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Bryan, Scott Edward, Orozco-Esquivel, Teresa, Ferrari, Luca, & Lopez-Martinez, Margarita(2013)Pulling apart the mid to late Cenozoic magmatic record of the Gulf of Cali-fornia : is there a Comondú arc?Orogenic Andesites and Crustal Growth, 385.
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1
Pulling Apart the Mid to Late Cenozoic Magmatic Record of the Gulf of California: Is 1 there a Comondú Arc? 2
3 4 Bryan S.E.1, Orozco-Esquivel T.2, Ferrari L.2,3, López-Martínez, M.4 5 6 1. School of Earth, Environmental and Biological Sciences, Queensland University of 7 Technology, GPO Box 2434, Brisbane, 4001, Australia. Email: [email protected] 8 9 2. Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Blvd Juriquilla 3001, 10 Querétaro, 76230, Mexico. 11 12 3. Instituto de Geología, Universidad Nacional Autonoma de Mexico, Circuito Investigacion 13 Cientifica, Ciudad Universitaria, Mexico City, 04510, Mexico. 14 15 4. CICESE, Carretera Ensenada-Tijuana No 3918, Zona Playitas, Ensenada, Baja 16 California, 22860, México. 17 18 19 Abstract: 20
21
The composition of the lithosphere can be fundamentally altered by long-lived subduction 22
processes such that subduction-modified lithosphere can survive for 100's Myrs. Incorrect 23
petrotectonic interpretations result when spatial-temporal-compositional trends of, and source 24
contributions to, magmatism are not properly considered. Western Mexico has had protracted 25
Cenozoic magmatism developed mostly in-board of active oceanic plate subduction beneath 26
western North America. A broad range of igneous compositions from basalt to high-silica 27
rhyolite were erupted with intermediate to silicic compositions in particular, showing calc-28
alkaline and other typical subduction-related geochemical signatures. A major Oligocene 29
rhyolitic ignimbrite “flare-up” (>300,000 km3) switched to a bimodal volcanic phase in the 30
Early Miocene (~100,000 km3), associated with distributed extension and opening of 31
numerous grabens. Extension became more focussed ~18 Ma resulting in localised volcanic 32
activity along the future site of the Gulf of California. This localised volcanism (known as the 33
Comondú “arc”) was dominantly effusive and andesite-dacite in composition. Past tectonic 34
interpretations of Comondú-age volcanism may have been incorrect as these regional 35
temporal-compositional changes are alternatively interpreted as a result of increased mixing 36
of mantle-derived basaltic and crust-derived rhyolitic magmas in an active rift environment 37
rather than fluid flux melting of the mantle wedge above the subducting Guadalupe Plate. 38
39
2
Supplementary material: References from which whole-rock geochemical and radiometric 40
age data have been compiled in this paper are available at www.geolsoc.org.uk/SUP00000. 41
42
Introduction 43
44
Mid- to Late Tertiary magmatism along the continental margin of western Mexico (Fig. 1) is 45
an example where interpretations on the tectonic setting of magmatism have been strongly 46
influenced by the continent-margin position, calc-alkaline affinity, relatively primitive 47
isotopic characteristics, the presence of andesitic or intermediate composition volcanic rocks, 48
and general association with continued subduction beneath western North America (e.g., 49
Cameron et al., 1980; Lanphere et al., 1980; Hausback, 1984; Wark et al., 1990; Umhoefer et 50
al., 2001). A feature of the magmatic record in western Mexico is the general continuity of 51
erupted/igneous compositions from basalt through to high-silica rhyolite (Fig. 2). This raises 52
the question of whether the presence of calc-alkaline “andesitic” igneous compositions offers 53
any constraint on the tectonic setting of magmatism. Past studies have widely interpreted the 54
mid- to Late Tertiary (~38-12 Ma) volcanism as recording a supra-subduction zone volcanic 55
arc (e.g., Cameron et al., 1980; Hausback, 1984; Sawlan & Smith, 1984; Wark et al., 1990; 56
Ferrari et al., 1999; Martín-Barajas et al., 1995, Martín et al., 2000; Umhoefer et al., 2001). 57
However, for Oligocene to Early Miocene volcanic activity (~38-18 Ma), the overwhelming 58
silicic composition, the eruptive scale, volume and output rate, and the rhyolite ignimbrite-59
dominated character of the erupted products sourced from multiple calderas and fissures (e.g., 60
Swanson & McDowell, 1984; Aguirre-Diaz & Labarthe-Hernandez, 2003; Swanson et al., 61
2006) are inconsistent with modern expressions of supra-subduction zone volcanism. 62
Equally, using narrow, silicic-magma dominated continental arc- to back-arc rifts like the 63
Taupo Volcanic Zone (Cole, 1990; Parson & Wright, 1996) as analogues are also not 64
appropriate (Bryan et al., 2008). 65
66
In contrast, mid-Miocene volcanism (~18-12 Ma) has been reported to be predominantly 67
intermediate in composition, with a restricted occurrence along the margins of the recently 68
opened Gulf of California (Fig. 1). When combined with a facies comparison to 69
stratovolcanic assemblages (Hausback, 1984), this has been the basis for arguing a westward 70
migration and reestablishment of supra-subduction zone arc volcanism along eastern Baja 71
3
California before the final termination of subduction along western Mexico at ~12.3-12.5 Ma 72
(Gastil et al., 1979; Stock & Hodges, 1989; Lonsdale, 1991). This interpretation is significant 73
because: 1) prevailing models for the opening and development of the Gulf of California, 74
despite differing in rifting kinematics, all imply Gulf extension began after the termination of 75
andesitic volcanism and subduction at 12.3 Ma (Stock & Hodges, 1989; Atwater and Stock, 76
1998; Fletcher et al., 2007); and 2) the Gulf of California has been considered an 77
anomalously rapid zone of continental rupture based on the onset of seafloor spreading 78
interpreted to be only ~6-10 Myrs after the cessation of subduction and arc volcanism 79
(Umhoefer, 2011). Such interpretations are predicated on extension beginning after 12 Ma 80
because Middle Miocene andesitic volcanism has been universally considered a nonextending 81
supra-subduction zone volcanic arc. This, however, is at odds with seismic data, well 82
information and paleontological data indicating rift basin development in the Gulf of 83
California began much earlier (see Karig & Jensky, 1972), and now supported by new studies 84
(Ferrari et al., 2012; in preparation) from the Gulf region that demonstrate active extension 85
beginning at least 18 Ma and spatially coincident with the andesitic volcanism. This is 86
consistent with structural studies further east (e.g., Gans, 1997) that indicate extension 87
occurred mainly in the Oligo-Miocene, well before the cessation of subduction (see also 88
Henry, 1989). Several studies have demonstrated how extension can strongly influence 89
magmatism in terms of magma generation (location, processes, rates, storage), and eruptive 90
composition and style (e.g., Gans et al., 1989; Axen et al., 1993), such that intermediate 91
magma compositions can be promoted by active extensional faulting (e.g., Johnson & 92
Grunder, 2000). 93
94
To understand better the origin and tectonic setting of the mid-Miocene andesitic volcanism 95
in western Mexico, it is critical to constrain the spatial-temporal-compositional record and 96
trends of magmatism and how this relates to the timing and location of extension across 97
western Mexico. An important limitation on our understanding of the tectonomagmatic 98
setting of mid- to late Tertiary volcanism in western Mexico has been that all current tectonic 99
and petrogenetic interpretations have relied on “discovery-phase” mapping begun in the 100
1970s, and while at least 90% of the Sierra Madre Occidental (SMO) remains unmapped 101
(Swanson et al., 2006); this has particularly been the case for the western margins of the 102
province through the states of Nayarit and Sinaloa. A recent NSF Margins focus on the 103
4
opening of the Gulf of California has resulted in a closer examination by us of how 104
magmatism transitioned from the huge SMO silicic Large Igneous Province through 105
interpreted arc volcanism to crustal rupturing to open the Gulf of California in the Late 106
Miocene. The examination here will begin by a review of the volcanic record of the SMO 107
silicic Large Igneous Province that immediately preceded the mid-Miocene andesitic 108
volcanism, referred to as the “Comondú “arc” or the middle and upper Comondú Group of 109
Umhoefer et al. (2001). Constraints on the development of the Gulf of California will then be 110
reviewed followed by an examination of the spatial-temporal compositional trends of 111
magmatism across this region from ~40 Ma to better understand the tectonic and structural 112
context of the mid-Miocene andesitic volcanism. 113
114
Geologic Background 115
116
Sierra Madre Occidental 117
118
The Sierra Madre Occidental (SMO, Fig. 1) is the largest silicic igneous province in North 119
America (McDowell & Keizer, 1977; McDowell & Clabaugh, 1979; Ward, 1995; McDowell 120
& McIntosh, 2012), and although principally contained within western Mexico, is contiguous 121
with silicic volcanism through the Basin and Range Province of western USA to the north 122
(Lipman et al., 1972; Gans et al., 1989; Best & Christiansen, 1991) and also with the 123
ignimbrite province of the Sierra Madre Sur, south of the Trans Mexican Volcanic Belt 124
(Martiny et al., 2000; Moran-Zenteno et al., 2007). At least 400,000 km3 of dominantly 125
rhyolitic ignimbrite was erupted, mostly between ~38 and 18 Ma, but age dating over the last 126
40 years has identified two main pulses or ‘flare-ups’ of ignimbrite activity (Fig. 3): at ~34-127
28 Ma and ~24-18 Ma (Ferrari et al., 2002, 2007; Bryan et al., 2008; McDowell & McIntosh, 128
2012). The Oligocene pulse is thought to be responsible for at least three-quarters of this 129
erupted volume, whereas at least 100,000 km3 was erupted in the Early Miocene. 130
131
Rhyolitic to dacitic ignimbrite represents at least 85-90% of the erupted volume with the 132
remaining volume being rhyolitic lavas/domes, basaltic lavas and lesser andesitic lavas. Age 133
dating has revealed brief but intense episodes of volcanism (~1 Myr duration) emplacing 134
kilometre thick sections of ignimbrite across the province (e.g., McDowell & Keizer, 1977; 135
5
Ferrari et al., 2002; Swanson et al., 2006; McDowell & McIntosh, 2012), attesting to rapid 136
rates of silicic magma generation and eruption (Bryan et al., 2008). Ignimbrite sections within 137
the central part of the province are flat-lying and define an unextended core to the SMO (Fig. 138
1), and this region also corresponds to the highest crustal thicknesses, up to 55 km (Bonner 139
and Herrin, 1999). In contrast, ignimbrite sections are faulted and tilted along both the 140
western and eastern flanks of this unextended core (Henry & Aranda-Gomez, 2000; Ferrari et 141
al., 2002). Basaltic andesite lavas, referred to as the Southern Cordilleran Basaltic Andesite, 142
or SCORBA (Cameron et al., 1989), appear to be widespread throughout the SMO, and in the 143
northern SMO, are commonly found intercalated with, or overlying the youngest ignimbrites 144
in each section (Swanson et al., 2006). 145
146
Early Miocene volcanic activity was largely superimposed on the Oligocene pulse of 147
volcanism, except in the northern SMO, but also extended further west (Fig. 1) to be present 148
on Baja California (e.g., Umhoefer et al., 2001). No apparent shift in the eastern limit of 149
volcanism occurred for the central and southern SMO, but a westward shift is more 150
pronounced for the northern SMO (Fig. 1). Recent dredge surveys and age dating of 151
recovered rocks through the southern Gulf of California have confirmed the presence of Early 152
Miocene bimodal volcanic and exhumed intrusive rocks offshore (Fig. 1) improving the pre-153
rift connection between Baja California and mainland Mexico (Orozco-Esquivel et al., 2010; 154
Ferrari et al., 2012; in preparation). The Early Miocene phase shows significant differences 155
from north to south despite showing a similar north-south extent to the previous Oligocene 156
pulse. While silicic volcanism appears to have been more volumetrically dominant in the SW 157
part of the SMO (Ferrari et al., 2002; Bryan et al., 2008), Early Miocene volcanism was less 158
abundant and dominantly mafic in composition across the northern SMO (McDowell et al., 159
1997; Murray et al., 2010, submitted). In the central and southern SMO, volcanism was more 160
bimodal with thick rhyolitic ignimbrite packages, similar to the Oligocene sections, 161
characterising some areas (eg., Espinazo del Diablo and El Salto successions, McDowell & 162
Keizer, 1977), whereas elsewhere, graben-focussed bimodal volcanism is characteristic 163
(Ferrari et al., 2002; Ramos Rosique, 2012). Graben margins in the southern SMO are 164
commonly defined by rhyolite domes, whereas basaltic lava packages up to 200 m thick and 165
rhyolitic ignimbrites partly infill the Early Miocene grabens (Ramos Rosique et al., 2010; 166
Ramos Rosique, 2012). Recent zircon chronochemical studies of the Early Miocene rhyolites 167
6
in the SW SMO have shown a very distinct zircon inheritance signature where the inherited 168
zircon ages indicate remelting of silicic igneous rocks formed during the Oligocene and Early 169
Miocene ignimbrite pulses (Bryan et al., 2008; Ramos Rosique, 2012; Ferrari et al., 2012; in 170
preparation). Mid to upper crustal remelting is interpreted to have been driven by basaltic 171
magmatism invading high structural levels in the crust, which was aided by active crustal 172
extension. 173
174
The Comondú “Arc” 175
176
The eruption of andesitic rocks in the Early to Middle Miocene principally along eastern Baja 177
California (the Comondú arc; Hausback, 1984; Sawlan & Smith, 1984; Sawlan, 1991; 178
Umhoefer et al., 2001) has widely been interpreted to mark the termination of the SMO and 179
its broad zone of silicic dominant magmatism and extension beginning ~40 Ma, with the re-180
establishment of typical supra-subduction zone arc magmatism (e.g., Ferrari et al., 2007). 181
This was a re-establishment because as observed along the length of the western North 182
American margin (McQuarrie & Oskin, 2011), no well-defined frontal ‘andesitic’ arc had 183
existed during the Oligocene, when instead a broad zone (up to 1000 km) of volumetrically 184
dominant silicic volcanism occurred (e.g., Lipman et al., 1972; Gans et al., 1989; McDowell 185
& Mauger, 1994; McDowell, 2007). This was the case for Baja California, which had 186
remained attached to mainland Mexico until the Late Miocene. Along Baja California, 187
Cretaceous rocks are generally overlain by lower Tertiary marine and local nonmarine 188
sedimentary rocks (recording this absence of frontal ‘andesitic’ arc volcanism), or late 189
Oligocene to Miocene volcanic rocks above a regional unconformity (e.g., Beal, 1948; 190
Hausback, 1984; Umhoefer et al., 2001). The Upper Oligocene to Middle Miocene 191
volcanosedimentary units in Baja California are known as the Comondú Group (Umhoefer et 192
al., 2001), and have been widely interpreted to be forearc basin and volcanic arc deposits 193
formed immediately prior to plate boundary reorganization and rifting that opened the Gulf of 194
California (Hausback, 1984; Umhoefer et al., 2001; Conly et al., 2005; Godinez et al., 2010; 195
Umhoefer, 2011). The Comondú Group is currently divided into three informal stratigraphic 196
units (Umhoefer et al., 2001). A lower unit (<500 m thick, ~30 - 19.5 Ma) is dominated by 197
quartz sandstones and conglomerate including aeolian sandstone, and interbedded 198
resedimented pyroclastic units (tuffaceous sandstone), whereas several rhyolitic ignimbrites 199
7
and localised basaltic lavas are prominent in the upper parts. Detrital zircon U-Pb ages on 200
aeolian sandstones suggest a maximum depositional age of ~25 Ma (Godinez et al., 2010), 201
and new U/Pb and Ar/Ar ages bracket ignimbrite emplacement between ~24 and 19.50 ± 0.05 202
Ma (Drake, 2005; Godinez et al., 2010). This lower unit is thus temporally and 203
compositionally correlated with Early Miocene bimodal volcanism offshore in the Gulf 204
(Orozco-Esquivel et al., 2010) and on mainland Mexico. These Comondú Group ignimbrites 205
are outflow facies and generally deposited at medial to distal distances from their sources 206
interpreted to occur to the east (Umhoefer et al., 2001; Drake, 2005). The middle and upper 207
‘Comondú’ units (~19.5 - 12 Ma; ~1 km thick) are more intermediate in composition and 208
dominated by sedimentary breccia with interspersed andesite-dacite lavas/domes, particularly 209
in southern Baja California, and cross-cut by dykes and minor porphyry intrusions (Sawlan & 210
Smith, 1984; Hausback, 1984;Umhoefer et al., 2001; Godinez et al., 2010). A few 211
interbedded rhyolitic ignimbrites have also been reported (Hausback, 1984; ash flow tuff D of 212
Drake, 2005). Some zircon U-Pb ages on biotite granodiorite porphyry intrusions indicate 213
emplacement ages of 19.9 ± 0.73 and 16.3 ± 0.49 Ma (Godinez et al., 2010), whereas 214
andesite-dacite lavas range in age from ~19.5 to ~11 Ma (Sawlan & Smith, 1984; Hausback, 215
1984; Martín-Barajas et al., 1995; Martín et al., 2000; Umhoefer et al., 2001; Drake, 2005). 216
The lava dome and flow units are vent to proximal facies, but critically, several stratigraphic 217
studies have shown the andesite-dacite lavas to be interbedded with sedimentary rocks rather 218
than forming thick stacked piles of lava as expected in the construction of stratocones. 219
220
Gulf of California 221
222
The Gulf of California (GoC) has been one of the focus sites for the Rupturing Continental 223
Lithosphere initiative of the NSF-funded MARGINS program from 2004-2010. It is a ~1400 224
km long, highly sedimented, oblique rift characterized by long transform faults and short 225
spreading centres (Lonsdale, 1989; Lizarralde et al., 2007). Despite different models (see 226
review in Fletcher et al., 2007), rifting has been considered to have developed rapidly 227
following cessation of subduction of the Guadalupe Plate (and arc volcanism) at about ~12.3-228
12.5 Ma (Stock & Hodges, 1989; Ferrari et al., 2007; Fletcher et al., 2007; Lizarralde et al., 229
2007; Umhoefer, 2011). Since rift inception, the Gulf has opened between ~300 and 500 km 230
(Oskin et al., 2001; Fletcher et al., 2007), with the Gulf progressively unzippering to the north 231
8
such that active extension of continental crust is currently occurring at the northern end of the 232
Gulf. The timing of Gulf inception and rifting at ~12.3 Ma is generally consistent with the 233
switch to more alkaline and tholeiitic basaltic volcanism, and the reappearance of more 234
rhyolitic volcanism at ~12-13 Ma (Fig. 3; e.g.., Martín-Barajas et al., 1995; Stock et al., 1999; 235
Martín et al., 2000; Vidal-Solano et al., 2007). Most dates for initiation of marine 236
sedimentation in the Gulf region are at 8 Ma and younger (e.g., Oskin & Stock, 2003), but an 237
earlier marine incursion and the development of a seaway in the middle Miocene has been the 238
basis for the concept of a “Proto-gulf” (Karig & Jensky, 1972). While the timing of marine 239
incursion remains contentious and based principally on onshore exposures, comprehensive 240
stratigraphic revision (Carreño & Smith, 2007) and recent micropaleontological studies of 241
several deep wells drilled in the Wagner, Consag, and Tiburón basins of the GoC document 242
at least 1870 m of marine sediments deposited before 11.2 Ma (Helenes et al., 2009; Helenes, 243
2012). 244
245
Seafloor-spreading centres formed at variable times from ~6 to 2 Ma (Umhoefer, 2011), with 246
a spreading centre in the Guaymas sub-basin (central Gulf) interpreted to have began ~ 6 Ma 247
(Fig. 1) based on the width of the new igneous crust observed in seismic refraction profiles 248
(Lizarralde et al., 2007). The Alarcón spreading center began forming proto-oceanic crust ca. 249
3–3.5 Ma (DeMets, 1995), and true sea-floor spreading at present rates began at 2.4 Ma 250
(Sutherland, 2006; Umhoefer et al., 2008). 251
252
Summary 253
254
Two important outcomes on the Miocene history of the GoC region are apparent from the 255
diversity of recent studies in the region. The first is that recent dating studies from the SMO, 256
the GoC and Baja California confirm eruptive ages on rhyolites and basalts related to the 257
Early Miocene pulse of the SMO extend to as young as ~17 Ma (Hausback, 1984; Martín et 258
al., 2000; Umhoefer et al., 2001; Drake, 2005; Bryan et al., 2008; Ramos Rosique, 2012; 259
Ferrari et al., 2012; in preparation). This continued bimodal volcanism thus overlaps the onset 260
of interpreted arc volcanism along Baja California at ~19.5 Ma (Umhoefer et al., 2001), 261
whereas others have considered ‘arc’ volcanism began earlier in northern Baja California at 262
~21 Ma (e.g., Martín-Barajas et al., 1995). This age overlap thus suggests no abrupt 263
9
termination to SMO bimodal volcanism when rejuvenation of supra-subduction zone arc 264
volcanism was apparently initiated, despite the bimodal and andesitic volcanism spatially 265
overlapping (Fig. 1). However, regional temporal-compositional patterns in erupted magma 266
compositions do indicate a strong compositional shift from dominant bimodal volcanism to 267
more intermediate composition volcanism beginning ~19 Ma (Figs 3, 4). The second 268
outcome is that biostratigraphic studies, particularly from the offshore basins are recording a 269
more protracted history of rift basin subsidence and sediment accumulation in the GoC and 270
that marine sedimentation began at least in the middle Miocene (~12 Ma; Helenes, 2012; cf. 271
Sutherland et al., 2012). This would require unrealistically rapid crustal thinning and 272
subsidence if all extension began ~12 Ma. Collectively, these observations are at odds with 273
the notion of rapid switchings from bimodal volcanism across the western SMO and southern 274
Baja California, to establishing a subduction-related andesitic magmatic arc along the eastern 275
side of Baja California and westernmost mainland Mexico from ~22-19 Ma, and which then 276
abruptly terminated at 13-12 Ma to switch back again to predominantly bimodal alkaline 277
volcanism (Fig. 3), coincident with the onset of rifting of the GoC at this time (e.g., Stock & 278
Hodges, 1989; Umhoefer et al., 2001; Sutherland et al., 2012). 279
280
Temporal-compositional trends of Oligocene-Miocene magmatism across western 281
Mexico 282
283
Published and unpublished whole-rock compositional and age data for 40-0 Ma igneous rocks 284
from Western Mexico, occurring in a northwest-trending belt from ~22°N 103°W, to 32°N 285
116°W have been compiled in this study (see Supplementary Material) and serves as an 286
update to that of Ferrari et al. (1999). The dataset comprises three types of data: 1) whole-287
rock compositional data for which radiometric ages have been obtained from the same 288
samples (n = 461); 2) whole-rock compositional data that have stratigraphic control and a 289
stratigraphic age can be assigned (e.g., Oligocene, Late Miocene; n = 1567); and 3) samples 290
that have radiometric age data and lithological information (e.g., basalt lava, rhyolite 291
ignimbrite), but may lack corresponding whole-rock compositions (n = 1520). For samples 292
with radiometric ages from 40-0 Ma, this includes K/Ar (whole rock, groundmass, feldspar, 293
biotite, hornblende), 40Ar/39Ar (whole rock, groundmass, feldspar, hornblende, biotite), and 294
zircon fission track and U/Pb ages. Of the samples with whole-rock compositional data, 1486 295
10
samples have major element analyses with the remainder having whole-rock isotopic (Sr, Pb, 296
Nd) compositions ± some trace element abundances. Overall, trace element data are more 297
sparse with only 288 samples having full rare earth element analyses. Most of the whole-rock 298
geochemical data have been measured by XRF, but ICP-MS trace element data have 299
increased over the last 10 years (e.g., Martín et al., 2000; Till et al., 2009). In the following 300
figures, igneous compositions are grouped into the following: 1) basalts, with 45-52 wt% 301
SiO2; 2) basaltic andesite, 52-56 wt% SiO2; 3) andesite, 56-63 wt% SiO2; 4) dacite, 63-68 302
wt% SiO2; and 5) rhyolite, 68-79 wt% SiO2. 303
304
Compiled whole-rock compositional data from the SMO for which age data are available 305
(Figs 3, 4) illustrate the dominantly silicic compositional character of the Oligocene 306
ignimbrite pulse, but with an increasing appearance and abundance of basaltic lavas from ~30 307
Ma. In contrast, the Early Miocene pulse was distinctly bimodal with a paucity of volcanic 308
compositions between 64 and 68 wt% SiO2 (Fig. 4). Of note is the broad silicic peak for both 309
the Oligocene and Early Miocene pulses or two compositional groupings of dacitic to low-310
silica rhyolite and high-silica rhyolite suites with the latter being dominant and occurring as 311
lavas/domes and ignimbrites. 312
313
The disappearance of rhyolite and high-silica rhyolite compositions across the region is 314
pronounced at ~19-18 Ma (Fig. 3). From 18-13 Ma, erupted magmas were more intermediate 315
in composition as recognised in many previous studies, with a paucity of both basalt (<52 316
wt% SiO2) and rhyolitic (>68 wt% SiO2) magmas. Basaltic andesite and low-silica andesite 317
compositions dominate with andesite-dacite compositions also important (Fig. 4), and almost 318
all have been emplaced as lavas. Consequently, there is not only a strong compositional 319
change at 19-18 Ma, but also a change from mixed explosive-effusive to dominantly effusive 320
eruptive styles. However, the period of dominant andesitic volcanism (~18-13 Ma) that 321
defines the basis for a subduction-related andesitic magmatic arc is actually much shorter 322
than is widely argued (Fig. 3); andesitic arc volcanism has previously been interpreted from 323
~22-16 Ma and 22-12 Ma across northern and southern Baja California, respectively (Sawlan 324
& Smith, 1984; Hausback, 1984; Sawlan, 1991). The reappearance of rhyolite and high-silica 325
rhyolite at 13-12 Ma (Fig. 3) is distinctive because these magma compositions are also 326
peralkaline and mark the recurrence of explosive eruptions emplacing ignimbrites (Vidal-327
11
Solano et al., 2008) such as the 12.6 Ma alkali rhyolite Tuff of San Felipe (Stock et al., 1999). 328
The alkali rhyolite eruptions coincide with the eruption of true basaltic compositions 329
regionally that are also more alkaline in composition. From 13-5 Ma (Fig. 4) magma 330
compositions became fundamentally more basaltic and alkalic with alkalic and tholeiitic 331
basaltic eruptions around the margins of the GoC eventually coinciding with the onset of 332
seafloor-spreading from 6-3.5 Ma. As with the preceding Miocene phase, dacitic 333
compositions (64-68 wt % SiO2) are significant between 12 and 8 Ma (Figs 3, 4). 334
335
Spatial-Temporal-Compositional Trends 336
337
Important insights can also be gained if the long-term temporal compositional trends 338
discussed above can be placed in a spatial context. The age dataset for which lithological ± 339
whole-rock compositional data are available is used here. Age data are plotted for E-W 340
transects across the GoC for the region between 20° and 26°N and 26° and 32°N as shown in 341
Figure 5. Dated igneous rocks have been grouped into four compositional groupings: basalt 342
(<56 wt% SiO2), andesite (56-63 wt% SiO2), dacite (63-68 wt% SiO2) and rhyolite (>68 wt% 343
SiO2). Several key features are apparent from Figure 5: 344
345
1. As observed in Figures 3 and 4, Oligocene to Early Miocene (~40-20 Ma) volcanism is 346
silicic-dominant, and widely distributed (up to 800 km width). Across this broad zone of 347
silicic volcanism between 40-30 Ma, low volumes of basalts and andesites were also erupted. 348
349
2. By the Early Miocene, volcanism became strongly bimodal, but this switch occurred 350
earlier in the northern region (~30 Ma), and basaltic magmas dominated from 30-20 Ma. In 351
the north (Sonora-Chihuahua), basalts (SCORBA of Cameron et al., 1989) commonly marked 352
the termination of silicic volcanism in an area and since ~28 Ma were emplaced on top of 353
rhyolitic ignimbrites and into developing extensional basins (until ~15 Ma; McDowell et al., 354
1997). In the south (Jalisco, Nayarit, Sinaloa), the basalts appear to represent the triggering of 355
the Early Miocene silicic ignimbrite pulse (Bryan et al., 2008). 356
357
3. Rather than a westward migration of volcanic activity as is widely reported from the 358
Oligocene to Miocene (e.g., Damon et al., 1981; Ferrari et al., 1999), volcanism did extend 359
12
westwards at ~30 Ma, but importantly, there was no corresponding shift of the eastern limit 360
of volcanism at this time, which would be expected if volcanic activity was intimately linked 361
to back-arc extensional processes and driven by slab roll back (e.g., Clift et al., 1995). A 362
westward shift in the location of the eastern limit of volcanism at ~25 Ma is more pronounced 363
in the northern region, and did not occur until ~20 Ma in the southern SMO. 364
365
4. The extensive Early Miocene bimodal volcanism was associated with distributed extension 366
and the formation of numerous grabens along the western and southern SMO (Fig. 1). 367
Inception of most grabens had begun by 24 Ma in the southern SMO (Ferrari et al., 2002; 368
Ramos Rosique, 2012) and by 27 Ma across the northern and western SMO (McDowell et al., 369
1997). Graben development occurred over a horizontal distance of up to 450 km (Fig. 1) and 370
at least 600 km in board of the paleocontinental margin at that time. Despite the size and 371
extent of the grabens, the magnitude of Latest Oligocene-Early Miocene extension associated 372
with them is inferred to be modest (<10% stretching, Ferrari et al., 2002). Soon after the 373
onset of this wide zone of rifting, a relatively narrow belt of high magnitude extension 374
producing metamorphic core complexes from ~25-15 Ma (Nourse et al., 1994; Wong et al., 375
2010) also developed across the NW SMO (Fig. 1). The prominent westward shift in volcanic 376
activity at ~25 Ma and subdued silicic volcanism from ~27 Ma for the northern region may in 377
part reflect suppression of volcanism in areas of high magnitude extension (Gans & Bohrson, 378
1998) that was occurring along the eastern limits of this volcanism. 379
380
5. In both regions, andesite-dacite volcanism becomes more abundant beginning ~19 Ma (see 381
also Fig. 3), and in contrast to earlier episodes of andesite eruption, are much more spatially 382
restricted to the location of the nascent GoC. 383
384
6. Although andesite-dacite volcanism was significant from ~20 to 10 Ma, as observed along 385
Baja California, it was coeval with (basalt-dominant) bimodal volcanism occurring 386
regionally. This is particularly the case during the period 20-16 Ma for the southern region. 387
Rhyolitic volcanism producing ignimbrites and lavas persisted until at least 16 Ma in the 388
southern region with rhyolites dated from Baja California (Hausback, 1984; Hosack, 2006), 389
Nayarit (Ferrari et al.2012; in preparation) and Jalisco (Nieto-Obregón et al., 1981; Bryan et 390
al., 2008). Consequently, spatial-temporal distinction between the termination of syn-391
13
extensional bimodal volcanism across the SMO and the beginnings of more intermediate 392
composition magmatism around the GoC is blurred. 393
394
7. The style, composition and extent of volcanism changes significantly at ~13 Ma. Bimodal 395
volcanism characterises the northern region (coastal Sonora and Baja California) and 396
rhyolitic compositions became more peralkaline (e.g., Tuff of San Felipe, Stock et al., 1999; 397
Vidal-Solano et al., 2007). In contrast, volcanism is more basaltic and widespread in the 398
southern region with few rhyolites dated younger than 13 Ma. Despite the widespread 399
eruption of basalts across the southern region, andesite-dacite volcanism remains principally 400
focussed around the margins of the nascent GoC. Andesitic compositions are particularly 401
spatially restricted in the northern region from 10-2 Ma despite other magma compositions 402
being erupted over a broader region (Figs 4,5). 403
404
Timing Relationships to Extension 405
406
An important constraint in understanding the significance of the spatial-temporal 407
compositional trends described above is understanding how these magmatic trends 408
correspond spatially and temporally to the onset, or changes to the rate and breadth, of 409
extension. Most of the SMO has been affected by different episodes of dominantly 410
extensional deformation that began in the Oligocene and potentially at the end of Eocene 411
(Henry, 1989; Ferrari et al., 2007). However, the central zone to the SMO remains unaffected 412
by extension (Fig. 1) where ignimbrite sections rise to high altitudes (>3000 m ASL), remain 413
flat-lying (Fig. 6) and crustal thicknesses are high (Bonner and Herrin, 1999). This 414
unextended core of the SMO represents a physiographic boundary between what has been 415
defined as the “Mexican Basin and Range,” to the east, and the “Gulf Extensional Province,” 416
to the west (Henry & Aranda-Gómez, 2000). At the northern and southern ends of the SMO 417
(i.e., northern Sonora and Chihuahua and Nayarit-Jalisco, respectively), these two provinces 418
merge where extension has affected the entire width of the SMO (Ferrari et al., 2007). 419
420
The age of extension in the Gulf Extensional Province is thought to have been largely Late 421
Miocene to Recent and associated with the termination of subduction and onset of rifting to 422
open the GoC at ~12.3 Ma (e.g., Stock & Hodges, 1989; Henry & Aranda-Gomez, 2000). 423
14
However, an intense phase of extension resulting in up to 100% crustal thinning and 424
producing metamorphic core complexes occurred from ~25-15 Ma in Sonora (e.g., Nourse et 425
al., 1994; Gans, 1997; Gans et al., 2003; Wong et al., 2010). These core complexes slightly 426
post-date the onset of a milder phase of extension producing sediment and basalt-filled 427
graben depressions (Baucarit Formation of King, 1939) through Sonora. Basaltic to andesitic 428
lavas ranging in age between 27 and 20 Ma are common toward the base of these rift basin 429
successions (McDowell et al., 1997; Paz-Moreno et al., 2003). 430
431
The age of extension along the southeastern margin of the GoC through Sinaloa and Nayarit 432
has been less clear. Recent studies by us (Ferrari et al., 2012; Ferrari et al., 2012; in 433
preparation) have revealed that kilometre-thick ignimbrite sections of the SMO, dated as 434
young as 20 Ma, have been tilted by up to 35° (Fig. 6) with some blocks remaining at high 435
elevations (~2000 m). However, along the low-relief coastal plain are extensive, flat-lying 436
and undeformed basaltic lava fields dated between 12-9 Ma extending for at least 700 km 437
along the eastern margin of the GoC (Fig. 1; Ferrari et al., 2012; in preparation; see also 438
Gastil et al., 1979). Similar aged basalts have also been dredged from the submerged 439
continental margins to the southern GoC (Ferrari et al., 2012; in preparation). 440
441
Important insights also come from the Baja California sections of the Comondú Group where 442
interpreted vent and proximal facies lava flows and domes are buried by thick successions of 443
medial facies bedded sedimentary conglomerate/breccia and sandstone, and distal ignimbrites 444
(e.g., Hausback, 1984; Umhoefer et al., 2001; Drake, 2005). In particular, positive relief lava 445
domes with thicknesses of up to 300 m have been shown to be buried by sedimentary 446
deposits (Drake, 2005). These depositional and facies relationships demonstrate that andesitic 447
to dacitic lavas and domes of the Comondú Group were being emplaced into actively 448
subsiding sedimentary basins. 449
450
In summary, field, stratigraphic and now new age data from the eastern margin of the GoC 451
indicate that large-magnitude extension must have occurred between ~25 and 12 Ma to the 452
north (e.g., Gans, 1997) and between ~18 and 12 Ma in the central and southern Gulf region. 453
This extension must have post-dated the final phases of bimodal and ignimbrite-dominant 454
activity of the Early Miocene pulse of the SMO, and preceded the widespread eruption of 455
15
flat-lying, (undeformed) basaltic lavas along the eastern margin of the GoC beginning ~12 456
Ma (Fig. 1). Importantly, as recognised in previous studies, the present-day crustal thickness 457
beneath the unextended core of the SMO is between 40 and 55 km, whereas along the 458
margins of the GoC it is between 18 and 26 km (Gomberg et al., 1988; Couch et al. 1991; 459
Bonner & Herrin, 1999; Persaud et al., 2006; 2007). Consequently, most of this crustal 460
thinning occurred prior to the termination of subduction at ~12.3-12.5 Ma, and was 461
coincident with the major period of andesitic volcanism (middle and upper Comondú Group 462
of Umhoefer et al., 2001). 463
464
Discussion 465
466
Origin of the Middle Miocene Comondú Andesites 467
468
In determining the origin of the Comondú Group andesites and in particular, why there was a 469
discrete period of dominantly intermediate volcanism between ~18-13 Ma, several key points 470
must be considered from the observations and discussion given above. First, there has been 471
no preceding history of andesitic volcanism or existence of a well-defined frontal arc in the 472
region since at least the Eocene, despite ongoing subduction. Andesitic volcanism occurred, 473
but 1) was diffuse and low-volume occurring over a very broad width in-board of the margin 474
(>600 km) where it was closely associated with silicic and basaltic volcanism (Fig. 5); 2) 475
when eruptions of andesitic-dacitic magma compositions became dominant in the mid-476
Miocene, they were localised in space and time around the nascent GoC (Figs 1, 5); 3) this 477
andesitic phase was coincident with the major period of crustal extension through the Gulf 478
Extensional Province with crustal thinning up to 100%; and 4) both immediately before and 479
after, and regionally, volcanism was clearly bimodal in character (Fig. 3). 480
481
Limited petrologic and geochemical data have been published on the Comondú Group 482
andesites and much interpretation has been based on major element chemistry (Gastil et al., 483
1979; Sawlan & Smith, 1984; Hausback, 1984). These previous studies have established the 484
general ‘calc-alkaline’ chemistry, intermediate composition and similarity to modern 485
subduction-related arc successions for the Comondú Group andesites. However, alkalic 486
basalts have also been reported from these suites (Martín-Barajas et al., 1995), which are 487
16
atypical of modern, unextended arc settings. The Comondú Group andesites are commonly, 488
moderately to very crystal-rich and vary from hornblende to pyroxene andesites (e.g., Sawlan 489
& Smith, 1984; Martín et al., 2000). However, magmatic heterogeneity of these andesites is 490
apparent from the brief petrographic descriptions with reports of resorbed olivine (Sawlan & 491
Smith, 1984), quartz and alkali feldspar (Martín et al., 2000); complexly zoned plagioclase 492
(Sawlan & Smith, 1984); megacrystic hornblende (Drake, 2005); enclaves (Drake, 2005); and 493
granitic and upper crustal xenoliths (Martín et al., 2000). Recent U-Pb zircon dating of a 494
Miocene porphyry intrusion that intrudes the Lower Comondú Group (Godinez et al., 2010) 495
has revealed the presence of abundant Cretaceous zircons indicating remelting of the 496
underlying calc-alkaline Cretaceous batholith. Collectively, these characteristics indicate the 497
importance of open system processes, magma mixing (involving basaltic magmas) and 498
crustal assimilation/partial melting in Comondú Group andesite genesis. Importantly, the 499
crustal materials involved have a strong subduction heritage given the long-lived history of 500
subduction along the western margin of North America since the Triassic. Therefore no 501
reliance should be placed on the whole-rock geochemical signatures for interpreting the 502
tectonic environment in which the magmas were emplaced where crustal assimilation has 503
demonstrably occurred (cf. Conly et al., 2005). Furthermore, recent studies of arc andesites 504
are emphasising that most if not all crystals are xenocrysts or antecrysts, being picked up by 505
initially aphyric mafic melts on their way to the surface (Zellmer et al., submitted) such that 506
the bulk rock compositions will incorrectly image the mantle source, and prove unreliable for 507
tectonic discrimination. Consequently, porphyritic intermediate compositions likely represent 508
remobilized igneous protoliths (Zellmer, 2009), and much of their bulk chemical 509
characteristics can be inherited from those source materials. As has been discussed elsewhere, 510
there is no a priori requirement for I-type calc-alkaline magmatism to be directly related to 511
active subduction processes (e.g., Roberts & Clemens, 1993; Morris & Hooper, 1997). 512
513
Mineralogical evidence suggests the importance of magma mixing being an important 514
process. Previous studies have illustrated linear variation trends for major elements (e.g., 515
Sawlan & Smith, 1984; Hausback, 1984). Plotting of several trace element ratio combinations 516
also reveals linear patterns and apparent mixing trends (Fig. 7). The role of mixing is a 517
dominant process at the regional scale and the existence of a subduction-related signature is 518
examined in Figure 7. The use of Ba/Nb as a subduction-related signature is similar to the 519
17
approach of Till et al. (2009) who used Ba/Ta, but because there are few samples with Ta 520
abundances, we use the Ba/Nb ratio here. However, elevated Ba/Nb ratios are also a crustal 521
signature. The Ba/Nb-La/Nb data show a spectrum from “intraplate” basaltic compositions, 522
as represented by the Early and Late Miocene alkali basalts in the region, to more arc-like 523
and crustal compositions with high Ba/Nb ratios. The trends for the different compositional 524
groupings are not readily explainable by fractional crystallisation processes since the mafic, 525
intermediate and silicic igneous compositions overlap. Of note is that from the Early Miocene 526
onwards, a distinctly high Ba/Nb suite of rocks were emplaced (Ba/Nb>150), which are 527
absent from the Eocene and Oligocene suites. These high Ba/Nb and La/Nb rocks are almost 528
entirely small-volume andesite-dacite lavas geographically restricted to the northern GoC 529
(western Sonora, and Baja California); some published whole-rock isotopic data (e.g., Mora-530
Klepeis & McDowell, 2004) indicate relatively radiogenic Sr (87Sr/86Sr = 0.70564) and 531
unradiogenic Nd (143Nd/144Nd = 0.51258) compositions compared to other volcanics in the 532
region. The high Ba/Nb ratios and Sr and Nd isotopic compositions are most similar to 533
Paleozoic to Proterozoic silicic orthogneisses from north central Chihuahua (Cameron et al., 534
1992). We raise the possibility that these high Ba/Nb intermediate composition rocks 535
emplaced since the Early Miocene are small-volume crustal partial melts of potentially 536
Paleozoic-Proterozoic crustal materials along the southwestern edge of the North American 537
craton. 538
539
We therefore conclude that the dominant period of andesitic volcanism focussed around the 540
margins of the GoC, is at the regional scale, the product of magma mixing. Our model 541
relieves issues around the apparent lack of geochemical discrimination of interpreted syn- and 542
post-subduction (<14 Ma) magmatism in the region (Till et al., 2009) as the entire Oligocene-543
Miocene igneous record was unrelated to subduction. Evidence for this comes from 544
uncontaminated Early Miocene basalts that show within-plate trace element compositions 545
(Fig. 7). A similar asthenospheric fingerprint has also been recognised by Cameron et al. 546
(1989) for Oligocene basaltic andesites (SCORBA) erupted across the northern SMO. Based 547
on the chemical variation within the mid-Miocene (~18-13 Ma) suite, magma end-members 548
are interpreted to be: 1) an asthenosphere-derived, within-plate alkali basalt similar to those 549
erupted in the region during the Late Miocene to Quaternary (e.g., Luhr et al., 1995; Pallares 550
et al., 2007), 2) crustal partial melts derived from Mesozoic-Cenozoic igneous crust, 551
18
including the Jurassic-Cretaceous Peninsular Ranges Batholith (e.g., Silver & Chappell, 552
1998) and Eocene-Oligocene batholithic rocks, as informed by inherited zircon ages (Bryan 553
et al., 2008; Godinez et al., 2010; Ramos Rosique, 2012); and 3) small-volume crustal partial 554
melts of dacitic composition derived from Proterozoic-Paleozoic crust (highlighted by the 555
extreme Ba/Nb ratios, Fig. 7). Importantly, Figure 7 illustrates no apparent temporal change 556
in basaltic magma composition from the Early to Late Miocene. Basaltic rocks with more 557
“arc-like” signatures are interpreted to be either crust/lithospheric mantle contaminated, or 558
possibly derived from a previously subduction-modified lithospheric mantle, but where 559
lithospheric mantle melting was in response to basaltic magma input from the asthenosphere 560
and decompression due to lithospheric thinning. 561
562
How was mixing promoted? We believe that extensional faulting causing the large-563
magnitude crustal thinning during the Middle Miocene began actively disrupting bimodal 564
magma systems in the GoC region (Fig. 8). Magma inputs (i.e. basaltic and rhyolitic magmas 565
erupted during the Early Miocene) had not fundamentally changed, and regionally, away 566
from the Gulf, basaltic and rhyolitic magmas generally continued to erupt (Fig. 5). However, 567
locally around the GoC where extension was now being focussed and was greatest, this 568
tectonic influence had a major effect on the composition of magmas being erupted. 569
Consequently, there was a spatial-temporal focussing of andesitic volcanism along the 570
nascent GoC rift. Importantly, this switch from widespread Early Miocene bimodal 571
volcanism to more localised andesitic volcanism during the Middle Miocene (Fig. 8) appears 572
to correspond to a switch from wide to narrow rifting beginning ~18 Ma. This switch had an 573
important effect on silicic magma generation rates, which appears to have significantly 574
decreased at ~19 Ma (Fig. 3) as mafic magma inputs to the crust became more focussed in the 575
Gulf region. The Early Miocene grabens that had developed across a ~500 km width of 576
western Mexico and which had significant volcanic fills, became magmatically abandoned by 577
~18 Ma (e.g., Bolaños Graben, Ramos Rosique, 2012), and continued magmatism was now 578
largely restricted to the GoC and its immediate margins (Fig. 8). The spatial-temporal-579
compositional patterns of magmatism through the Miocene of western Mexico are thus 580
providing us with an important record of changes to the extensional style, intensity and 581
location. 582
583
19
Active extensional faulting, particularly during the period 18-13 Ma, modified the erupted 584
magma compositions that became more intermediate in composition. Concomitant with this 585
compositional change was a change in eruption styles that were dominantly effusive, 586
producing scattered lavas and domes as well as dyking. Eruption location shifted to be 587
concentrated around the nascent GoC rift. We therefore conclude the Comondú Group 588
andesites are a larger-scale regional example of syn-volcanic extensional fault-driven magma 589
mixing processes such as described by Johnson & Grunder (2000). 590
591
Concluding Remarks 592
593
Magmatism is used widely as an instant tracer of the geodynamic setting, and magma 594
composition as a proxy for a specific tectonic setting (e.g., arc/supra-subduction zone, slab 595
melting, slab tearing, crustal rifting). In particular, calc-alkaline chemistries ± relative Nb 596
depletions and blind use of tectonic discrimination diagrams often form the basis for 597
interpreting an active subduction-related setting (e.g., Zhu et al., 2012). However, subduction 598
leaves a strong chemical imprint on the lithosphere such that subduction-related geochemical 599
signatures can persist for 100's of millions of years (e.g., Morris & Hooper, 1997) and stamp 600
the chemistry of new magmas produced at much younger times and unrelated or far removed 601
from any active subduction zone. The low-Ti flood basalts of the Karoo and Paraná-Etendeka 602
LIPs are classic examples of this (Duncan, 1987; Ewart et al., 1998). 603
604
Our study demonstrates the importance of understanding the regional tectonic setting both at 605
the time of volcanism and for the preceding and following magmatic history in interpreting 606
the origin of the middle Miocene Comondú Group andesites. As shown in many previous 607
studies, when considered in isolation and focussing only on the general chemical 608
characteristics, these andesitic rocks do resemble the products of a supra-subduction zone arc 609
(e.g., Hausback, 1984; Till et al., 2009). However, the andesites are spatially restricted to the 610
nascent GoC rift, there was no preceding history of andesitic arc volcanism in this region, the 611
region was clearly undergoing wide rifting and bimodal volcanism immediately beforehand, 612
and their emplacement coincides in space and time with focussed rifting and large magnitude 613
(~100%) crustal thinning. The stratigraphic relationships and architecture of Middle Miocene 614
andesite-dacite lavas and domes (proximal and vent volcanic facies) being buried by thick 615
20
successions of syn-volcanic sedimentary deposits including sandstones demonstrates lava 616
emplacement into actively subsiding rift basins. This facies architecture is more consistent 617
with biostratigraphic and paleontological evidence for earlier basin rifting and subsidence and 618
a marine incursion into the GoC by 12 Ma (Helenes, 2012). Andesite-dacite lavas and domes 619
are subordinate to the volcanic-derived sedimentary rocks and isolated to scattered 620
lavas/domes thus should not be used to define an arc. We extend the recent arguments by 621
Castillo (2008) that the compositionally diverse suite of mafic rocks including adakitic and 622
high-Nb basalts erupted in Baja California are unrelated to subduction or slab melting in the 623
Late Miocene, but that asthenospheric mantle involvement in regional magmatism occurred 624
much earlier, being prevalent in the Oligocene (Cameron et al., 1989) and Early Miocene 625
(Fig. 7). 626
627
Our fundamental conclusion is that the Comondú Group does not represent a supra-628
subduction zone arc and this challenges all previous studies on the Comondú Group. We 629
caution that andesites in ancient extended continental margins may bear no relationship to 630
active subduction and magma chemistry cannot be solely relied on to interpret tectonic 631
setting. Alternatively, as shown here, the andesitic volcanism and the switch from widespread 632
bimodal to more focussed andesitic volcanism can potentially provide much more important 633
insights into the extensional history of a continental margin. 634
635
Finally, our study raises questions about genetic links between the Sierra Madre Occidental 636
silicic volcanism and the Gulf of California, as these have previously been considered two 637
separate phenomena. We suggest the Sierra Madre Occidental Silicic LIP is the pre- to syn-638
rift volcanic event to the GoC rift. This is because from ~30 Ma and certainly by 24 Ma, 639
SMO volcanism and extension were overlapping spatially and temporally. It therefore raises 640
the question of when did rupturing of the Gulf of California really begin? Was it at the onset 641
of wide rifting and bimodal volcanism at ~30-24 Ma, or at the onset of narrow rifting at ~18 642
Ma? Regardless, the new regional timing and stratigraphic-structural relationships evident 643
along the eastern margin of the GoC show that the onset of rifting to open the GoC was 644
clearly older than the ~12.3 Ma end of subduction along this segment of the western North 645
American margin. Consequently, the GoC has had a much longer history of rifting and 646
21
associated magmatism before the onset of seafloor-spreading, and may not be such an 647
anomalously rapid zone of crustal rupture (cf. Umhoefer, 2011). 648
649
Acknowledgements 650
651
S.B. has been supported by a Vice Chancellor’s Research Fellowship from QUT. We 652
acknowledge support of grant CONACyT 82378 to LF. The submarine samples shown on 653
Figure 1 were collected on cruises supported by the US NSF (grants 0203348 and 0646563 to 654
co-PIs Peter Lonsdale and Paterno Castillo), as well as grants to Peter Lonsdale and Jared 655
Kluesner for the BEKL, ROCA and DANA cruises in the Gulf of California. Discussions 656
with John Clemens, Georg Zellmer and Christoph Schrank on aspects of this manuscript are 657
appreciated. We thank Jim Cole and Paterno Castillo and Susan Straub for constructive 658
reviews of this manuscript. 659
660
22
661
662
Figure Captions 663
664
Figure 1. Tectonic map of northwestern Mexico showing the lithospheric variation across the 665
region including unextended and extended continental regions, and transitional to new 666
oceanic crust formed by the propagating spreading centre in the Gulf of California. 667
Superimposed on this tectonic map are the preserved extents of the Oligocene-Early Miocene 668
silicic-dominant volcanic activity of the SMO (Ferrari et al., 2002; Bryan et al., 2008), and 669
the dominantly bimodal phase during the Early Miocene that coincided with the wide 670
development of grabens and rift basins (McDowell et al., 1997; Ferrari et al., 2002) and a 671
restricted belt of metamorphic core complexes in the state of Sonora (Nourse et al., 1994; 672
Wong et al., 2010). Distribution of Comondú Group andesites from Umhoefer et al. (2001). 673
Offshore Miocene igneous rocks from Ferrari et al. (2012; in preparation). Rift basin 674
segments to the Gulf of California are labelled; Car., Carmen, and Pesc., Pescadero basins. 675
NAY., Nayarit; EPR, East Pacific Rise; H., Hermosillo. Red boxed areas near Mazatlán and 676
Tepic refer to locations of photographs in Figure 6. 677
678
Figure 2. Total alkali-silica diagram (TAS; Le Maitre et al., 1989) for Oligocene-Miocene 679
(~34-13 Ma) igneous rocks from NW Mexico, plotted on an anhydrous basis, showing the 680
spectrum of compositions generated during this broad interval. The field of the Southern 681
cordillera basaltic andesites (SCORBA; Cameron et al., 1989) is from McDowell et al. 682
(1997). Diagram contains 606 analyses. 683
684
Figure 3. Probability density plot of igneous ages from western Mexico for the period 40-0 685
Ma. Dated rocks have been grouped into four main compositional groupings - basalt 686
(includes basaltic andesites and tholeiitic, calc-alkaline and alkaline varieties), andesite 687
(includes calc-alkaline and adakitic-type compositions), dacite and rhyolite (includes high-688
silica rhyolites and peralkaline compositions). Important features of the diagram are: 1) the 689
silicic dominant character of the Oligocene ignimbrite pulse; 2) the appearance of basalts 690
during the Oligocene silicic ignimbrite pulse and increase in the frequency of basaltic 691
eruptions up to the start of the Early Miocene pulse; these basaltic eruptions correspond to 692
23
SCORBA of Cameron et al. (1989); 3) the bimodal character of the Early Miocene pulse; 4) 693
the increase in andesitic compositions beginning ~20 Ma until ~13 Ma; 5) the abrupt decline 694
in rhyolite magma generation and eruption beginning ~19 Ma when andesite-dacite eruptions 695
were more predominant; 6) an abrupt return to bimodal volcanism at ~13 Ma with a 696
concomitant decline in andesitic eruptions; and 7) after 10 Ma, volcanism becomes 697
fundamentally basaltic to coincide with the onset of seafloor spreading in the GoC beginning 698
as early as ~6 Ma (Lizarralde et al., 2007). Diagram based on 1496 radiometric ages, and age 699
data plotted using Isoplot (Ludwig, 2003). 700
701
Figure 4. SiO2 histograms of Oligocene-Miocene igneous rocks of NW Mexico. A) The 702
Oligocene pulse of the Sierra Madre Occidental (n = 346 analyses)is silicic dominant and in 703
particular, high-silica rhyolite-dominant. Many of the ignimbrites and lavas are high-silica 704
rhyolites (>75 wt% SiO2), and it is seldom appreciated that high-silica rhyolites, although 705
common in intracontinental settings, are uncommon in continental margin arcs (Hildreth & 706
Fierstein 2000). B) The Early Miocene bimodal pulse of the Sierra Madre Occidental (n = 707
106 analyses). C) Middle Miocene volcanism that is principally focussed around the margins 708
of the Gulf of California (n = 159 analyses), where in contrast to previous episodes, silicic 709
volcanism was subdued, and basaltic andesite to dacite compositions dominant. D) Late 710
Miocene volcanism (n = 321 analyses) is marked by a return to significant silicic volcanism 711
and true basaltic compositions. Note change in vertical scale for each time interval. 712
713
Figure 5. Time-space plots of igneous activity across western Mexico and around the Gulf of 714
California region. In A) age data are for the region between 20° and 26° N; and in B) between 715
26° and 30°N (see boxed areas in inset). Dated samples from Baja California have been 716
displaced to their “pre-rift” positions whereby for each sample the latitude has been adjusted 717
by 2.3°S and the longitude by 3.9°E. The approximate position of the Gulf of California in 718
both sections is marked by the grey arrow. Igneous rocks have been grouped into four main 719
compositional groupings - basalt (includes basaltic andesites and tholeiitic, calc-alkaline and 720
alkaline varieties), andesite (includes calc-alkaline and adakitic-type compositions), dacite 721
and rhyolite (includes high-silica rhyolites and peralkaline compositions). See text for 722
discussion. Inset map of western Mexico from Google Earth. 723
724
24
Figure 6. Field relationships between the SMO and GoC showing the progressive tilting and 725
deformation of the Early Miocene silicic ignimbrite successions in the central and western 726
SMO. A) view west from the Mazatlán-Durango old highway (23° 39.927'N 105° 43.340'W) 727
to the flat-lying ~23 Ma Espinazo-El Salto sequence of Waitt (1970) and McDowell & Keizer 728
(1977), and part of the undeformed core to the SMO. B) Tilted Early Miocene ignimbrite 729
successions approximately 30 km to the west of A viewed to the north of the Mazatlán-730
Durango old highway (23° 24.329'N 105° 54.634'W). C) Tilt blocks of Early Miocene 731
ignimbrites 27 km east of Tepic with ignimbrites dipping up to 35°E (21° 34.030'N 104° 732
37.880'W). The ignimbrites continue to the SSE below younger lavas and where they crop 733
out again have been dated at 20.46 Ma (Ferrari et al., in preparation). 25 km to the NW are 734
flat-lying (post-extensional) basaltic lavas dated between 11 and 10 Ma and form part of an 735
extensive series of flat-lying 9-12 Ma basaltic lavas along the coastal plain of Nayarit and 736
Sinaloa (see Fig. 1). These field relationships bracket the timing of major extension in the 737
GoC to between ~18 and ~12 Ma. 738
739
Figure 7. Ba/Nb vs La/Nb plots for Oligocene-Miocene (34-13 Ma) igneous rocks from NW 740
Mexico, with data organised into compositional groupings: basalts (<52 wt% SiO2); basaltic 741
andesite (52-56 wt% SiO2); andesite (56-63 wt% SiO2); and dacite-rhyolite (>63 wt% SiO2). 742
Stars labelled 1, 2 and 3 are E-MORB, N-MORB (Sun & McDonough, 1989) and averaged 743
calc-alkali andesite compositions (based on 2865 analyses from the GeoRoc database), 744
respectively. PRB, is the average composition of the Peninsular Ranges Batholith from Silver 745
& Chappell (1988). Grey shaded region labelled LMB represents compositions of Late 746
Miocene alkali basalts from the region (e.g., Henry & Aranda Gomez, 2000; Pallares et al., 747
2007). The anomalously high Ba/Nb ratios (>200) approach values for Paleozoic-Proterozoic 748
silicic orthogneiss and Tertiary plutonic-derived, glass-rich silicic xenoliths analysed from 749
the La Olivina basaltic centre in northern Chihuahua by Cameron et al. (1992), which have 750
Ba/Nb ratios between ~90 and 371 and 100-726, respectively. Compare with figure 12 of 751
Ewart et al. (1992). 752
753
Figure 8. A schematic model for the Early to Middle Miocene tectonomagmatic evolution of 754
western Mexico and the initial stages of rifting and opening of the Gulf of California. The 755
cross-sections illustrate: 1) the change from wide to narrow rifting and the focussing of 756
25
magmatism into the gulf region, which occurred ~18 Ma; 2) the general crustal structure such 757
as the development of thickened crust beneath the Sierra Madre Occidental core, and 758
requisite thinning of the lower crust and mantle lithosphere in the gulf region beginning at 759
least by the Early Miocene; 3) the main mafic magma source regions - the involvement of 760
asthenosphere-derived basaltic magmas is evident from at least the Early Miocene (e.g., see 761
Fig. 7) whereas more "calc-alkali” or subduction signature-bearing basalts are either 762
crust/mantle contaminated, or have been derived from subduction-modified lithospheric 763
mantle; 4) Early Miocene silicic magmas are predominantly mid-upper crustal melts with 764
significant material contributions from earlier formed batholithic type granitic rocks (as 765
informed by the zircon inheritance, Bryan et al., 2008; Ramos Rosique, 2012; Ferrari et al., 766
2012; in preparation); 5) for the Middle Miocene, additional silicic magma source regions 767
include the lower crust to account for the anomalously high Ba/Nb dacites/rhyolites (Fig. 7) 768
where lower crustal melting may have been promoted by thinning, decompression and 769
basaltic intrusion; and 6) active faulting interrupts bimodal magma systems (see inset) 770
existing regionally (red= rhyolite, black = basalt) to produce the andesitic magmas (green) in 771
the Middle Miocene around the margins of the Gulf of California. 772
773
774
775
26
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