José Rosas-Elguera

Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, Mexico

The middle to late Cenozoic tectonic-magmatic evolution of the Sierra Madre Occidental volcanic province south of the Tropic of Cancer is summarized and analyzed for the first time, based on new geologic and structural work and published information. In the eastern part of the study region (Mesa central physiographic province) silicic volcanism occurred in a short-lived episode culminating at ca. 30 Ma and was followed by crustal-scale extension between 30 and 27 Ma. In the western part of the study area (Sierra Madre Occidental physiographic province) a voluminous episode of ignimbrite volcanism at 24‐21 Ma was succeeded by east-west extension that produced regularly spaced grabens affecting only the upper crust. In the westernmost part of the study region, an andesitic to rhyolitic arc, formed between 17 and 12 Ma, was affected by crustal-scale, north-northwest‐trending, extensional faulting, leading to the formation of the Gulf of California. In the Mesa central the maximum extension was oriented approximately east-west and amounted to ~20%. In the eastern Sierra Madre Occidental physiographic province extension was only 8% and oriented approximately east-west. We observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late Oligocene, early Miocene, and late Miocene time. Comparison with the offshore tectonics indicates that the first two tectonic-magmatic pulses coincide with periods of fast spreading at the Pacific-Farallon boundary, south of the Shirley fracture zone. We propose that increases in the spreading rate are related to periods of high subduction rate, which in turn correspond to episodes of retreating subduction. A retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge, producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate. Boundary conditions (i.e., plate tectonics) ultimately determined timing, magnitude, and orientation of extension, whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism.

Ignimbrite flare‐up and deformation in the southern Sierra Madre Occidental, western Mexico: Implications for the late subduction history of the Farallon plate

geologic mapping, and structural data for the southern part of the SMO demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare-up in Oligocene (31.5–28 Ma) and early Miocene (23.5–20 Ma) time, and that extensional deformation occurred mostly before the transfer of Baja California to the Pacific plate. Extensive ignimbrite successions, with 40 Ar/ 39 Ar ages clustering at � 23 and � 21 Ma, cover most of the southern SMO, thus correlating in age with ignimbrites exposed in southern Baja California and central Mexico. Grabens with a 020� to N-S orientation developed in the east almost concurrently with this volcanic episode. Half grabens and NNW striking listric normal fault systems formed at the end of middle Miocene as far as 150 km from the present coast. A belt of left-lateral transpressional structures formed along the southern boundary of the SMO during the same period. We link these magmatic and tectonic events to the evolution and dynamics of the Farallon and North America plates during the Miocene. Particularly, we propose that a first detachment of the lower part of the Farallon plate in early Miocene time produced a transient thermal event and partial melting of the crust via mafic underplating. Middle Miocene extension would be related to a second detachment event, resulting from the slowing subduction that preceded the final capture of the Magdalena microplate by the Pacific plate at 12.5 Ma. Transpression at the southernmost end of the SMO occurred along the inland projection of the MagdalenaCocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the Magdalena plate. INDEX TERMS: 5480 Planetology: Solid Surface Planets: Volcanism (8450); 8150 Tectonophysics: Evolution of the Earth: Plate boundary—general (3040); 8109 Tectonophysics: Continental tectonics—extensional (0905); KEYWORDS: ignimbrite flare-up, Sierra Madre Occidental, western Mexico, extensional tectonics, slab detachment

Late Miocene to Quaternary extension at the northern boundary of the Jalisco Block, western Mexico: The Tepic-Zacoalco Rift revised

In the last decade several tectonic models have considered the Jalisco block (JB) as an incipient microplate which is rifting away from mainland Mexico since Pliocene time due to an eastward “jump” of the East Pacific Rise. These models predict normal and right-lateral faulting along the northern boundary of the JB, called the TepicZacoalco rift (TZR). However, the Plio-Quaternary kinematics of the Jalisco block has remained unclear due to the scarcity of structural data along its boundaries. We present a new picture of the structure, the kinematics and time of deformation along the TZR obtained by geological and structural mapping integrated with subsurface stratigraphic data provided by deep geothermal drilling. What has previously been defined as the TZR is actually a combination of different fault systems developed during Late Miocene (12‐9 Ma), Early Pliocene (5.5‐3.5 Ma) and, to a lesser extent, in Late Pliocene to Quaternary times. These structures can be grouped in three branches: 1) a northwestern branch, named the Pochotitan fault system, consisting of listric faults belonging to the Gulf Extensional Province; 2) a central branch made of en echelon grabens which reactivated the boundary between the JB and the Sierra Madre Occidental; 3) a southern branch constituted by detachment faults located inside the Jalisco block. The Pochotitan fault system is composed of northnorthwest‐trending, high angle normal faults which tilt up to 35° towards east-northeast blocks of the Sierra Madre Occidental succession. These faults accommodate at least 2,000 m of vertical displacement related to 12‐9 Ma “Protogulf” extension. The central branch consists of two composite grabens developed along an older transcurrent deformation zone. The western one, the Compostela-Ceboruco graben, is a complex asymmetrical depression developed during Late Miocene and Pliocene time with vertical displacement exceeding 2,000 m. Toward the east is the Plan de BarrancasSanta Rosa graben, a west-southwest‐trending and 30-km-wide depression, bounded to the north by the Santa Rosa-Cinco Minas fault and to the south by the Plan de Barrancas fault and its buried southeastern prolongation detected by geophysical studies under the Tequila volcano and the southwestern part of La Primavera caldera. The graben displays a total vertical displacement of ~550 m mainly achieved during early Pliocene time. The southern branch is formed by the Amatlan de Canas half-graben

Continental boundaries of the Jalisco block and their influence in the Pliocene-Quaternary kinematics of western Mexico

Extensional faulting observed in southwestern Mexico has been related to the incipient rifting of the Jalisco block from the Mexican mainland since the Pliocene. On the basis of new structural and geophysical data, we propose that (1) the continental boundaries of the Jalisco block are ancient structures reactivated since the Pliocene at a low (<1 mm/yr) rate of deformation, and (2) Pliocene-Quaternary extensional faulting at the edges of Jalisco block is a basement-controlled intraplate deformation related to plate boundary forces rather than to active continental rifting. The Jalisco block boundaries first developed in response to the uplift of the Puerto Vallarta batholith in pre-Neogene time and underwent a complex contractile deformation before the Pliocene. During Pliocene-Quaternary times north-northeast extension reactivated the northern boundary, forming the Tepic-Zacoalco rift, whereas east-southeast extension formed the northern Colima rift. South of the Colima volcano, active extension is found only west of the so-called southern Colima rift and partly reactivates old northeast-trending basement faults. The parallelism between the subducted Rivera-Cocos plate boundary zone and the eastern neotectonic boundary of the Jalisco block supports east-southeastward motion of the southern Mexican blocks induced by the differential motion and oblique subduction of the Cocos and Rivera plates. On the other hand, we envisage Pliocene-Quaternary extension along the northern boundary as an upper-plate response to the low convergence rate and the steep subduction angle of the Rivera plate.

Paleomagnetic data from the Trans-Mexican Volcanic Belt: implications for tectonics and volcanic stratigraphy

We report a paleomagnetic and rock-magnetic study of Miocene volcanic rocks from the Trans-Mexican Volcanic Belt. A total of 32 sites (238 oriented samples) were collected from three localities: Queretaro, Guadalajara and Los Altos de Jalisco basaltic plateaux, which span from 11 to 7.5 Ma. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Microscopic observation of polished sections shows that the main magnetic mineral is Ti-poor titanomagnetite associated with exsolved ilmenite. Continuous susceptibility measurements with temperature yield in most cases reasonably reversible curves with Curie points close to that of magnetite. Judging from the ratios of hysteresis parameters, it seems that all samples fall in the pseudo-single domain (PSD) grain size region, probably indicating a mixture of multidomain (MD) and a significant amount of single domain (SD) grains. Based on our paleomagnetic and available radiometric data, it seems that the volcanic units have been emplaced during a relatively short time span of 1 to 2 My at each locality. The mean paleomagnetic directions obtained from each locality differ significantly from that expected for the Middle Miocene. The mean paleomagnetic direction calculated from 28 sites discarding those of intermediate polarity is I = 32.46°, D = 341.2°, k = 7.2 and α95 = 11.6°. Comparison with the expected direction indicates some 20° anticlockwise tectonic rotations for the studied area, in accordance with the proposed left-lateral transtensional tectonic regime already proposed for this period.

Combined Paleomagnetic and Petromagnetic Study of the Upper Cretaceous Volcanic Sequence in Western Mexico: Implications for Tectonics and Magnetostratigraphy of the Jalisco Block

Results of detailed paleomagnetic studies of the Upper Cretaceous Autlan volcanic sequence from the Sierra Cacoma area (Jalisco block, western Mexico) are reported. The 67.4 ± 1.2 Ma whole-rock K-Ar date for these lavas and the magnetic polarity stratigraphy indicate that flows were emplaced after the Cretaceous Normal Superchron and just prior to the Cretaceous-Tertiary boundary. Fifteen sites (~150 oriented samples) were collected along two vertical profiles. Rock-magnetic experiments permit identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements versus temperature in most cases yield reasonably reversible curves with Curie points close to magnetite. Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain grain-size region, probably indicating a mixture of multi-domain and a significant amount of single-domain grains. Reliable paleomagnetic directions were obtained for 14 sites, corresponding to 2 normal and 12 reverse polarity sites. Lowermost flows in the sequence yield reverse polarity magnetization, and probably formed during Chron 31r of the reference geomagnetic polarity time scale. The remaining flows, including those belonging to radiometrically dated sites, correspond to Chron 30n. These tentative magnetic correlations suggest that the entire volcanic sequence was emplaced during a time span of about 2 Ma. The mean paleo-direction obtained from 14 sites is Inc = 44.2°, Dec = 320.6°, k = 45, a95 = 6.0°. The mean inclination is in reasonably good agreement with the expected value for 65 and 70 Ma, as derived from reference poles for the North American craton. Declination is significantly different from expected (D = 345.8° for 65 Ma and D = 346.2° for 70 Ma), which suggests a counterclockwise tectonic rotation of about 25° for this region within the Jalisco block.

A detailed paleomagnetic and rock-magnetic investigation around Cretaceous-Paleogene boundary: the Autlan (Western Mexico) volcanic sequence revisited

We present a detailed rock-magnetic and paleomagnetic survey from Autlan volcanic succession in western Mexico. The principal aim of this study is to extend paleomagnetic data from Autlan lavas in order to confirm vertical-axis rotation observed in reconnaissance study and to evaluate long-term variation of the geomagnetic field strength based on existing and global data. The mean inclination (44.7°) is in agreement with the expected inclination for 60 and 70 Ma, as derived from available reference poles for the North American craton. The declination (333.6°), however, is significantly different from those expected, which suggests a statistically significant counterclockwise tectonic rotation ranging between 10° ± 6° and 14° ± 7°. As a measure of paleosecular variation (PSV), we obtained a geomagnetic field dispersion of 9.6° (upper and lower limits: 7.2°–11.9°) in perfect agreement with the previously published PSV compilation of selected Cretaceous data from lavas. The mean virtual dipole moments available for Autlan lavas are about 65% of the present geomagnetic axial dipole but are in reasonably good agreement with other comparable quality determinations between 5 and 90 Ma. This reinforces the hypothesis that low geomagnetic field strengths persisted for the entire Jurassic extending into the Upper Cretaceous.

Rock-magnetic and paleomagnetic survey on dated lava flows erupted during the Bruhnes and Matuyama chrons: the Mascota Volcanic Field revisited (Western Mexico)

A rock magnetic and paleomagnetic investigation was performed on some selected, radiometrically dated lava flows from the Mascota Volcanic Field (MVF), western Trans- Mexican Volcanic Belt. A set of rock-magnetic experiments and standard paleomagnetic analysis were carried out on 19 sites spanning the time interval from 2268 to 72 kyr. The paleomagnetic directions are anchored to absolute radiometric ages while no such information was available in previous studies. This makes possible to correctly evaluate the fluctuation of Earth’s magnetic field from Pliocene to Pleistocene and reveal the firm evidence of possible Levantine excursion. Both Ti-poor and Ti-rich titanomagnetites seem to carry the remanent magnetization with Curie temperatures ranging from 350°C to 537°C. Thirteen flows correspond to the Brunhes chron, one of them exhibits transitional directions, while the remaining six sites belong to the Matuyama chron. New and existing dataset for MVF were used to estimate the paleosecular variation parameters. The selected data include 35 Plio-Quaternary lava flows. After excluding the poor quality data, as well as the transitional directions, the mean paleodeclination is 356.1° and oaleoinclination 39.9°, which agree well with the geocentric axial dipole (GAD) and the expected paleodirections for the Plio-Pleistocene, as derived from the reference poles for the stable North America. The corresponding mean paleomagnetic poles are paleolongitude 226.7° and paleolatitude 86.0°. The virtual geomagnetic pole scatter for the MVF is 15.2°, which is consistent with the value expected from model G at latitude of 20° (this model provides an interpretation of the paleosecular variation at different latitudes for the time of interest). The combined paleomagnetic data, supported by positive reversal test, indicate no paleomagnetically detectable vertical-axis rotations in the study area. The evidence of one transitional directions was detected, which may correspond to the Levantine excursion (360-370 kyr) or unnamed event between 400-420 kyr.