plate motion and a dextral transtensional deformation was

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superimposed on an already thinned and weakened crust along the Gulf of California rift.

Offshore Baja California the Pacific oceanic floor shows evidence of the Miocene extinguished spreading centers and the abandoned trench (Figure 1),

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where fragments of the Farallon plate (Guadalupe and Magdalena micro-plates) subducted beneath the Baja California micro-plate (Lonsdale, 1991; Stock and Hodges, 1989; Fletcher et al., 2007). Since 12.5 Ma ago, the kinematics of Baja

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California micro-plate is dominated by boundary forces, in contrast with the buoyance forces that dominated during Mesozoic to mid-Miocene convergent

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history of the western margin of North American plate (Ferrari et al., 2017).

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Figure 1. Tectonic setting of northwestern Mexico. The Baja California Peninsula contains a regionally lineated magnetic anomaly here outlined by a dark band (Langenheim et al., 2014). Black dashed lines represent suture zones between distinct crustal terranes accreted to western North America (after, Sedlock, 2003). White dashed lines show the extinguished Miocene spreading centers and the abandoned trench and microplates. The Tosco-Abreojos fault parallel the abandoned trench is also indicated with a longer dash line. Active trans-tensive rifting occuring along the Gulf of California is indicated with continuous with line. Dashed contour lines show Magdalena Fan, in the southernmost edge of the abandoned trench (after Fletcher et al., 2007).

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3. Magnetotelluric Profile We conducted a magnetotelluric (MT) profile consisting of 37 observation sites along a 190-km long transect across central Baja California (Figure 2).

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Temporal variations of the natural electromagnetic fields, within a broad frequency band (0.001 to 100 Hz), were registered at every site. Observations were made with a remote reference array and the surface impedances calculated

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using a robust estimation technique (Chave and Thomson, 1989). Figure 2 shows the location of the observation sites as well as curves representing

selected sites along the profile.

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apparent resistivity and impedance phase, as functions of period, for eight

The profile was designed to perpendicularly cross the main lithological contacts and structures known by geological and geophysical observations

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(Figures 1 and 2). The motivation for this design lies in the intention to interpret the magnetotelluric profile in terms of 2D resistivity models. As shown in Figure 1, the tectonic structures characterizing the crust of Baja California are linear

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structures oriented NW-SW consistent with the orientation of the peninsular mainland and the Gulf of California. The geological map (Figure 2) also displays

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how our profile crosses major rock units that correspond to distinct terranes described here before. Thus, the subsurface structure is expected to be roughly 2D, coherent with known major tectonic and lithological structures. The belt of magnetic highs along the peninsula (Figure 1), supports the assumption that our MT profile goes across these structures and may be interpreted considering that resistivity changes are along the profile and with depth.

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Figure 2. Geologic map of Central Baja California (from Martin-Barajas and Delgado-Argote, 1995), location of magnetotelluric observation sites and representative examples of observed data. Apparent resistivity and phase curves estimated with field components in the measurement coordinates: X north, Y east.

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3.1. Vertical magnetic field The vertical magnetic field Hz, which is sensitive to lateral contrasts in ground resistivity, was also measured during the data acquisition in the field. We

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estimate the tipper vector, which is the response function between vertical and horizontal magnetic field components (Romo et al., 1999), as well as the socalled induction arrows. The tipper vector is a complex two-component vector

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whose magnitude is large near lateral resistivity contrasts. In a homogeneous medium or in a medium with weak lateral contrasts, the vertical magnetic field is

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close to zero and the tipper is not significant. However, where lateral contrasts are strong or very close to the measurement site, Hz is large and tipper estimation is more reliable. Figure 3b represents the tipper magnitudes as a function of period along the profile, significant magnitudes (larger than 0.5) occur in three zones at periods larger than 0.3 to1.0 seconds (-0.5 to 0.0 in logarithmic

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scale). These zones are (1) the western edge, from 0 to 40 km; (2) the central part, between 70 and 130 km; and (3) the eastern edge, from 180 to 200 km. As discussed later, these tipper anomalies can be associated with high conductivity

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anomalies found in the resistivity model.

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The real part of both tipper components is used to estimate the in-phase induction arrows (Schmucker, 1970; Parkinson, 1959, 1962), which points toward the conductive features and away from the resistive ones (Wiese, 1962). Inphase induction arrows were calculated at each observation site for periods larger than 1 s, in agreement with the occurrence of the larger tipper magnitude anomalies. In each observed site, we show an azimuth diagram showing the magnitude and azimuth of the induction arrows calculated for periods greater than 1 s (Figure 3a). The radius of the circles, indicated next to each circle with a

ACCEPTED MANUSCRIPT small number, vary from site to site. Most diagrams show induction arrows with highly consistent azimuths in the selected period band. Azimuth changes from site to site along the profile, but several coherent groups can be distinguished.

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The westernmost sites (01 to 04) have azimuths between 135° and 180°; sites 5 through 11 show azimuths between 30 and 90 degrees. In the central part of the profile, between sites 12 and 19, there are eleven sites that show azimuths between 180° and 240°; sites 18 to 27 show azimuths between 90° and 180°;

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from site 28 to the eastern edge of the profile (site 37) ten sites indicate very consistent azimuth between 45° and 90°. The signifi cance of the azimuth

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estimation can be judged by taking into account the magnitude of the induction vector arrow, as is the case for site 27 where the azimuth seems somewhat erratic, and the magnitude of the induction arrow is of the order of 0.25, this is too small for a reliable estimate.

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Summarizing, the consistency obtained in the azimuths at each site for different periods and along the profile, as well as the tipper magnitude anomalies located in certain zones of the profile, suggest that the geometry of the structures

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at depth is not too complex, allowing an interpretation based on 2D models that

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yield valid results.

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Figure 3. Induction arrows and tipper magnitudes. a) Azimuthal diagrams showing in-phase induction arrows for periods larger than 1 s. In a given site, azimuths are quite consistent. Azimuth variation along the profile can be grouped in five groups (see text). b) Tipper magnitude as a function of period and distance along the profile. Magnitudes larger than 0.5 are localized in three zones: from 0 to 40 km in the western edge; in the central part of the profile between 70 and 130 km; and in the eastern edge from 180 to 200 km. The tipper anomalies detect zones with lateral resistivity contrast.

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3.2. 2-D inversion Interpretation was based on the 2-D inversion of the apparent resistivity and phase responses for the TE and TM polarization modes, as estimated from the

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impedance tensor, by the Groom-Bailey’s (GB) decomposition process (Groom and Bailey, 1989). The basic assumption in this approach is that the geologic structure is appropriately represented by a 2D regional geometry, possibly

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distorted by 3D local effects that must be removed, as it is assumed that they are produced by shallow and local heterogeneities, with no geologic-interest but

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affecting the observed data. The measured impedance tensor is decomposed, using a non-linear optimization process, in a regional 2D-interpretable response along with four distortion parameters: anisotropy, shear, twist and azimuth. The first three of these parameters account for distortions affecting the measured

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electric field, whereas the azimuth is associated to the preferential direction of current flow. The optimization process solves for the combination of 2Dimpedance response and distortion parameters that better fit the observed

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impedance data. In a first attempt, all parameters are free to change, but restrictions on some parameters are imposed in the ensuing process in order to

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reduce the degrees of freedom of the problem. In our case, the only constrain imposed is an azimuth independent of frequency, i.e. we assumed that the measured impedance is the response of a 2D resistivity distribution, only distorted by local effects (e.g. shear, twist and anisotropy). In preparation for the 2D inversion the apparent resistivity GB-responses were corrected by a static shift factor estimated from transient electromagnetic soundings measured at the same locations of the MT observations (Flores et al., 2013).

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3.3. Resistivity model The GB impedances were interpreted in terms of a 2-D earth resistivity model, using a regularized 2-D inversion algorithm based on Gauss-Newton

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iterations (Rodi and Mackie, 2001). A 2-D half-space consisting of an arrangement of rectangular cells with uniform resistivity represented the initial model. The electromagnetic fields at the surface of the model were computed by

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an approximate finite-differences solution of Maxwell equations (Madden, 1972). The algorithm minimizes the misfit between observed data and model response

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in the usual least square sense, as well as constrains the roughness of the model by minimizing the Laplacian of the cells’ resistivities. The tradeoff between misfit and roughness is controlled by the so-called regularization parameter. The GB apparent resistivity and phase data for both polarization modes were inverted simultaneously, assuming a 5% uncertainty at every data point. We

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explore the solution space using different values of the regularization parameter. The solution shown in Figure 4a was obtained after 100 iterations, starting with a homogenous half space of 100 Ohm-m. The global misfit between observations

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and model response was 32%. The bar graph at the top of the model shows the

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misfit obtained at every site. The site misfit ranges from 3 to 10 standard deviations (sd), i.e. 15 to 50%. This value is obtained considering four responsecurves: apparent resistivity and phase for two polarization modes. The bottom panel of Figure 4 shows the misfit at every site for the whole

frequency range. The sections show the misfit distribution for apparent resistivity and phase responses in both polarization modes. The TE apparent resistivity shows the larger misfits, particularly at site 26. In this site, the predicted curve is a factor of two lower than the observed data, for the whole period range. Even

ACCEPTED MANUSCRIPT so, the shape of the observed curve is well fitted, as revealed by the corresponding phase misfit. When the four responses are considered, the resulting site misfit is 48%, as plotted in the bar graph at the top of panel (a). In

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contrast, misfits with similar magnitude (~50%) in sites 4 and 7 seem to be distributed in the four responses. In general, the main features present in the observed responses are reproduced in the predicted responses. As expected, the larger misfit is in the TE response, as the 2-D model cannot reproduce some

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possible 3-D effects persistent in the GB estimates.

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A limitation inherent to the physics of electromagnetic induction is that resolution decreases logarithmically with depth. As a consequence, the resistivity model of Figure 4a must be viewed as a blurred image of reality. In the model, conductivity anomalies are imaged smoother as they are deeper. On the other hand, the wave energy is strongly absorbed by low-resistivity zones, while it

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travels practically undisturbed through high-resistivity zones. Consequently, the tops of conductive anomalies are better resolved than their bottoms. In addition,

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zones.

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spurious anomalies may appear as shadows beneath intense low-resistivity

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Figure 4. a) Resistivity model and misfits. Conductive anomalies (hot colors) are labeled A to C and boundaries (dashed lines) are discussed in the text. The bar diagram represents the misfit between observed and calculated responses at each site expressed as standard deviations (sd). b) Seudo-sections representing the misfit between observed and calculated data, for apparent resistivity (above) and phase (below) in both polarization modes.

3.4. Sensitivity analysis In order to test the sensitivity of our data to the presence of the deep conductive zones B and C (Figure 4), we designed the following experiment: the

ACCEPTED MANUSCRIPT model of Figure 4a was modified in such a way that the entire structure below 20 km was removed and replaced by a half-space with a resistivity of 1000 Ohm-m; then, we calculated the magnetotelluric response of this modified model, and

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estimated its misfit. Figure 5a compares the misfit obtained for both models: the one that includes the conductors of Figure 4a and the model with conductors removed. We noted that conductive anomalies below 20 km are significant and needed to reduce the misfit. As expected, the increase in misfit is very noticeable

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for sites in the eastern half of the profile, where the high resistivity in the upper 20 km (~ 1000 Ohm-m) allows a greater sensitivity at larger depth. In this area the

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presence of deep conductors produces an improvement of more than 10 standard deviations (50%) in most of the sites. In contrast, in the western half of the profile, the improvement is still significant but less intense. In this region, the highly conductive sedimentary cover (1-3 Ohm-m) shields the effect of deep

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conductors, as it dissipates part of the electromagnetic wave energy, resulting in a loss of sensitivity at depth. Even so, the presence of a deep conductor in this zone improves the fit in about 1 to 3 standard deviations (5 to 15%). Finally, sites

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36 and 37, at the eastern edge of the profile, are the only sites whose fit deteriorates by including the deep conductors. In this case we must take into

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account that these sites are at the end of the profile, and there we have more uncertainty about deep structures. Figure 5b shows misfit seudo-sections obtained for the impedance response of a model without conductors, to be compared with Figure 4b that shows the corresponding seudo-sections from our final model.

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a) 24

20

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model with conductive anomalies model with conductors removed

12

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Misfit ( σ )

16

8

0 1

3 2

5 4

7 6

9 8

11 10

13 12

SW

17

16

22

23

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Apparent resistivity misfit

19

21

24

18

26

25

27

28

30 29

32 31

34 33

36 35

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Apparent resistivity misfit

Phase misfit

37

NE TM Mode

TM Mode (degrees)

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Phase misfit

log period (s)

15

14

site number

log period (s)

b)

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4

distance (km)

distance (km)

Figure 5. a) Misfit obtained at each site by the model shown in Figure 4a compared to that obtained when > 20 km deep conductive anomalies are removed, and substituted by a 1000 Ohm-m half space. In this case the misfit is represented by the number of standard deviations (σ), (one s.d equals 5%). b) Misfit seudo-sections obtained with the model with the deep conductors removed. Each seudo-section represents the results from apparent resistivity and phase, in both polarization modes. Here, the misfit is represented in percentage (5% equals 1 s.d.) for apparent resistivity and in degrees for the phase.

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4. Discussion Four main anomalies are visible in the resistivity model shown in Figure 4a.

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The shallow (0 - 3 km) low resistivity layer (<3 Ohm-m) between sites 4 and 21 can be associated with highly porous Quaternary sediments overlying Mid Tertiary sedimentary deposits chiefly composed of sandstone and shale

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(Helenes, 1984; García‐Abdeslem et al., 2005). The presence of salty soil at the surface is an evidence for seawater invasion in the highly permeable shallow

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sediments, which increases significantly the conductivity. Sites located around kilometer 40 in our profile are very close to the southernmost edge of a large coastal lagoon (Ojo de Liebre), which is a salty body of seawater that penetrate almost 60 km inland from the southern coast of Bahía Vizcaino (Figure 2). In fact,

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our profile goes across supra-tidal evaporitic flats, where inflow of marine brines and dolomite formation is taking place (Pierre et al., 1984). Deeper and more interesting anomalies are labeled as A, B, C and D (Figure

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4a). At the western edge of the model, close to the fossil trench, the conductive anomaly A extends to the surface emerging beneath sites 1 to 7. This anomaly is

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likely associated to the Triassic-Jurassic arc-ophiolite complex and serpentinitematrix mélange accreted to Baja California during Cretaceous time and subsequently exhumed. Sedlock, (1988; 1996) claims that the Jurassic accretion prism was metamorphosed in blueschist conditions, exhumed at the footwalls of regional normal faults during syn-subduction extension, and currently exposed at the western tip of Vizcaíno Peninsula and in Cedros Island. The regional fault sketched by Sedlock (1996; 2003) as separating the Jurassic exhumed terrane

ACCEPTED MANUSCRIPT from the western PRB terrane (westernmost black dashed line along southern Baja California in Figure 1), is shown in our resistivity model (Figure 4a) as a regional normal fault between sites 8 and 9. This structure is clearly coincident

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with the western limit of Vizcaino Basin (anomaly D discussed later) and extends at depth through a slightly lower resistivity feature that seems to separate the high-resistivity basement beneath Vizcaino basin.

Anomaly B is interpreted as produced by fluids released by the oceanic slab

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and trapped at the base of the continental crust. The subduction of the

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Guadalupe and Magdalena microplates likely provided considerable amount of fluids to the lower crust. On the other hand, it is believed that, as the PacificFarallon ridge came close to the trench, the subduction rate slowed down and finally stopped, with a consequent warming of both, the oceanic slab and the overriding plate. The fluids released in this process may be trapped above the

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oceanic slab, just below the Moho. It is known that fluids can be trapped in the lower crust during periods as long as 108 years (Bailey, 1990; Park, 1992). The flat east-dipping geometry and the depth of anomaly B are additional arguments

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supporting our interpretation. Persaud et al. (2007) estimate the depth to the Moho at the latitude of our profile in between 27 and 34 km, based on teleseismic

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receiver functions, which is in very good agreement with the ~30 km depth to the top of conductor B in our survey. Persaud et al. (2007) also found indications of the presence of a stalled slab beneath the Moho in one of their measurement sites located ~75 km south of our profile. It should be mentioned that the top of anomaly B is accurately resolved, but not the thickness. As discussed before, our methodology allows a good resolution for the top of the conductive interval but not for the bottom. Conductor B behaves as an energy-diffusing layer that

ACCEPTED MANUSCRIPT definitely precludes the detection of a relatively resistive layer beneath it, as could be the case for a fragment of oceanic slab stalled underneath the Baja California crust. Either way, the presence of fluids in horizontally extended zone

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represents a weakness zone at the base of the continental crust, which supports the idea that Baja California lithosphere is not entirely coupled to the Pacific plate (Fletcher et al., 2007; Dixon et al., 2000).

Anomaly C bends up beneath site 18 just where rocks of the PBR crops out

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and have their westernmost expressions (Figure 2). Its location coincides with

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the eastern edge of a regional magnetic anomaly associated with the westernPBR magnetite-bearing crust (Langenheim and Jachens, 2003; Langenheim et al., 2014). This magnetic anomaly is a 70-km-wide belt of magnetic highs that

extends all along Baja California Peninsula (Figure 1). It is worth mentioning that towards the north of our profile, the eastern edge of the high-magnetic belt

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coincides with the magnetite-ilmentita compositional change of the PBR identified by Gastil (1990), as well as with the suture zone between Alisitos arc (Alisitos terrane) and the old Paleozoic-Precambrian continental crust (Caborca terrane),

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as outlined by Sedlock (2003) and represented by the black-dashed line in Figure 1, close to the Gulf of California coastline. Structural evidence of a suture zone at

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the latitude of our profile and further south is missing, even though the magnetic anomaly belt continues south to the tip of the peninsula. The upward bent of anomaly C may correspond to the compositional boundary between the western and eastern PRB, as suggested by the magnetic anomaly as well as by the fact that it separates a thicker resistive crust (~30 km thick beneath site 19), and a thinner crust (~20 km depth) that extends from site 26 to the eastern edge of our profile (Figure 4). The crustal thickness imaged by the resistivity model is in good

ACCEPTED MANUSCRIPT agreement with the values reported by Persaud et al., (2007). As discussed before, the depth to the top of a conductor is a relatively well-solved feature in our model. The difference in thickness at both sides of anomaly C is consistent

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with the idea of a thinner crust beneath eastern Baja California. Finally, we associate anomaly D to the Cretaceous and younger sedimentary, volcanic and meta-volcanic rocks filling the Vizcaíno fore-arc basin (Kimbrough et al., 2001). The Late Cretaceous to Tertiary siliciclastic fore arc sequence

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overlays Jurassic-Cretaceous turbidite deposits on the top of ophiolite rocks and

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blueschists assemblage that constitute the basement in the fore-arc basin, which originated in similar tectonic conditions as the Great Valley basin in California. The Vizcaino and Purisima basins may thus contain potential hydrocarbon reservoirs.

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5. Conclusions

Our resistivity model provides new geophysical information about the physical conditions of the crust and upper mantle in central Baja California. Our findings

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are in good agreement with current knowledge provided by geological and geophysical information. The horizontally extended anomaly in the lower crust

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(anomaly B) is interpreted as fluids trapped beneath the Moho, which were released by the subducted slab during convergence of the Farallon-related microplates. This anomalous conductive zone can be considered as a weak zone beneath the lower crust of Baja California, supporting the idea that the peninsular lithosphere is not entirely coupled to the Pacific plate. Being conductive, this anomaly behaves like a diffusing layer that absorbs electromagnetic energy and prevents the detection of a relatively resistive body at greater depth, as may be a stalled fragment of the oceanic plate.

ACCEPTED MANUSCRIPT Triassic-Jurassic arc-ophiolite complex and serpentinite-matrix mélange associated to the Cochimí terrane (after Sedlock, 2003) produces the conductive anomaly A in the western edge of the MT profile. The boundary sketched by

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Sedlock (1996; 2003) as separating the Jurassic exhumed Cochimi terrane, from the western PRB, coincides with the western limit of Vizcaino basin (anomaly D), as well as with a slightly lower resistivity feature beneath sites 8-9, which separates the high-resistivity structure imaged beneath Vizcaino basin between

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10 and 30 km depth.

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The bending up of anomaly C beneath site 18 is interpreted as the boundary between the western and eastern PRB terranes, as suggested by the magnetic anomaly belt reported by Langenheim and Jachens (2003), and Langenheim et al. (2014). This conductive anomaly also separates a thicker resistive crust (~30

of our profile.

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km beneath site 19), from a thinner crust (~20 km depth) eastwards of site 26 in

Acknowledges

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This work was possible thanks to the financial support of Conacyt (project grant: 25792-T). J.M. Romo thanks Conacyt for the scholarship granted for the

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completion of his PhD studies at CICESE. We also thank the support of the technical staff at the Department of Applied Geophysics at CICESE during the field survey. We thank A. Martín-Barajas who reviewed and commented our manuscript, particularly the geological implications. The authors are especially grateful for the comments of two reviewers, which contributed substantially to the improvement of the manuscript.

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References Alsleben, H., Wetmore, P. H., Schmidt, K., Paterson, S., and Melis, E., 2008. Complex deformation during arc–continent collision: Quantifying finite strain

Journal of Structural Geology, 30(2), 220-236.

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in the accreted Alisitos arc, Peninsular Ranges batholith, Baja California.

Alsleben, H., Wetmore, P., & Paterson, S., 2014. Structural evidence for mid-

SC

Cretaceous suturing of the Alisitos arc to North America from the Sierra Calamajue, Baja California, Mexico. Geological Society of America

M AN U

Memoirs, 211, 691-711.

Bailey, R.C., 1990. Trapping of aqueous fluids in the deep crust, Geophys. Res. Lett., 17(8), 1129-1132.

Bennett, S.E. and Oskin, M.E., 2014. Oblique rifting ruptures continents:

218.

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Example from the Gulf of California shear zone. Geology, 42(3), pp.215-

Bohannon, R.G. and Parsons, T., 1995. Tectonic implications of post-30 Ma Pacific and North American relative plate motions, Geol. Soc. Amer. Bull.

EP

107, 937-959.

AC C

Brasse, H., Kapinos, G., Mütschard, L., Alvarado, G.E., Worzewski, T. and Jegen, M., 2009. Deep electrical resistivity structure of northwestern Costa Rica. Geophysical Research Letters, 36(2).

Busby, C., 2004. Continental growth at convergent margins facing large ocean basins: a case study from Mesozoic convergent-margin basins of Baja California, Mexico. Tectonophysics, 392(1), 241-277. Busby, C., Adams, B. F., Mattinson, J., & Deoreo, S., 2006. View of an intact oceanic arc, from surficial to mesozonal levels: Cretaceous Alisitos arc, Baja

ACCEPTED MANUSCRIPT California. Journal of Volcanology and Geothermal Research, 149(1), 1-46. doi.org/10.1016/j.jvolgeores.2005.06.009. Busby, C.J., 2012. Extensional and transtensional continental arc basins: Case

Basins: Recent Advances, 382-404.

RI PT

studies from the southwestern United States. Tectonics of Sedimentary

Busby, C., Smith, D., Morris, W. and Fackler-Adams, B., 1998. Evolutionary model for convergent margins facing large ocean basins: Mesozoic Baja

SC

California, Mexico. Geology, 26(3), pp.227-230.

Chave, A. D., Jones, A., 2012. The magnetotelluric method, theory and practice.

M AN U

Cambridge University Press, New York, 544 pp.

Chave, A. and Thomson, D.J., 1989. Some comments on magnetotellurics response function estimation, J. Geophys. Res., 94, 14215-14225. Dickinson, W.R., and Lawton, T.F., 2001. Carboniferous to Cretaceous assembly

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and fragmentation of Mexico: Geol. Soc. Amer. Bull., 113, 1142-1160. Dixon, T., Farina, F., DeMets, C., Suarez-Vidal, F., Fletcher, J., Marquez-Azua, B., Miller, M., Sanchez, O., Umhoefer, P., 2000. New kinematics for Pacific-

EP

North America motion from 3 Ma to present, II: evidence for a “Baja California shear zone”. Geophysical Research Letters. 27, 3961–3964. DOI:

AC C

10.1029/2000GL008529

Ferrari, L., Orozco-Esquivel, T., Bryan, S.E., López-Martínez, M. and SilvaFragoso, A., 2017. Cenozoic magmatism and extension in western Mexico: Linking the Sierra Madre Occidental silicic large igneous province and the Comondú Group with the Gulf of California rift. Earth-Science Reviews. Fletcher, J.M., Grove, M., Kimbrough, D., Lovera, O. and Gehrels, G.E., 2007. Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon

ACCEPTED MANUSCRIPT U-Pb ages from the Magdalena fan and adjacent areas. Geological Society of America Bulletin, 119(11-12), pp.1313-1336.

Flores, C., Romo, J.M. and Vega, M., 2013. On the estimation of the maximum

RI PT

depth of investigation of transient electromagnetic soundings: the case of the Vizcaino transect, Mexico. Geofísica internacional, 52(2), pp.159-172.

García‐Abdeslem, J., Romo, J.M., Gómez‐Treviño, E., Ramírez‐Hernández, J., Esparza‐Hernández, F.J. and Flores‐Luna, C.F., 2005. A constrained 2D

M AN U

Geophysical Prospecting, 53(6), pp.755-765.

SC

gravity model of the Sebastián Vizcaíno basin, Baja California Sur, Mexico.

Gastil, R.G., 1993. Prebatholithic history of peninsular California. Geological Society of America Special Papers, 279, pp.145-156.

Gastil, G., Diamond, J., Knaack, C., Walawender, M., Marshall, M., Boyles, C., Chadwick, B. and Erskine, B., 1990. The problem of the magnetite/ilmenite

TE D

boundary in southern and Baja California California. Geological Society of America Memoirs, 174, pp.19-32.

Gastil, R.G., Morgan, G.J. and Krummenacher, D., 1981. The tectonic history of

EP

peninsular California and adjacent Mexico. The Geotectonic Development of California: Englewood Cliffs, New Jersey, Prentice-Hall, pp.284-306.

AC C

Gastil, R.G., Phillips, R.P. and Allison, E.C., 1975. Reconnaissance geology of the state of Baja California. Geological Society of America Memoirs, 140,

pp.1-201.

Groom, R.W. and Bailey, R.C., 1989. Decomposition of magnetotelluric impedance tensors in the presence of local three‐dimensional galvanic distortion. Journal of Geophysical Research: Solid Earth, 94(B2), pp.19131925.

ACCEPTED MANUSCRIPT Helenes J. 1984. Dinoflagellates from Cretaceous to early Tertiary rocks of the Sebastian Vizcaino basin, Baja California, Mexico. in: Geology of the Baja California Peninsula (ed. V.A. Frizzell), No. 39, pp. 89–106. Pacific Section,

RI PT

Society of Economic Paleontologists and Mineralogists.

Ichiki, M., Baba, K., Toh, H. and Fuji-Ta, K., 2009. An overview of electrical conductivity structures of the crust and upper mantle beneath the

Gondwana Research, 16(3), pp.545-562.

SC

northwestern Pacific, the Japanese Islands, and continental East Asia.

Jödicke, H., Jording, A., Ferrari, L., Arzate, J., Mezger, K. and Rüpke, L., 2006.

M AN U

Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico: Implications for the generation of volcanism and subduction dynamics. Journal of Geophysical Research: Solid Earth, 111(B8).

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Johnson, S.E., Tate, M.C. and Fanning, C.M., 1999. New geologic mapping and SHRIMP U-Pb zircon data in the Peninsular Ranges batholith, Baja California, Mexico: Evidence for a suture?. Geology, 27(8), pp.743-746.

EP

Johnson, S.E., Fletcher, J.M., Fanning, C.M., Vernon, R.H., Paterson, S.R. and Tate, M.C., 2003. Structure, emplacement and lateral expansion of the San

AC C

José tonalite pluton, Peninsular Ranges batholith, Baja California, México. Journal of Structural Geology, 25(11), pp.1933-1957.

Kimbrough, D.L., Smith, D.P., Mahoney, J.B., Moore, T.E., Grove, M., Gastil, R.G.,

Ortega-Rivera,

A.

and

Fanning,

C.M.,

2001.

Forearc-basin

sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of southern and Baja California. Geology, 29(6), pp.491-494.

ACCEPTED MANUSCRIPT Lonsdale, P., 1991. Structural patterns of the Pacific floor offshore of Peninsular California, in: The Gulf and peninsular province of the Californias, edited by J.P. Dauphin and B.T. Simoneit, Amer. Assoc. Petr. Geol., Memoir 47, 87-

RI PT

125. Langenheim, V.E. and Jachens, R.C., 2003. Crustal structure of the Peninsular Ranges batholith from magnetic data: Implications for Gulf of California rifting. Geophysical Research Letters, 30(11).

SC

Langenheim, V.E., Jachens, R.C. and Aiken, C., 2014. Geophysical framework of the Peninsular Ranges batholith—Implications for tectonic evolution and

M AN U

neotectonics. Geological Society of America Memoirs, 211, pp.1-20. Madden, T.R., 1972. Transmission systems and network analogies to geophysical forward and inverse problems, ONR Technical Report, 72-3. Mammerickx, J. and Klitgord, K.D., 1982. Northern East Pacific Rise: Evolution

TE D

from 25 my BP to the present. Journal of Geophysical Research: Solid Earth, 87(B8), pp.6751-6759.

Martın-Barajas, A. and Delgado-Argote, L., 1995. Inventario de Recursos

EP

Minerales del Estado de Baja California. Cap, 2, pp.20-77. ID:1654 Matsuno, T., Seama, N., Evans, R.L., Chave, A.D., Baba, K., White, A., Goto,

AC C

T.N., Heinson, G., Boren, G., Yoneda, A. and Utada, H., 2010. Upper mantle electrical resistivity structure beneath the central Mariana subduction system. Geochemistry, Geophysics, Geosystems, 11(9).

Newton, R.C., 1990. Fluids and shear zones in the deep crust. Tectonophysics, 182(1-2), pp.21-37. Normark, W.R., Spencer, J.E., and Ingle, J., 1987, Geology and Neogene history of the Pacific continental margin of Baja California Sur, Mexico, in: Scholl, D.W., et al., eds., Geology and resource potential of the continental margin

ACCEPTED MANUSCRIPT of western North America and adjacent ocean basins—Beaufort Sea to Baja California: Houston, Texas, Circum-Pacific Counsel for Energy and Mineral Resources, Earth Science Series, 6, p. 449–472.

RI PT

Park, S.K., Biasi, G.P., Mackie, R.L. and Madden, T.R., 1991. Magnetotelluric evidence for crustal suture zones bounding the southern Great Valley, California. Journal of Geophysical Research: Solid Earth, 96(B1), pp.353376.

SC

Park, S.K., 1992. Magnetotelluric evidence for a brittle-ductile transition,

Letters, 19(21), 2143-2146.

Parkinson,

W.D.,

1959.

M AN U

Peninsular Ranges batholith, southern California?, Geophysical Research.

Directions

of

rapid

geomagnetic

fluctuations.

Geophysical Journal International, 2(1), pp.1-14.

Parkinson, W.D., 1962. The influence of continents and oceans on geomagnetic

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variations. Geophysical Journal International, 6(4), pp.441-449. Pierre, C., Ortlieb, L. and Person, A., 1984. Supratidal evaporitic dolomite at Ojo de Liebre lagoon: mineralogical and isotopic arguments for primary

EP

crystallization. Journal of Sedimentary Research, 54(4). Reynard, B., Mibe, K. and Van de Moortèle, B., 2011. Electrical conductivity of

AC C

the serpentinised mantle and fluid flow in subduction zones. Earth and Planetary Science Letters, 307(3), pp.387-394.

Rodi, W. and Mackie, R.L., 2001. Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion. Geophysics, 66(1), pp.174-187.

Romo, J.M., Gomez-Trevino, E. and Esparza, F.J., 1999. An invariant representation for the magnetic transfer function in magnetotellurics. Geophysics, 64(5), pp.1418-1428.

ACCEPTED MANUSCRIPT Santos, F.A.M., Mateus, A., Almeida, E.P., Pous, J. and Mendes-Victor, L.A., 2002. Are some of the deep crustal conductive features found in SW Iberia caused by graphite?. Earth and planetary science letters, 201(2), pp.353-

RI PT

367. Schmucker, U., 1970, Anomalies of geomagnetic variations in the southwestern United States, Bull. Scripps Inst. of Oceanography, Univ. of California San Diego, v.13, Univ. of California Press.

SC

Sedlock, R.L., 1988. Tectonic setting of blueschist and island-arc terranes of west-central Baja California, Mexico. Geology, 16(7), pp.623-626.

M AN U

Sedlock, R.L., Ortega-Gutiérrez, F. and Speed, R.C., 1993. Tectonostratigraphic terranes and tectonic evolution of Mexico. Geological Society of America Special Papers, 278, pp.1-153.

Sedlock,

R.L.,

1996.

Syn-subduction

forearc

extension

and

blueschist

TE D

exhumation in Baja California, México, in: Subduction: top to bottom, edited by Bebout, G.E., D.W. Scholl, S.H. Kirby and J.P. Platt, Amer. Geophys. Un., Geophysical Monograph 96, 155-162,1996.

EP

Sedlock, R.L., 2003. Geology and tectonics of the Baja California Peninsula and adjacent areas, in: Tectonic evolution of northwestern Mexico and the

AC C

Southwestern USA, Johnson, S.E. ed., Geological Society of America, 374.

Seiler, C., Fletcher, J.M., Quigley, M.C., Gleadow, A.J. and Kohn, B.P., 2010. Neogene structural evolution of the Sierra San Felipe, Baja California: Evidence for proto-gulf transtension in the Gulf Extensional Province?.

Tectonophysics, 488(1), pp.87-109.

Soyer, W. and Unsworth, M., 2006. Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids. Geology, 34(1), pp.53-56.

ACCEPTED MANUSCRIPT Spencer, J.E. and Normark, W.R., 1979. Tosco-Abreojos fault zone: A Neogene transform plate boundary within the Pacific margin of southern Baja California, Mexico. Geology, 7(11), pp.554-557.

RI PT

Stock, J.M. and Hodges, K.V., 1989. Pre‐Pliocene extension around the Gulf of California and the transfer of Baja California to the Pacific Plate. Tectonics, 8(1), pp.99-115.

Sutherland, F.H., Kent, G.M., Harding, A.J., Umhoefer, P.J., Driscoll, N.W.,

SC

Lizarralde, D., Fletcher, J.M., Axen, G.J., Holbrook, W.S., GonzálezFernández, A., Lonsdale, P., 2012. Middle Miocene to early Pliocene

M AN U

oblique extension in the southern Gulf of California. Geosphere. http://dx.doi.org/10.1130/GES00770.1.

Thomson, C.N. and Girty, G.H., 1994. Early Cretaceous intra‐arc ductile strain in Triassic‐Jurassic and Cretaceous continental margin arc rocks, Peninsular

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Ranges, California. Tectonics, 13(5), pp.1108-1119.

Todd, V.R., B.G. Erskine and Morton, D.M., 1988. Metamorphic and tectonic evolution of the northern Peninsular Ranges batholith, in Metamorphism and

EP

Crustal Evolution of the Western United States, edited by W.G. Ernst, 894937, Prentice-Hall Inc., Englewood Cliffs, New Jersey.

AC C

Upton, P. and Craw, D., 2008. Modelling the role of graphite in development of a mineralised mid-crustal shear zone, Macraes mine, New Zealand. Earth and

Planetary Science Letters, 266(3), pp.245-255.

Walawender, M. J., , A synthesis of recent work in the Peninsular Ranges batholith, in Geological Excursions in southern California and Mexico, edited by M.J. Walawender and B.B. Hanan, 297-318, Geol. Soc Amer. Guidebook, 1991.

ACCEPTED MANUSCRIPT Walawender, M.J., Girty, G.H., Lombardi, M.R., Kimbrough, D., Girty, M.S. and Anderson, C., 1991. A synthesis of recent work in the Peninsular Ranges Batholith, in: Geological Excursions in southern California and Mexico,

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edited by M.J. Walawender and B.B. Hanan, 297-318, Geol. Soc Amer. Guidebook,

Wetmore, P.H., Schmidt, K.L., Paterson, S.R. and Herzig, C., 2002. Tectonic implications for the along-strike variation of the Peninsular Ranges batholith,

SC

southern and Baja California. Geology, 30(3), pp.247-250.

Wannamaker, P.E., Caldwell, T.G., Jiracek, G.R., Maris, V., Hill, G.J., Ogawa, Y.,

M AN U

Bibby, H.M., Bennie, S.L. and Heise, W., 2009. Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand. Nature, 460(7256), p.733.

Wiese, H., 1962. Geomagnetische Tiefentellurik Teil II: die Streichrichtung der

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Untergrundstrukturen des elektrischen Widerstandes, erschlossen aus geomagnetischen Variationen. Pure and applied geophysics, 52(1), pp.83103.

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Worzewski, T., Jegen, M., Kopp, H., Brasse, H. and Castillo, W.T., 2011. Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone.

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Nature Geoscience, 4(2), 108-111.

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Electrical conductivity of the crust in central Baja California, México, based on magnetotelluric observations

•

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Highlights

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Romo, J.M., Gómez-Treviño E., Flores-Luna C. and García-Abdeslem J.

A magnetotelluric profile across Baja California, México, reveals several

peninsular crust. •

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electrical conductivity anomalies associated to the tectonic history of the

A striking sub-horizontal conductivity anomaly reveled in our model can be explained by the presence of high-salinity fluids released after dehydration of

•

TE D

the subducted Magdalena microplate.

The presence of fluids at the base of the peninsular crust may produce a zone of weakness, which supports the idea that Baja California lithosphere has not

•

EP

been entirely coupled to the Pacific plate. Some conductivity anomalies present in our model are associated to highly

AC C

sheared zones resulting of some of the major accretion events occurred during the geologic evolution of western North America.

•

The closely spaced array of magnetotelluric observation sites allowed us to obtain a sufficiently constrained resistivity model, as well as reach geological conclusions that agree with some other independent results.