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Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation
Journal Pre-proof Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation. Thalia Avilés Esquivel, Carlos Flores, Valeria Reyes Ortega, Steven Constable, Enrique Gómez-Treviño, Antonio González-Fernández PII:
S0895-9811(20)30014-6
DOI:
https://doi.org/10.1016/j.jsames.2020.102501
Reference:
SAMES 102501
To appear in:
Journal of South American Earth Sciences
Received Date: 8 July 2019 Revised Date:
10 January 2020
Accepted Date: 12 January 2020
Please cite this article as: Esquivel, T.A., Flores, C., Ortega, V.R., Constable, S., Gómez-Treviño, E., González-Fernández, A., Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation., Journal of South American Earth Sciences, https://doi.org/10.1016/j.jsames.2020.102501. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
December 14th, 2019 To whom correspond, I attach the author’s statements, where I describe the contribution of each one of the authors to this publication.
Authors Thalia Anaid Avilés Esquivel (Corresponding author) Carlos Flores Luna
Formal Analysis, visualization, validation, writing the original draft Conceptualization, Investigation, writing the original draft
Valeria Reyes Ortega
Formal analysis, validation, visualization
Steven Constable
Methodology, software, resources, supervision
Enrique Gómez Treviño
Methodology, software
Antonio González Fernández
Proyect administration, found resources, funding acquisition.
Best regards,
Thalia Anaid Avilés Esquivel
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation. Thalia Avilés-Esquivel a,b,*, Carlos Flores a,b, Valeria Reyes-Ortega a,b,d , Steven Constable c, Enrique Gómez-Treviño a,b and Antonio González-Fernández a,b a
División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, Baja California 22860, México. b Centro Mexicano de Innovación en Energía Geotérmica (CeMIEGeo), Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, Baja California 22860, México. c Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, 92093, USA. d Now at c * Corresponding author: [email protected] Keywords: Marine Magnetotellurics, electrical resistivity – heat flow correlation Abstract
24
Different geological and geophysical disciplines have suggested the Wagner Basin as a
25
promising location of geothermal resources. As a reconnaissance tool Magnetotelluric
26
(MT) data were measured at 10 sites along a profile over this basin in the northern Gulf
27
of California. The subsurface distribution of electric resistivity was estimated with two-
28
dimensional smooth inversion using the apparent resistivities and phases from both field
29
polarizations. We found a deep conductor underneath the center of the basin,
30
interpreted as a region of partial melt feeding the accretion zone of this incipient oceanic
31
crust. From six MT sites with close-by heat flow measurements, we also found a
32
positive correlation between the interpreted shallow (first 100 m) low resistivities and
33
high heat flows. This shows that the MT method can be used to map submarine heat
34
sources.
35 36
Introduction
37
The Wagner Basin is the northernmost rifting basin in the Gulf of California.
38
Several facts suggest this basin as a potential source of geothermal resources. First, its
39
tectonic similarity with the Mexicali-Imperial valley to the north, which has important
40
geothermal fields such as Cerro Prieto, Salton Sea, Brawley, East Mesa, and Heber.
41
The combined installed capacity of these fields in 2015 was 1.5 GWe (Boyd et al., 2015;
42
Gutiérrez-Negrín et al., 2015). Second, the high temperatures and heat flow
43
measurements reported in this area since the earliest 70’s (Henyey and Bischoff, 1973;
44
Grijalva, 1986; Prol-Ledesma et al., 2013; Neumann et al., 2017). The measured heat
45
flows in the Wagner and neighboring Consag basins are up to 15 times the average
46
value for oceanic crust. Third, the intense submarine gas discharges reported by Canet
47
et al. (2010) and the geochemical signature of high-temperature pore water measured
48
in sediment cores by Batista Cruz et al. (2019). The mapped seismic events (González-
49
Escobar et al., 2010) and the active distributed deformation (Persaud et al., 2003)
50
suggest the presence of active tectonism. Finally, the incipient formation of new crust is
51
suggested by the possible presence of intrusive bodies in a seismic reflection profile
52
over the Consag Basin (González-Escobar et al., 2014) and in the gravity anomaly in a
53
profile over the Wagner Basin (Pérez, 1982).
54 55
Regarding the geologic setting, the Gulf of California – San Andreas fault system
56
defines the boundary between the Pacific and North American plates (Atwater, 1970;
57
Lonsdale, 1989). The Wagner Basin is the northernmost rifting basin in the Gulf of
58
California. The Consag, Upper and Lower Delfín, and Guaymas are among other basins
59
to the south. Important differences exist between the northern and southern Gulf basins.
60
Well developed spreading centers occur at the mouth of the Gulf, with clearly
61
observable magnetic anomaly stripes, evidence of new oceanic crust being created
62
since about 4 Ma at a full spreading rate of 6 cm/year (Larson et al., 1968). In contrast,
63
in the northern part no magnetic lineaments are observed (Klitgord et al., 1974;
64
Sanchez et al., 1991) and thick sedimentary deposits, supplied mainly by the Colorado
65
River, fill these basins (Dorsey, 2010). The lack of magnetic anomalies in the northern
66
basins is the main evidence for the absence of new oceanic crust. However, the region
67
of the northern basins is considered a continent-ocean transition zone, where diffuse
68
continental deformation exists (Nagy and Stock, 2000; Persaud et al., 2013).
69
70
In this work, we present the results of a Magnetotelluric (MT) reconnaissance
71
survey carried out in the southern portion of the Wagner Basin (Figure 1) with the
72
purpose of assessing its geothermal potential. This project is part of a national effort to
73
increase the use of renewable energy sources (geothermal, wind, solar, etc.) to diminish
74
the hydrocarbon dependence (Avilés Esquivel, 2016). Technologic and environmental
75
obstacles should be overcome for future sustainable exploitation of the submarine
76
geothermal resource in this environmental protected zone.
77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93
94
95 96
Figure 1: Regional map of the northern Gulf of Baja California. Lines denote the normal
97
faults inferred from seismic reflection data from PEMEX (Petróleos Mexicanos) (Martín-
98
Barajas et al., 2013). CF=Consag Fault, WF=Wagner Fault, CPF=Cerro Prieto Fault,
99
CPSF=Cerro Prieto South Fault, ADF=Adair Fault, AMF=Amado Fault, WB=Wagner
100
Basin, CB=Consag Basin. Black dots indicate the location of the MT sites. The
101
bathymetry is from G. Díaz Méndez (pers. comm.). The boundaries between the Pacific
102
(PP) and North American (NAP) plates, as well as their direction of displacement are
103
shown in the inset.
104 105
106
The use of electromagnetic (EM) methods to study the marine subsurface has
107
shown an important increase in the last years (Key, 2012). These works have either
108
academic or resource exploration goals, mainly for the characterization of offshore
109
hydrocarbon reservoirs, but also for studying the oceanic lithosphere (e.g., Evans et al.,
110
1991; Constable et al., 1997; Heinson et al., 2000; Baba et al., 2006a; Key et al., 2013).
111
The MT method uses low-frequency EM waves originated by sun-ionosphere
112
interactions and far-away lightning sources. These waves impinge upon the sea
113
surface, diffusing into the seawater and seabed. Measured electric and magnetic fields
114
on the seafloor are then used to estimate the frequency-dependent impedances, which
115
are inversely modeled to estimate the subsurface electrical resistivity. The presence of
116
a thick sedimentary layer in the Gulf of California dissipates the signal from seismic and
117
gravity sources, preventing to obtain deeper information about the characteristics of the
118
underlying high velocity and denser structure. Using MT marine studies, we will
119
penetrate below the thick sedimentary layer and describe for the first time the electrical
120
structures related to a possible intrusive magma body.
121 122
The relationships between high temperatures and electrical resistivity and their
123
implication with marine MT studies have not been studied. So we explore if MT marine
124
models are sensitive to detect changes in the shallow resistivity associated with high
125
heat flows. The previous results may allow us to have a better understanding of the
126
electrical resistivity influence on the high heat flows and their relationship with the
127
intrusive body. This approach might initiate a wider use of MT marine methods as an
128
instrument for assessing geothermal potential in this area.
129 130
Magnetotelluric data
131
We collected 10 marine magnetotelluric (MT) stations for 18 days in May 2015
132
over the Wagner basin (Figure 1). The data were acquired using an MT system
133
developed at the Scripps Institution of Oceanography (Constable et al., 1998), obtaining
134
earth response functions in the period band from 0.12 to 6000 s. The time variations of
135
the two horizontal magnetic and electric fields were recorded. The Ocean Bottom
136
Electromagnetic (OBEM) instruments were deployed along a profile consisting of 10
137
stations distributed along 80 km. This profile has an azimuth of 65o, it is approximately
138
perpendicular to both coasts of the Gulf of California (Figure 1), it starts near San
139
Felipe, Baja California, and ends near the town of Puerto Peñasco, Sonora. The
140
separation of the different sites is not uniform, varying from 7 to 15 km. The compasses
141
attached to each OBEM did not work properly. The orientations of the receivers at the
142
seafloor were estimated with cross-correlations of the magnetic field measured at the
143
Pinon Flat Observatory, southern California, with the rotated magnetic fields recorded at
144
the receivers. This observatory is about 350 km north from the MT profiles. The data of
145
one of the electric field components at Site 8 was lost before the OBEM recovery.
146 147
The impedance tensors were calculated using a robust, multi-station transfer-
148
function estimation routine (Egbert, 1997). Figure 2 shows the impedance polar
149
diagrams (Reddy et al., 1977) at the MT sites for some selected periods. These
150
diagrams plot the normalized absolute values of the impedances Z x y and Z x x for a
151
360o rotation angle. Figure 3 helps to discuss the main features of the data and displays
152
the observed apparent resistivities and phases for the transverse electric (TE) and
153
transverse magnetic (TM) modes obtained by rotating the impedances to an angle
154
coincident with the azimuth of this profile (65o).
155 156 157 158 159 160 161 162
163
164 165
Figure 2. Impedance polar diagrams of the MT sites for some selected periods (north
166
upwards). The absolute values of the impedances Zxy (blue lines) and Zxx (red lines) are
167
plotted as a function of the rotation angle.
168
The general behavior of the data can be divided into four-period ranges. From
169
0.13 to 0.5 s the Z x y diagrams are nearly circular (Figure 2), and the responses of the
170
two modes almost coincide (Figure 3), suggesting a 1D resistivity distribution associated
171
with the shallow portion of the basin sediments. The apparent resistivities are low,
172
varying from 0.3 to 0.9 Ωm, except at sites 9 and 10, where they are 3.1 and 2 Ωm.
173
From 0.7 to 11 s the polar diagrams suggest the presence of 3D subsurface structure;
174
however, the coherences between the fields were significantly lower in these periods.
175
We inferred that this band is contaminated by noise, probably the combination of low
176
intensity of the ionospheric signal and the effect of sea waves in this relatively shallow
177
sea. We discarded practically all the data contained within this period band. From 16 to
178
130 s, the strike directions are not homogeneous, but an NW-SE strike can be identified
179
as slightly dominant, and the apparent resistivities of the two modes are either
180
coincident or start to separate. These features suggest that the structure is mainly 2D.
181
Finally, for periods longer than 130 s the relative increase of Z x x , the shapes of Z x y ,
182
and the separation of the TE and TM modes, all suggest that these responses are the
183
product of a 3D structure.
184 185 186 187 188 189 190 191
192
193
Figure 3. Apparent resistivities ( Ω m) and phases (degrees) for the MT soundings. The
194
observed data are shown with blue dots (TE mode) and red circles (TM mode). The
195
bars indicate +/- one standard error. The responses calculated from the inverted model
196
displayed in Figure 4 are depicted as blue (TE mode) and red (TM mode) solid lines.
197
The Root Mean Squared (RMS) misfit error between calculated and measured
198
responses is 1.7
199 200
The data were inverted with the MARE2DEM program, a 2D code (Key, 2016)
201
that incorporates the smooth-structure approach (Constable et al., 1987) using an
202
unstructured finite-element method to calculate the forward problem (Key and Weiss,
203
2006; Key and Ovall, 2011). The bathymetry, measured with an echo sounder during
204
the research cruise (Díaz-Méndez, pers. comm.), was incorporated into the model. The
205
water depth along the profile varies from 31 to 208 m. The shallowest zone lies at the
206
eastern end of the profile, and the deepest region is located near sites 4 and 5, at the
207
center of the basin. Isotropic resistivity was considered in all inversions. An error floor of
208
10% in apparent resistivity and phases was considered.
209 210
Results
211
Figure 4b shows the inverted 2D model obtained after seven iterations using as
212
input data the apparent resistivities and phases for the TE and TM polarization modes.
213
The fit between observed and calculated responses (Figure 3) is good for periods less
214
than 400 s, but not so good for longer periods, with a value of 1.7 for the overall Root
215
Mean Squared (RMS) misfit error. Figure 4c displays the interpretation by Aragón
216
Arreola (2006) of a seismic reflection line almost coincident with our electromagnetic
217
profile.
218 219
Bouguer Anomaly (mGal)
220
221 222 223 224 225 226 227 228 229
Figure 4. a) Bouguer gravity anomaly (mGal) from Pérez (1982). b) 2D model estimated by the inversion of the MT data. The rms misfit error is 1.69. The main resistivity features are indicated: shallow conductor (SC), shallow resistor (SR), deep resistor (DR), deep central conductor (DCC), and deep eastern conductor (DEC). The red outline around the DDC structure denotes the perturbed area in the sensitivity test. c) Structural interpretation from a seismic profile close to our MT line (Aragón Arreola, 2006). Vertical axis is two-way travel time. The approximate depth range covered by this seismic section is indicated by the dashed line of the section in a).
230 231
The main features of the model are indicated in Figure 4b as shallow conductor
232
(SC), shallow resistor (SR), deep resistor (DR), deep central conductor (DCC). The
233
shallow conductor (SC) is associated with the high conductivities of the basin
234
sediments, with resistivities from 0.3 to 3.3 Ωm. This conductor has variable
235
thicknesses, reaching values of 5.5 km. The depths of maximum positive vertical
236
resistivity gradients are not at the base of the basin but at shallower depths of about 3
237
km, possibly associated with a decrease in the porosity of the sediments. Between sites
238
7 and 8 the conductor completely disappears, being replaced by a resistive zone, here
239
named as the shallow resistor (SR). There is no evidence in the seismic section (Figure
240
4c) that under this region the acoustic basement is closer to the surface to produce a
241
resistive zone. We believe the presence of this resistive body is artificial because the
242
inversion program tends to create shallow resistive bodies when the separation
243
between two adjacent sites is large. A similar situation, although less pronounced,
244
occurs at shallow depths between sites 8 and 9.
245 246
Lewis et al. (2001) estimated, using the receiver function approach, crustal
247
thicknesses at this latitude of 18 and 15 km at the western and eastern shores of the
248
Gulf of California, respectively. At depths corresponding to the upper mantle, the
249
electrical resistivity structure in this model is asymmetrical. In the western part of the
250
model, belonging to the Pacific Plate, the upper mantle is resistive, illustrated by the
251
deep resistor (DR) zone of Figure 4b. In contrast, the eastern part of the inverted model,
252
which belongs to the North American Plate, is more conductive.
253 254
The most interesting geophysical feature in the model of Figure 4b is the
255
presence of the deep central conductor (DCC). It has resistivities from 2 to 6 Ωm and is
256
surrounded by a more resistive host. It is located under site 5, in the northeastern limit
257
of the Wagner basin, extending from the base of the basin to greater depths. The
258
geometry of this conductor shows a dip toward the NE; at a depth of 15 km apparently
259
displays a horizontal branch toward sites 8 and 9. However, inferences about the
260
deepest structure should be taken with caution as 3D effects, revealed by the long-
261
period impedance polar diagrams of Figure 2, since they are not considered in the 2D
262
inversion approach. We interpret the low resistivity, location, and depth extension of this
263
conductive body as evidence of the presence of partial melt at the boundary between
264
the Pacific and North American Plates.
265
With the purpose of verifying the presence of the DCC anomaly, we carried out
266
two testing approaches. In the first one, we increased the resistivities of the outlined
267
area framing the DCC anomaly in Figure 4b to those of the surrounding host (12 and 24
268
Ωm), in such a way that the anomalous character of the DCC feature disappears. We
269
then calculated the forward response of this perturbed model and compared it with the
270
calculated response from the inverted model. If there is no difference between both
271
responses means that the DCC is not well constrained by the data. However, if there is
272
a significant difference means that this resistivity feature is well constrained by the
273
measured data. We used the normalized residuals as a measure to quantify the
274
response differences, defined by
275
resistivity at a given period and
276
period of the inverted model or the perturbed model, both logarithmically defined. The
277
residuals in Site 5 for eight periods, from 500 to 6,000 seconds, for the TE and TM
278
models are shown in Figure 5. Site 5 is directly above the DCC anomaly. For each
279
period the left bar corresponds to the residual with the inverted or chosen model, the
280
right bar is for the perturbed model. The elimination of the DCC results in a slight
281
decrease in the perturbed residuals. However, the residuals are significantly higher for
282
the TM mode, indicating that the DCC feature is well constrained by the data.
283 284
, where
is the observed apparent
is the calculated apparent resistivity at the same
285 286
Figure 5. Residuals from the inverted (chosen) and perturbed models at Site 5 for
287
several long-period data. (a). TM mode, (b) TE mode.
288
An additional methodology was applied to support the existence of the DCC. To
289
this end, we use the approximate formula
290
where
291
the depths of penetration
292
,
represents the average of conductivity over a depth window bounded by and
. Apparent conductivity
,
corresponds to the period
, and
to
, with
stands for the reciprocal of apparent resistivity
.
293
Unlike models that fit the data, the averages are unique and have a meaningful variance
294
(Gómez-Treviño, 1996). The wider the depth window is, the smaller the variance and
295
vice versa. As
296
(Jones, 1983), and the variance increases without bound. Different averages with their
297
corresponding variances can be computed at will, just by increasing the steps between
298
periods. The results are typical Backus-Gilbert trade-offs between variance and
299
resolution.
300
determinant of the impedance tensor for station 5 and its four neighbors, stations 3, 4,
301
6, and 7. We assumed the same error floor of 10% as we did for the 2D inversion.
the formula reduces to the familiar Niblett-Bostick transformation
We applied this approach to apparent conductivities derived from the
302 303
The results are shown in Figure 6 for the five contiguous stations. Consider first
304
station 5 and follow the averages and their variances as they develop with increasing
305
depth. It can be observed that there is definitively a decrease in resistivity below 9 km
306
since a horizontal line cannot be drawn within the region defined by the error bars. This
307
complies with the view at depth below station 5 as shown in Figure 4.
308
309 310
Figure 6. Depth averages of electrical resistivity calculated with the method proposed by
311
Gómez-Treviño (1996). The results have shown that station 5 is sensible to lower
312
resistivities, which are related to the presence of a conductive body beneath the depth
313
of 9 km.
314
315
Figure 7 shows the location of the heat flow measurements in the area (Henyey
316
and Bischoff, 1973; Prol-Ledesma et al., 2013; Neumann et al., 2017), together with the
317
mapped faults interpreted from seismic reflection profiles (Martín-Barajas et al., 2013)
318
and the MT sites. Notable features of the heat flow measurements are their high values
319
and their strong spatial variability, indicators of the presence of an intense system of
320
hydrothermal circulation, occurring mainly along faults and fractures (Prol-Ledesma et
321
al., 2013; Neumann et al., 2017). Most of the higher heat flow values occur in the
322
vicinity of the Wagner fault and smaller faults that mark the eastern boundary of the
323
basin. According to the seismic reflection information, most of the current differential
324
movement is occurring along this fault (Aragón-Arreola and Martín-Barajas, 2007).
325
Persaud et al. (2003), in an independent seismic reflection survey of the area, also have
326
found that at the southeastern side of the basin the faults are more active and the fault
327
density is higher. It is interesting to note that the three MT soundings (sites 5, 6, and 7)
328
that give the lowest resistivities at shallow levels are located close to this zone of high
329
heat flow values, strongly suggesting the presence of a correlation between low
330
resistivities and high heat flows. Figure 8 presents this correlation. In the x-axis are the
331
most superficial resistivities of six MT sites (Table 1), which are practically the same as
332
the apparent resistivities measured at the shortest period of 0.13 s (Figure 3); the
333
remaining four MT sites are not considered in this analysis because they do not have
334
any nearby heat flow measurements. In the y-axis are the average values estimated
335
from 27 heat flows measured by Neumann et al. (2017), considering in this estimation
336
only heat flow points located less than 4 km from the MT sites. The standard deviations
337
are displayed as error bars. The correlation between low resistivities and high heat
338
flows is clearly high. A controlled-source electromagnetic study (Reyes Ortega, 2016)
339
also produced lower resistivities in the same region of high heat flows.
340 341
Figure 7. Color dots indicate the locations and intensities of heat flow measurements
342
reported by Henyey and Bischoff (1973), Prol-Ledesma et al. (2013), and Neumann et
343
al. (2017). The colored bar at right is the logarithmic heat flow scale (in mW/m2). Black
344
dots denote the MT sites (the inset indicates the MT site numbers). The fault pattern is
345
from Martín-Barajas et al. (2013).
346 347 348
349 350 351
Figure 8. Mean heat flow values against the estimated shallow resistivities under six MT sites. The standard deviations are represented by bars.
352 353
The electrical conductivity in geothermal environments may be produced not only by
354
high temperatures, but also by a number of other factors. A commonly used relation for
355
the bulk conductivity
356
357
of a porous rock is given by Ward (1990),
, where a and m are factors depending on the rock texture,
is the fractional porosity,
358
is the liquid saturation,
is the fluid conductivity, and
represents an increase in
359
the bulk conductivity produced by the presence of clays. The first term is the familiar
360
Archie’s law. The second term is produced by the surface conductivity due to ions in the
361
double layer at the surface of clay particles, which also depends on several factors as
362
the clay content and the cation exchange capacity of the particular clay mineral.
363
In this work, we are interested in the relationship between the conductivities of
364
the rock and the fluid contained in their pores. The fluid conductivity essentially depends
365
on the amount of dissolved salts and temperature. To that aim, in the above expression
366
we adopt three approximations for the conductivity of the shallow sediments deposited
367
in the Wagner basin: a) these sediments are fully saturated, that is,
368
assume a fractional porosity of 0.5, and a and m parameters of 1 and 2, respectively,
369
which are reasonable assumptions (Nafe and Drake, 1957), and c) the contribution to
370
the bulk conductivity from the surface conductivity is negligible. The effect of clays in
371
unsaturated or saturated rocks with freshwater may be significant. However, when the
372
saturation fluid has high salinities, such as seawater, its effect is small (Boadu and
373
Seabrook, 2006). Under these considerations the relation between the rock resistivity
374
and the fluid resistivity is
, b) we
.
375
The resulting fluid resistivities of the six MT sites of Figure 8 are listed in Table 1.
376
We then used the Keller and Frischknecht (1966) graphs describing the resistivity
377
variation of a sodium chloride solution with concentration and temperature (Figure 9)
378
and plotted the six pore-water resistivities in two extreme situations. In the first one, we
379
assume the resistivities vary only with the temperature and not by the effect of the
380
salinity, so we plot them along the vertical line corresponding to the typical seawater
381
salinity of 35 g/kg or 35,000 ppm. In an independent way, we calculated the
382
temperature of seawater from the fluid resistivity using the relationship proposed by
383
Constable et al. (2009). At sites 3 and 6 the calculated temperatures are 18o C and 58o
384
C, respectively, agreeing well with the curves drawn in Figure 9. These temperatures
385
are listed in Table 1. In the second situation, we assume the resistivity variation is not
386
due to temperature but only to changes in the pore-water salt concentration. The points
387
are now plotted (Figure 9) along the curve of 15 oC, which is approximately the
388
temperature measured at the bottom of the water column (Neuman et al., 2017). The
389
resistivity of the pore-water under site 6 could be produced by a concentration of 200
390
g/kg, a salinity almost six higher than that of marine water (Table 1).
391 392
MT site
Rock resistivity (Ωm)
Fluid resistivity (Ωm)
Temperature (°C) at a 35 g/kg salinity
2 3 4 5 6 7
0.465 0.878 0.672 0.593 0.311 0.472
0.116 0.22 0.168 0.148 0.078 0.118
61 18 35 40 92 60
Salinity (g/kg) at a 15oC temperature 95 39 56 67 200 93
393 394
Table 1. Shallow rock resistivities interpreted with the MT data, pore-water resistivities
395
estimated with Archie’s law, temperatures at a salinity of 35 g/kg, and salinities at a
396
temperature of 15 oC under six MT sites.
397 398
We cannot know how much the temperature and the salinity contribute to the
399
anomalous pore-water resistivities; any combination of these variables enclosed within
400
the triangular area bounded by the plotted points of Figure 9 is possible. However, we
401
could attempt to constrain the salinities. With this goal, we used the brine
402
concentrations reported by Harthill (1978) of five geothermal fields located to the north
403
of the Wagner basin: four in the Imperial Valley, USA (Salton Sea, Burec, Wilson, and
404
Heber) and one in the Mexicali Valley, Mexico (Cerro Prieto). All these fields are in the
405
region of continental aperture at the boundary between the North America and Pacific
406
plates. In Figure 9, the sum of these concentrations plus that of seawater is plotted. The
407
lower (48 g/kg) and upper (164 g/kg) bounds of these concentrations could serve to limit
408
the
Ω 409 410 411 412 413 414 415 416
Figure 9. Resistivity of a sodium chloride solution as a function of temperature and salt concentration (after Keller and Frischknecht, 1966). The fluid resistivity estimates from the six Wagner basin MT sites are plotted assuming a constant salinity and a constant temperature. The salinity ranges from five geothermal fields in the Mexicali and Imperial Valleys (Harthill, 1978) are indicated.
417
possible combination of temperature and salinity that could produce such resistivities.
418
Another source of information on the salinities is the chemical analysis of pore-water
419
samples taken from the first three meters of sediments (R. Batista, pers. comm.). In a
420
location 3 km northward of our MT site 6, the mean salinity of 11 samples is 37.1 g/kg
421
with a standard deviation of 3.8 g/kg. At this MT site, a low resistivity and high heat flow
422
were measured (Figure 8), suggesting that the anomalously low resistivities might be
423
due mainly to high temperatures, and not to high salinities.
424 425
Discussion
426
We interpret the deep central conductor as a zone of partial melt and the ultimate
427
source of heat for the surface hydrothermal upwelling. It is the first time such structure
428
has been sensed in the Gulf of California using an electromagnetic method. The
429
conductor shows an apparent dip toward the East, probably related to the Wagner fault
430
(González-Escobar et al., 2009), which marks the eastern boundary of the depocenter
431
and is tectonically more active than the western limit of the basin (Aragón-Arreola and
432
Martín-Barajas, 2007). Information supporting the presence of magmatic intrusions at
433
the base of the sedimentary deposits, which is contributing to the accretion of oceanic
434
crust, comes from two sources. From the Bouguer anomaly (shown in Figure 4a)
435
measured along a line almost coincident with our MT profile by Petróleos Mexicanos
436
(Pemex), Pérez (1982) has interpreted a high local gravity as produced by mafic
437
intrusions at the base of the basin. Furthermore, in a seismic reflection line about 15 km
438
to the south of our profile, over the Consag basin González et al. (2014) detected a
439
reflection interpreted as a volcanic intrusion at a depth of 6 or 7 km.
440 441
Considering the resistivity of this conductor (4 Ωm) as the effective value of a
442
two-component medium we can have an estimate of the amount of melt in this mixture.
443
These two components are magma, with a 0.25 Ωm resistivity (Constable, 2007), and
444
the 30 Ωm normal resistivity surrounding the conductor in the model of Figures 4b.
445
Using the bounds proposed by Hashin-Shtrikman (1962) for a two-component system,
446
the bounds for the melt fraction vary from 8% to 70%, which is a range too wide to be
447
useful. If we now consider Archie’s law with a cementation factor of 2, we obtain an
448
estimate of 25 % melt in this conductor.
449 450
There have been several MT studies over mid-ocean spreading ridges where the
451
conductor associated with the zone of partial melt is vertical (Heinson et al., 2000; Key
452
et al., 2013), asymmetric (Baba et al., 2006a) or even absent (Baba et al., 2006b). Its
453
presence depends on such factors as pressure, temperature, water content, and
454
connectivity between the melt pockets.
455 456
A number of studies describing the shear velocity structure of the Gulf area
457
based on Rayleigh wave dispersion have been carried out (e.g., Zhang et al., 2007;
458
Wang et al., 2009; Di Luccio et al., 2014). They show complex variations of the velocity
459
in both horizontal and vertical directions. In none of them, there is an agreement
460
between our deep central conductor and their low-velocity zones in the depth range
461
from 10 to 40 km.
462
horizontal resolution of 10 by 10 km of some of these seismic studies (Di Luccio et al.,
463
2014), which might not be good enough to resolve the 10 to 15 km width of the
464
conductor.
One possible reason for this discrepancy is the approximate
465 466
Conclusions
467
The inverted 2-D model using the data from both polarizations gave responses
468
with adequate fits with the observed data. The main features of this model are the low
469
resistivities associated with the sedimentary basin and a deep conductor. We obtained
470
a positive correlation between the resistivities estimated in the first 100 m and the
471
measured heat flow values. Most of the higher heat flows occur in the vicinity of the
472
Wagner fault and smaller associated faults, where present-day tectonic activity is
473
inferred from seismic reflection data. The two MT sites with the lowest shallow
474
resistivities are located in this zone. Although low resistivities might also be produced by
475
high salinities of the hydrothermal plumes, direct measurements of the salt content of a
476
few shallow pore-water samples might suggest that the anomalous resistivities are
477
mainly influenced by the high temperatures. The DCC is interpreted as a zone of partial
478
melt feeding the formation of new crust. It has a resistivity close to 4 Ωm, extends from
479
the base of the shallow conductor to 15 km depths, and has an apparent dip toward the
480
East. This dipping is probably related to the higher tectonic activity of the eastern side of
481
the basin. Assuming that its effective resistivity follows Archie’s law, we estimate a 25 %
482
melt in this conductor.
483 484
Acknowledgments
485
This project was funded by CeMIEGeo and CICESE. We acknowledge the captain and
486
crew of Alpha Helix, the CICESE research vessel, for their help during the data
487
acquisition stage. We thank David Myer, Ramón Batista, Guillermo Díaz, Mario
488
González, Arturo Martín, and Martín Pacheco for their technical advice in different
489
aspects of this work. T.A.E. and V.R.O. were supported by scholarships from CONACyT
490
and by UC-Mexus for a temporal stay at Scripps.
491 492
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Highlights • • •
The magnetotelluric marine method was used to explore the North Gulf of California with geothermal purposes. The 2D model of the magnetotelluric marine data reveals a deep central conductor in the center of the Wagner basin. We obtained a positive correlation between the low resistivities estimated in the first 100 m using 2D inversion of the magnetotelluric marine data with high heat flow measurements reported previously.
January 10th, 2020 Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0895-9811(20)30014-6
DOI:
https://doi.org/10.1016/j.jsames.2020.102501
Reference:
SAMES 102501
To appear in:
Journal of South American Earth Sciences
Received Date: 8 July 2019 Revised Date:
10 January 2020
Accepted Date: 12 January 2020
Please cite this article as: Esquivel, T.A., Flores, C., Ortega, V.R., Constable, S., Gómez-Treviño, E., González-Fernández, A., Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation., Journal of South American Earth Sciences, https://doi.org/10.1016/j.jsames.2020.102501. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
December 14th, 2019 To whom correspond, I attach the author’s statements, where I describe the contribution of each one of the authors to this publication.
Authors Thalia Anaid Avilés Esquivel (Corresponding author) Carlos Flores Luna
Formal Analysis, visualization, validation, writing the original draft Conceptualization, Investigation, writing the original draft
Valeria Reyes Ortega
Formal analysis, validation, visualization
Steven Constable
Methodology, software, resources, supervision
Enrique Gómez Treviño
Methodology, software
Antonio González Fernández
Proyect administration, found resources, funding acquisition.
Best regards,
Thalia Anaid Avilés Esquivel
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation. Thalia Avilés-Esquivel a,b,*, Carlos Flores a,b, Valeria Reyes-Ortega a,b,d , Steven Constable c, Enrique Gómez-Treviño a,b and Antonio González-Fernández a,b a
División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, Baja California 22860, México. b Centro Mexicano de Innovación en Energía Geotérmica (CeMIEGeo), Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, Baja California 22860, México. c Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, 92093, USA. d Now at c * Corresponding author: [email protected] Keywords: Marine Magnetotellurics, electrical resistivity – heat flow correlation Abstract
24
Different geological and geophysical disciplines have suggested the Wagner Basin as a
25
promising location of geothermal resources. As a reconnaissance tool Magnetotelluric
26
(MT) data were measured at 10 sites along a profile over this basin in the northern Gulf
27
of California. The subsurface distribution of electric resistivity was estimated with two-
28
dimensional smooth inversion using the apparent resistivities and phases from both field
29
polarizations. We found a deep conductor underneath the center of the basin,
30
interpreted as a region of partial melt feeding the accretion zone of this incipient oceanic
31
crust. From six MT sites with close-by heat flow measurements, we also found a
32
positive correlation between the interpreted shallow (first 100 m) low resistivities and
33
high heat flows. This shows that the MT method can be used to map submarine heat
34
sources.
35 36
Introduction
37
The Wagner Basin is the northernmost rifting basin in the Gulf of California.
38
Several facts suggest this basin as a potential source of geothermal resources. First, its
39
tectonic similarity with the Mexicali-Imperial valley to the north, which has important
40
geothermal fields such as Cerro Prieto, Salton Sea, Brawley, East Mesa, and Heber.
41
The combined installed capacity of these fields in 2015 was 1.5 GWe (Boyd et al., 2015;
42
Gutiérrez-Negrín et al., 2015). Second, the high temperatures and heat flow
43
measurements reported in this area since the earliest 70’s (Henyey and Bischoff, 1973;
44
Grijalva, 1986; Prol-Ledesma et al., 2013; Neumann et al., 2017). The measured heat
45
flows in the Wagner and neighboring Consag basins are up to 15 times the average
46
value for oceanic crust. Third, the intense submarine gas discharges reported by Canet
47
et al. (2010) and the geochemical signature of high-temperature pore water measured
48
in sediment cores by Batista Cruz et al. (2019). The mapped seismic events (González-
49
Escobar et al., 2010) and the active distributed deformation (Persaud et al., 2003)
50
suggest the presence of active tectonism. Finally, the incipient formation of new crust is
51
suggested by the possible presence of intrusive bodies in a seismic reflection profile
52
over the Consag Basin (González-Escobar et al., 2014) and in the gravity anomaly in a
53
profile over the Wagner Basin (Pérez, 1982).
54 55
Regarding the geologic setting, the Gulf of California – San Andreas fault system
56
defines the boundary between the Pacific and North American plates (Atwater, 1970;
57
Lonsdale, 1989). The Wagner Basin is the northernmost rifting basin in the Gulf of
58
California. The Consag, Upper and Lower Delfín, and Guaymas are among other basins
59
to the south. Important differences exist between the northern and southern Gulf basins.
60
Well developed spreading centers occur at the mouth of the Gulf, with clearly
61
observable magnetic anomaly stripes, evidence of new oceanic crust being created
62
since about 4 Ma at a full spreading rate of 6 cm/year (Larson et al., 1968). In contrast,
63
in the northern part no magnetic lineaments are observed (Klitgord et al., 1974;
64
Sanchez et al., 1991) and thick sedimentary deposits, supplied mainly by the Colorado
65
River, fill these basins (Dorsey, 2010). The lack of magnetic anomalies in the northern
66
basins is the main evidence for the absence of new oceanic crust. However, the region
67
of the northern basins is considered a continent-ocean transition zone, where diffuse
68
continental deformation exists (Nagy and Stock, 2000; Persaud et al., 2013).
69
70
In this work, we present the results of a Magnetotelluric (MT) reconnaissance
71
survey carried out in the southern portion of the Wagner Basin (Figure 1) with the
72
purpose of assessing its geothermal potential. This project is part of a national effort to
73
increase the use of renewable energy sources (geothermal, wind, solar, etc.) to diminish
74
the hydrocarbon dependence (Avilés Esquivel, 2016). Technologic and environmental
75
obstacles should be overcome for future sustainable exploitation of the submarine
76
geothermal resource in this environmental protected zone.
77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93
94
95 96
Figure 1: Regional map of the northern Gulf of Baja California. Lines denote the normal
97
faults inferred from seismic reflection data from PEMEX (Petróleos Mexicanos) (Martín-
98
Barajas et al., 2013). CF=Consag Fault, WF=Wagner Fault, CPF=Cerro Prieto Fault,
99
CPSF=Cerro Prieto South Fault, ADF=Adair Fault, AMF=Amado Fault, WB=Wagner
100
Basin, CB=Consag Basin. Black dots indicate the location of the MT sites. The
101
bathymetry is from G. Díaz Méndez (pers. comm.). The boundaries between the Pacific
102
(PP) and North American (NAP) plates, as well as their direction of displacement are
103
shown in the inset.
104 105
106
The use of electromagnetic (EM) methods to study the marine subsurface has
107
shown an important increase in the last years (Key, 2012). These works have either
108
academic or resource exploration goals, mainly for the characterization of offshore
109
hydrocarbon reservoirs, but also for studying the oceanic lithosphere (e.g., Evans et al.,
110
1991; Constable et al., 1997; Heinson et al., 2000; Baba et al., 2006a; Key et al., 2013).
111
The MT method uses low-frequency EM waves originated by sun-ionosphere
112
interactions and far-away lightning sources. These waves impinge upon the sea
113
surface, diffusing into the seawater and seabed. Measured electric and magnetic fields
114
on the seafloor are then used to estimate the frequency-dependent impedances, which
115
are inversely modeled to estimate the subsurface electrical resistivity. The presence of
116
a thick sedimentary layer in the Gulf of California dissipates the signal from seismic and
117
gravity sources, preventing to obtain deeper information about the characteristics of the
118
underlying high velocity and denser structure. Using MT marine studies, we will
119
penetrate below the thick sedimentary layer and describe for the first time the electrical
120
structures related to a possible intrusive magma body.
121 122
The relationships between high temperatures and electrical resistivity and their
123
implication with marine MT studies have not been studied. So we explore if MT marine
124
models are sensitive to detect changes in the shallow resistivity associated with high
125
heat flows. The previous results may allow us to have a better understanding of the
126
electrical resistivity influence on the high heat flows and their relationship with the
127
intrusive body. This approach might initiate a wider use of MT marine methods as an
128
instrument for assessing geothermal potential in this area.
129 130
Magnetotelluric data
131
We collected 10 marine magnetotelluric (MT) stations for 18 days in May 2015
132
over the Wagner basin (Figure 1). The data were acquired using an MT system
133
developed at the Scripps Institution of Oceanography (Constable et al., 1998), obtaining
134
earth response functions in the period band from 0.12 to 6000 s. The time variations of
135
the two horizontal magnetic and electric fields were recorded. The Ocean Bottom
136
Electromagnetic (OBEM) instruments were deployed along a profile consisting of 10
137
stations distributed along 80 km. This profile has an azimuth of 65o, it is approximately
138
perpendicular to both coasts of the Gulf of California (Figure 1), it starts near San
139
Felipe, Baja California, and ends near the town of Puerto Peñasco, Sonora. The
140
separation of the different sites is not uniform, varying from 7 to 15 km. The compasses
141
attached to each OBEM did not work properly. The orientations of the receivers at the
142
seafloor were estimated with cross-correlations of the magnetic field measured at the
143
Pinon Flat Observatory, southern California, with the rotated magnetic fields recorded at
144
the receivers. This observatory is about 350 km north from the MT profiles. The data of
145
one of the electric field components at Site 8 was lost before the OBEM recovery.
146 147
The impedance tensors were calculated using a robust, multi-station transfer-
148
function estimation routine (Egbert, 1997). Figure 2 shows the impedance polar
149
diagrams (Reddy et al., 1977) at the MT sites for some selected periods. These
150
diagrams plot the normalized absolute values of the impedances Z x y and Z x x for a
151
360o rotation angle. Figure 3 helps to discuss the main features of the data and displays
152
the observed apparent resistivities and phases for the transverse electric (TE) and
153
transverse magnetic (TM) modes obtained by rotating the impedances to an angle
154
coincident with the azimuth of this profile (65o).
155 156 157 158 159 160 161 162
163
164 165
Figure 2. Impedance polar diagrams of the MT sites for some selected periods (north
166
upwards). The absolute values of the impedances Zxy (blue lines) and Zxx (red lines) are
167
plotted as a function of the rotation angle.
168
The general behavior of the data can be divided into four-period ranges. From
169
0.13 to 0.5 s the Z x y diagrams are nearly circular (Figure 2), and the responses of the
170
two modes almost coincide (Figure 3), suggesting a 1D resistivity distribution associated
171
with the shallow portion of the basin sediments. The apparent resistivities are low,
172
varying from 0.3 to 0.9 Ωm, except at sites 9 and 10, where they are 3.1 and 2 Ωm.
173
From 0.7 to 11 s the polar diagrams suggest the presence of 3D subsurface structure;
174
however, the coherences between the fields were significantly lower in these periods.
175
We inferred that this band is contaminated by noise, probably the combination of low
176
intensity of the ionospheric signal and the effect of sea waves in this relatively shallow
177
sea. We discarded practically all the data contained within this period band. From 16 to
178
130 s, the strike directions are not homogeneous, but an NW-SE strike can be identified
179
as slightly dominant, and the apparent resistivities of the two modes are either
180
coincident or start to separate. These features suggest that the structure is mainly 2D.
181
Finally, for periods longer than 130 s the relative increase of Z x x , the shapes of Z x y ,
182
and the separation of the TE and TM modes, all suggest that these responses are the
183
product of a 3D structure.
184 185 186 187 188 189 190 191
192
193
Figure 3. Apparent resistivities ( Ω m) and phases (degrees) for the MT soundings. The
194
observed data are shown with blue dots (TE mode) and red circles (TM mode). The
195
bars indicate +/- one standard error. The responses calculated from the inverted model
196
displayed in Figure 4 are depicted as blue (TE mode) and red (TM mode) solid lines.
197
The Root Mean Squared (RMS) misfit error between calculated and measured
198
responses is 1.7
199 200
The data were inverted with the MARE2DEM program, a 2D code (Key, 2016)
201
that incorporates the smooth-structure approach (Constable et al., 1987) using an
202
unstructured finite-element method to calculate the forward problem (Key and Weiss,
203
2006; Key and Ovall, 2011). The bathymetry, measured with an echo sounder during
204
the research cruise (Díaz-Méndez, pers. comm.), was incorporated into the model. The
205
water depth along the profile varies from 31 to 208 m. The shallowest zone lies at the
206
eastern end of the profile, and the deepest region is located near sites 4 and 5, at the
207
center of the basin. Isotropic resistivity was considered in all inversions. An error floor of
208
10% in apparent resistivity and phases was considered.
209 210
Results
211
Figure 4b shows the inverted 2D model obtained after seven iterations using as
212
input data the apparent resistivities and phases for the TE and TM polarization modes.
213
The fit between observed and calculated responses (Figure 3) is good for periods less
214
than 400 s, but not so good for longer periods, with a value of 1.7 for the overall Root
215
Mean Squared (RMS) misfit error. Figure 4c displays the interpretation by Aragón
216
Arreola (2006) of a seismic reflection line almost coincident with our electromagnetic
217
profile.
218 219
Bouguer Anomaly (mGal)
220
221 222 223 224 225 226 227 228 229
Figure 4. a) Bouguer gravity anomaly (mGal) from Pérez (1982). b) 2D model estimated by the inversion of the MT data. The rms misfit error is 1.69. The main resistivity features are indicated: shallow conductor (SC), shallow resistor (SR), deep resistor (DR), deep central conductor (DCC), and deep eastern conductor (DEC). The red outline around the DDC structure denotes the perturbed area in the sensitivity test. c) Structural interpretation from a seismic profile close to our MT line (Aragón Arreola, 2006). Vertical axis is two-way travel time. The approximate depth range covered by this seismic section is indicated by the dashed line of the section in a).
230 231
The main features of the model are indicated in Figure 4b as shallow conductor
232
(SC), shallow resistor (SR), deep resistor (DR), deep central conductor (DCC). The
233
shallow conductor (SC) is associated with the high conductivities of the basin
234
sediments, with resistivities from 0.3 to 3.3 Ωm. This conductor has variable
235
thicknesses, reaching values of 5.5 km. The depths of maximum positive vertical
236
resistivity gradients are not at the base of the basin but at shallower depths of about 3
237
km, possibly associated with a decrease in the porosity of the sediments. Between sites
238
7 and 8 the conductor completely disappears, being replaced by a resistive zone, here
239
named as the shallow resistor (SR). There is no evidence in the seismic section (Figure
240
4c) that under this region the acoustic basement is closer to the surface to produce a
241
resistive zone. We believe the presence of this resistive body is artificial because the
242
inversion program tends to create shallow resistive bodies when the separation
243
between two adjacent sites is large. A similar situation, although less pronounced,
244
occurs at shallow depths between sites 8 and 9.
245 246
Lewis et al. (2001) estimated, using the receiver function approach, crustal
247
thicknesses at this latitude of 18 and 15 km at the western and eastern shores of the
248
Gulf of California, respectively. At depths corresponding to the upper mantle, the
249
electrical resistivity structure in this model is asymmetrical. In the western part of the
250
model, belonging to the Pacific Plate, the upper mantle is resistive, illustrated by the
251
deep resistor (DR) zone of Figure 4b. In contrast, the eastern part of the inverted model,
252
which belongs to the North American Plate, is more conductive.
253 254
The most interesting geophysical feature in the model of Figure 4b is the
255
presence of the deep central conductor (DCC). It has resistivities from 2 to 6 Ωm and is
256
surrounded by a more resistive host. It is located under site 5, in the northeastern limit
257
of the Wagner basin, extending from the base of the basin to greater depths. The
258
geometry of this conductor shows a dip toward the NE; at a depth of 15 km apparently
259
displays a horizontal branch toward sites 8 and 9. However, inferences about the
260
deepest structure should be taken with caution as 3D effects, revealed by the long-
261
period impedance polar diagrams of Figure 2, since they are not considered in the 2D
262
inversion approach. We interpret the low resistivity, location, and depth extension of this
263
conductive body as evidence of the presence of partial melt at the boundary between
264
the Pacific and North American Plates.
265
With the purpose of verifying the presence of the DCC anomaly, we carried out
266
two testing approaches. In the first one, we increased the resistivities of the outlined
267
area framing the DCC anomaly in Figure 4b to those of the surrounding host (12 and 24
268
Ωm), in such a way that the anomalous character of the DCC feature disappears. We
269
then calculated the forward response of this perturbed model and compared it with the
270
calculated response from the inverted model. If there is no difference between both
271
responses means that the DCC is not well constrained by the data. However, if there is
272
a significant difference means that this resistivity feature is well constrained by the
273
measured data. We used the normalized residuals as a measure to quantify the
274
response differences, defined by
275
resistivity at a given period and
276
period of the inverted model or the perturbed model, both logarithmically defined. The
277
residuals in Site 5 for eight periods, from 500 to 6,000 seconds, for the TE and TM
278
models are shown in Figure 5. Site 5 is directly above the DCC anomaly. For each
279
period the left bar corresponds to the residual with the inverted or chosen model, the
280
right bar is for the perturbed model. The elimination of the DCC results in a slight
281
decrease in the perturbed residuals. However, the residuals are significantly higher for
282
the TM mode, indicating that the DCC feature is well constrained by the data.
283 284
, where
is the observed apparent
is the calculated apparent resistivity at the same
285 286
Figure 5. Residuals from the inverted (chosen) and perturbed models at Site 5 for
287
several long-period data. (a). TM mode, (b) TE mode.
288
An additional methodology was applied to support the existence of the DCC. To
289
this end, we use the approximate formula
290
where
291
the depths of penetration
292
,
represents the average of conductivity over a depth window bounded by and
. Apparent conductivity
,
corresponds to the period
, and
to
, with
stands for the reciprocal of apparent resistivity
.
293
Unlike models that fit the data, the averages are unique and have a meaningful variance
294
(Gómez-Treviño, 1996). The wider the depth window is, the smaller the variance and
295
vice versa. As
296
(Jones, 1983), and the variance increases without bound. Different averages with their
297
corresponding variances can be computed at will, just by increasing the steps between
298
periods. The results are typical Backus-Gilbert trade-offs between variance and
299
resolution.
300
determinant of the impedance tensor for station 5 and its four neighbors, stations 3, 4,
301
6, and 7. We assumed the same error floor of 10% as we did for the 2D inversion.
the formula reduces to the familiar Niblett-Bostick transformation
We applied this approach to apparent conductivities derived from the
302 303
The results are shown in Figure 6 for the five contiguous stations. Consider first
304
station 5 and follow the averages and their variances as they develop with increasing
305
depth. It can be observed that there is definitively a decrease in resistivity below 9 km
306
since a horizontal line cannot be drawn within the region defined by the error bars. This
307
complies with the view at depth below station 5 as shown in Figure 4.
308
309 310
Figure 6. Depth averages of electrical resistivity calculated with the method proposed by
311
Gómez-Treviño (1996). The results have shown that station 5 is sensible to lower
312
resistivities, which are related to the presence of a conductive body beneath the depth
313
of 9 km.
314
315
Figure 7 shows the location of the heat flow measurements in the area (Henyey
316
and Bischoff, 1973; Prol-Ledesma et al., 2013; Neumann et al., 2017), together with the
317
mapped faults interpreted from seismic reflection profiles (Martín-Barajas et al., 2013)
318
and the MT sites. Notable features of the heat flow measurements are their high values
319
and their strong spatial variability, indicators of the presence of an intense system of
320
hydrothermal circulation, occurring mainly along faults and fractures (Prol-Ledesma et
321
al., 2013; Neumann et al., 2017). Most of the higher heat flow values occur in the
322
vicinity of the Wagner fault and smaller faults that mark the eastern boundary of the
323
basin. According to the seismic reflection information, most of the current differential
324
movement is occurring along this fault (Aragón-Arreola and Martín-Barajas, 2007).
325
Persaud et al. (2003), in an independent seismic reflection survey of the area, also have
326
found that at the southeastern side of the basin the faults are more active and the fault
327
density is higher. It is interesting to note that the three MT soundings (sites 5, 6, and 7)
328
that give the lowest resistivities at shallow levels are located close to this zone of high
329
heat flow values, strongly suggesting the presence of a correlation between low
330
resistivities and high heat flows. Figure 8 presents this correlation. In the x-axis are the
331
most superficial resistivities of six MT sites (Table 1), which are practically the same as
332
the apparent resistivities measured at the shortest period of 0.13 s (Figure 3); the
333
remaining four MT sites are not considered in this analysis because they do not have
334
any nearby heat flow measurements. In the y-axis are the average values estimated
335
from 27 heat flows measured by Neumann et al. (2017), considering in this estimation
336
only heat flow points located less than 4 km from the MT sites. The standard deviations
337
are displayed as error bars. The correlation between low resistivities and high heat
338
flows is clearly high. A controlled-source electromagnetic study (Reyes Ortega, 2016)
339
also produced lower resistivities in the same region of high heat flows.
340 341
Figure 7. Color dots indicate the locations and intensities of heat flow measurements
342
reported by Henyey and Bischoff (1973), Prol-Ledesma et al. (2013), and Neumann et
343
al. (2017). The colored bar at right is the logarithmic heat flow scale (in mW/m2). Black
344
dots denote the MT sites (the inset indicates the MT site numbers). The fault pattern is
345
from Martín-Barajas et al. (2013).
346 347 348
349 350 351
Figure 8. Mean heat flow values against the estimated shallow resistivities under six MT sites. The standard deviations are represented by bars.
352 353
The electrical conductivity in geothermal environments may be produced not only by
354
high temperatures, but also by a number of other factors. A commonly used relation for
355
the bulk conductivity
356
357
of a porous rock is given by Ward (1990),
, where a and m are factors depending on the rock texture,
is the fractional porosity,
358
is the liquid saturation,
is the fluid conductivity, and
represents an increase in
359
the bulk conductivity produced by the presence of clays. The first term is the familiar
360
Archie’s law. The second term is produced by the surface conductivity due to ions in the
361
double layer at the surface of clay particles, which also depends on several factors as
362
the clay content and the cation exchange capacity of the particular clay mineral.
363
In this work, we are interested in the relationship between the conductivities of
364
the rock and the fluid contained in their pores. The fluid conductivity essentially depends
365
on the amount of dissolved salts and temperature. To that aim, in the above expression
366
we adopt three approximations for the conductivity of the shallow sediments deposited
367
in the Wagner basin: a) these sediments are fully saturated, that is,
368
assume a fractional porosity of 0.5, and a and m parameters of 1 and 2, respectively,
369
which are reasonable assumptions (Nafe and Drake, 1957), and c) the contribution to
370
the bulk conductivity from the surface conductivity is negligible. The effect of clays in
371
unsaturated or saturated rocks with freshwater may be significant. However, when the
372
saturation fluid has high salinities, such as seawater, its effect is small (Boadu and
373
Seabrook, 2006). Under these considerations the relation between the rock resistivity
374
and the fluid resistivity is
, b) we
.
375
The resulting fluid resistivities of the six MT sites of Figure 8 are listed in Table 1.
376
We then used the Keller and Frischknecht (1966) graphs describing the resistivity
377
variation of a sodium chloride solution with concentration and temperature (Figure 9)
378
and plotted the six pore-water resistivities in two extreme situations. In the first one, we
379
assume the resistivities vary only with the temperature and not by the effect of the
380
salinity, so we plot them along the vertical line corresponding to the typical seawater
381
salinity of 35 g/kg or 35,000 ppm. In an independent way, we calculated the
382
temperature of seawater from the fluid resistivity using the relationship proposed by
383
Constable et al. (2009). At sites 3 and 6 the calculated temperatures are 18o C and 58o
384
C, respectively, agreeing well with the curves drawn in Figure 9. These temperatures
385
are listed in Table 1. In the second situation, we assume the resistivity variation is not
386
due to temperature but only to changes in the pore-water salt concentration. The points
387
are now plotted (Figure 9) along the curve of 15 oC, which is approximately the
388
temperature measured at the bottom of the water column (Neuman et al., 2017). The
389
resistivity of the pore-water under site 6 could be produced by a concentration of 200
390
g/kg, a salinity almost six higher than that of marine water (Table 1).
391 392
MT site
Rock resistivity (Ωm)
Fluid resistivity (Ωm)
Temperature (°C) at a 35 g/kg salinity
2 3 4 5 6 7
0.465 0.878 0.672 0.593 0.311 0.472
0.116 0.22 0.168 0.148 0.078 0.118
61 18 35 40 92 60
Salinity (g/kg) at a 15oC temperature 95 39 56 67 200 93
393 394
Table 1. Shallow rock resistivities interpreted with the MT data, pore-water resistivities
395
estimated with Archie’s law, temperatures at a salinity of 35 g/kg, and salinities at a
396
temperature of 15 oC under six MT sites.
397 398
We cannot know how much the temperature and the salinity contribute to the
399
anomalous pore-water resistivities; any combination of these variables enclosed within
400
the triangular area bounded by the plotted points of Figure 9 is possible. However, we
401
could attempt to constrain the salinities. With this goal, we used the brine
402
concentrations reported by Harthill (1978) of five geothermal fields located to the north
403
of the Wagner basin: four in the Imperial Valley, USA (Salton Sea, Burec, Wilson, and
404
Heber) and one in the Mexicali Valley, Mexico (Cerro Prieto). All these fields are in the
405
region of continental aperture at the boundary between the North America and Pacific
406
plates. In Figure 9, the sum of these concentrations plus that of seawater is plotted. The
407
lower (48 g/kg) and upper (164 g/kg) bounds of these concentrations could serve to limit
408
the
Ω 409 410 411 412 413 414 415 416
Figure 9. Resistivity of a sodium chloride solution as a function of temperature and salt concentration (after Keller and Frischknecht, 1966). The fluid resistivity estimates from the six Wagner basin MT sites are plotted assuming a constant salinity and a constant temperature. The salinity ranges from five geothermal fields in the Mexicali and Imperial Valleys (Harthill, 1978) are indicated.
417
possible combination of temperature and salinity that could produce such resistivities.
418
Another source of information on the salinities is the chemical analysis of pore-water
419
samples taken from the first three meters of sediments (R. Batista, pers. comm.). In a
420
location 3 km northward of our MT site 6, the mean salinity of 11 samples is 37.1 g/kg
421
with a standard deviation of 3.8 g/kg. At this MT site, a low resistivity and high heat flow
422
were measured (Figure 8), suggesting that the anomalously low resistivities might be
423
due mainly to high temperatures, and not to high salinities.
424 425
Discussion
426
We interpret the deep central conductor as a zone of partial melt and the ultimate
427
source of heat for the surface hydrothermal upwelling. It is the first time such structure
428
has been sensed in the Gulf of California using an electromagnetic method. The
429
conductor shows an apparent dip toward the East, probably related to the Wagner fault
430
(González-Escobar et al., 2009), which marks the eastern boundary of the depocenter
431
and is tectonically more active than the western limit of the basin (Aragón-Arreola and
432
Martín-Barajas, 2007). Information supporting the presence of magmatic intrusions at
433
the base of the sedimentary deposits, which is contributing to the accretion of oceanic
434
crust, comes from two sources. From the Bouguer anomaly (shown in Figure 4a)
435
measured along a line almost coincident with our MT profile by Petróleos Mexicanos
436
(Pemex), Pérez (1982) has interpreted a high local gravity as produced by mafic
437
intrusions at the base of the basin. Furthermore, in a seismic reflection line about 15 km
438
to the south of our profile, over the Consag basin González et al. (2014) detected a
439
reflection interpreted as a volcanic intrusion at a depth of 6 or 7 km.
440 441
Considering the resistivity of this conductor (4 Ωm) as the effective value of a
442
two-component medium we can have an estimate of the amount of melt in this mixture.
443
These two components are magma, with a 0.25 Ωm resistivity (Constable, 2007), and
444
the 30 Ωm normal resistivity surrounding the conductor in the model of Figures 4b.
445
Using the bounds proposed by Hashin-Shtrikman (1962) for a two-component system,
446
the bounds for the melt fraction vary from 8% to 70%, which is a range too wide to be
447
useful. If we now consider Archie’s law with a cementation factor of 2, we obtain an
448
estimate of 25 % melt in this conductor.
449 450
There have been several MT studies over mid-ocean spreading ridges where the
451
conductor associated with the zone of partial melt is vertical (Heinson et al., 2000; Key
452
et al., 2013), asymmetric (Baba et al., 2006a) or even absent (Baba et al., 2006b). Its
453
presence depends on such factors as pressure, temperature, water content, and
454
connectivity between the melt pockets.
455 456
A number of studies describing the shear velocity structure of the Gulf area
457
based on Rayleigh wave dispersion have been carried out (e.g., Zhang et al., 2007;
458
Wang et al., 2009; Di Luccio et al., 2014). They show complex variations of the velocity
459
in both horizontal and vertical directions. In none of them, there is an agreement
460
between our deep central conductor and their low-velocity zones in the depth range
461
from 10 to 40 km.
462
horizontal resolution of 10 by 10 km of some of these seismic studies (Di Luccio et al.,
463
2014), which might not be good enough to resolve the 10 to 15 km width of the
464
conductor.
One possible reason for this discrepancy is the approximate
465 466
Conclusions
467
The inverted 2-D model using the data from both polarizations gave responses
468
with adequate fits with the observed data. The main features of this model are the low
469
resistivities associated with the sedimentary basin and a deep conductor. We obtained
470
a positive correlation between the resistivities estimated in the first 100 m and the
471
measured heat flow values. Most of the higher heat flows occur in the vicinity of the
472
Wagner fault and smaller associated faults, where present-day tectonic activity is
473
inferred from seismic reflection data. The two MT sites with the lowest shallow
474
resistivities are located in this zone. Although low resistivities might also be produced by
475
high salinities of the hydrothermal plumes, direct measurements of the salt content of a
476
few shallow pore-water samples might suggest that the anomalous resistivities are
477
mainly influenced by the high temperatures. The DCC is interpreted as a zone of partial
478
melt feeding the formation of new crust. It has a resistivity close to 4 Ωm, extends from
479
the base of the shallow conductor to 15 km depths, and has an apparent dip toward the
480
East. This dipping is probably related to the higher tectonic activity of the eastern side of
481
the basin. Assuming that its effective resistivity follows Archie’s law, we estimate a 25 %
482
melt in this conductor.
483 484
Acknowledgments
485
This project was funded by CeMIEGeo and CICESE. We acknowledge the captain and
486
crew of Alpha Helix, the CICESE research vessel, for their help during the data
487
acquisition stage. We thank David Myer, Ramón Batista, Guillermo Díaz, Mario
488
González, Arturo Martín, and Martín Pacheco for their technical advice in different
489
aspects of this work. T.A.E. and V.R.O. were supported by scholarships from CONACyT
490
and by UC-Mexus for a temporal stay at Scripps.
491 492
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494
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Highlights • • •
The magnetotelluric marine method was used to explore the North Gulf of California with geothermal purposes. The 2D model of the magnetotelluric marine data reveals a deep central conductor in the center of the Wagner basin. We obtained a positive correlation between the low resistivities estimated in the first 100 m using 2D inversion of the magnetotelluric marine data with high heat flow measurements reported previously.
January 10th, 2020 Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: