Full vector magnetic dating of some pyroclastic rocks associated to the Colima volcano, western Mexico

"Se presentan los resultados de un estudio paleomagnético y de magnetismo de rocas realizado en rocas de flujos piroclásticos y algunos fragmentos de cerámica pre-Colombina asociados al complejo Volcánico de Colima. Con la finalidad de estimar la mineralogía magnética y determinar tanto la magnetización remanente característica, así como la intensidad absoluta, se realizó un conjunto de experimentos y mediciones magnéticas. Se reporta una edad absoluta determinada mediante el fechamiento magnético de dos erupciones volcánicas asociadas al volcán Colima (volcán de Fuego). Esta es la primera vez que el modelo de predicción de campo magnético SHADIF.14K, basado en el vector completo (declinación, inclinación y paleointensidad absoluta), es utilizado en depósitos de flujos piroclásticos intercalados con cerámicas. Los resultados obtenidos concuerdan con las edades radiométricas (C14) disponibles y constituyen una determinación más precisa de la edad del momento de enfriamiento de los flujos de lava.""A combined rock-magnetic and paleomagnetic researches were carried out on some pyroclastic rocks and pre-Columbian pottery fragments associated to the Colima Volcanic Complex. A full battery of magnetic measurements was performed in order to estimate magnetic mineralogy and to determine both the directions and absolute intensities of the characteristic remanent magnetization. In this paper we report absolute age determination using magnetic dating for two volcanic eruptions associated to the Colima volcano (Volcán de Fuego). The geomagnetic field prediction model SHA.DIF.14K, based on full vector (declination, inclination and absolute paleointensity), was used for first time in pyroclastic flows deposits and intercalated potteries. The results obtained agree reasonably well with available radiometric (C14) data, them provide a more accurate age determination of the cooling moment for the lava flows.

Paleomagnetism and aeromagnetic survey from Tancitaro volcano (Central Mexico) - paleo-secular variation at low latitudes during the past 1 Ma

"El volcán Tancítaro (TV) forma parte del campo volcánico monogenético MichoacánGuanajuato (MGVF) en el sector centro-oeste del Eje Volcánico Trans Mexicano (TMVB). Los resultados de un estudio paleo magnético de flujos de lava del volcán Tancítaro fechados radiométricamente, se utilizaron para investigar la variación paleo secular (PSV) y el campo promedio temporal (TAF) para latitudes bajas. El rango de fechado Ar-Ar fue de 70 a 960 kaños considerando los Chrones de polarización de Brunhers y Matuyama. Todas las muestras arrojaron una polaridad de magnetización normal bien definida. Dos flujos se correlacionaron con la polaridad del Evento Jaramillo, lo cual provee un marcador útil para la actividad volcánica en el MGVF. Para los análisis del PSV y TAF las paleo direcciones se combinaron con resultados anteriores de alta precisión. El estudio aeromagnético alrededor del volcán Tancítaro se caracterizó por una serie de anomalías tanto positivas como negativas. El Volcán Tancítaro presenta una gran anomalía positiva que sugiere la presencia de una gran fuente subterránea. El análisis espectral de este campo anómalo da una estimación promedio hacia la parte alta de los cuerpos que la originan entre 2-3 km.""The Tancitaro volcano (TV) is part of the Michoacan-Guanajuato monogenetic volcanic field (MGVF) in the central-western sector of the Trans-Mexican Volcanic Belt (TMVB). Results of a paleomagnetic study of radiometrically dated lava flows from Tancitaro volcano were used to investigate the paleosecular variation (PSV) and time averaged field (TAF) at low latitudes. Ar-Ar dates range from ~70 to 960 kyr spanning the Brunhes and Matuyama polarity chrons. All samples yielded well defined normal polarity magnetization. Two flows are correlated to the Jaramillo polarity event, which provide a useful marker for the volcanic activity in the MGVF. For the PSV and TAF analysis, paleodirections were combined with previously reported high standard results. The aeromagnetic survey around the Tancitaro volcano was characterized by a trend of regional anomalies over the volcanic structures. The residual field showed several positive and negative anomalies. The Tancitaro volcano is marked by a broad positive anomaly suggesting the presence of a large underground source. Spectral analysis of this anomaly field gives an average estimate to the top of the source bodies between 2-3km.

Alkaline lavas in the volcanic front of the Western Mexican Volcanic Belt: Geology and petrology of th Ayutla and Tapalpa volcanic fields

The Plio-Quaternary Ayutla and Tapalpa volcanic fields in the volcanic front of the western Mexican Volcanic Belt (WMVB) contain a wide variety of alkaline volcanic rocks, rather than only calc-alkaline rocks as found in many continental arcs. There are three principal rock series in this region: an intraplate alkaline series (alkali basalts and hawaiites), a potassic series (lamprophyres and trachylavas), and a calc-alkaline series. Phlogopite-clinopyroxenite and hornblende-gabbro cumulate xenoliths from an augite minette lava flow have orthocumulate textures. The phlogopite-clinopyroxenite xenoliths also contain apatite and titanomagnetite and probably formed by accumulation of minerals fractionated from an augite minette more primitive than the host. The intraplate alkaline series is probably generated by decompression melting of asthenospheric mantle as a result of corner flow in the mantle wedge beneath the arc. Alkaline magmas may be common in the WMVB as a result of prior metasomatism (during Tertiary Sierra Madre Occidental magmatism) of the Mexican sub-arc mantle. Generation of the more evolved andesites and dacites of the calc-alkaline series is due to either combined assimilation and fractional crystallization (AFC) or magma mixing. The preponderance of alkaline and hydrous lavas in this region demonstrates that these lava types are the norm, rather than the exception in western Mexico, and occur in regions that are not necessarily associated with active rifting

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 that 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 Tepic-Zacoalco 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 Pochotitán 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 Pochotitán fault system is composed of north-northwest-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 Barrancas-Santa 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 Amatlán de Cañas half-graben and the Ameca-San Marcos detachment fault. They are south- to southwest-dipping listric normal fault systems with a minimum of 1,400 m of vertical displacement largely produced during the Pliocene. Only the San Marcos faults show clear geologic evidence of Quaternary tectonic activity. The great majority of the 295 measured mesofaults of Late Miocene to Quaternary age have pitches higher than 45° and inclinations ranging between 45° and 75°, typical of normal faults. The paleo-stress field has been computed by fault-slip data inversion and cinder cone alignment at 40 locations and the computed stress tensors are always extensional (vertical maximum principal stress). The average direction of extension (sHmin) is 72° for the Late Miocene extension in the Gulf area, whereas for Pliocene and Quaternary time, it ranges from 35° to 2°. Displacement of dated geologic units constrains an average minimum deformation rate for each fault system which decreases from 0.75 mm/yr for the Late Miocene to 0.1 mm/yr for the Quaternary. These results confirm the absence of strike-slip deformation along the TZR in Plio-Quaternary times and indicate that the JB is not actively separating from the Mexican mainland. In our view, the TZR represents an intraplate deformation zone which reactivated the tectonic boundary between the Sierra Madre Occidental and the JB. These deformations are more likely related to plate boundary forces rather than to an eastward relocation of the East Pacific Rise under continental Mexico. The small divergent motion between the Rivera and Cocos plate and the steep subduction of the Rivera plate can account for the deformation observed at the boundaries of the Jalisco block

Tectonic control of the volcano-sedimentary sequence of the Chapala graben, Western Mexico

The Chapala graben forms part of a regional system of intra-arc and half-grabens located along the western and central portions of the Mexican Volcanic Belt. Results of a stratigraphic and tectonic study of the thick, widespread volcano-sedimentary sequence around the Chapala graben are reported. The study has concentrated on the distribution of lacustrine deposits and on the stratigraphy, geochronology, and deformation of the sequence. The volcano-sedimentary record suggests a probable link between the Tepic-Zacoalco and Chapala grabens through a basin system developed during late Miocene and early Pliocene time. The stratigraphie sections show a southward spatial migration of the basin (and lake) system. The main unit of the volcano-sedimentary succession is the Chapala Formation, which has been affected by northeast tilting, unlike the lacustrine deposits to the east in the Chapala plain (e.g., Ixtlan area), where the sequence is essentially flat lying. Two distinct units with different structural attitudes, separated by an angular unconformity, can be distinguished in the Chapala Formation. Based on differences between the stratigraphic sections and structural attitudes, we propose a model for development of the Chapala graben that involves a combination of left-lateral and extensional deformation, together with volcanic and erosional processes, all of which have contributed to shape the basin. Furthermore, we suggest that Lake Chapala is the remnant of a large Jalisco paleo-lake in west-central Mexico

Inverse optimal neural control with speed gradient for a power electric system with changes in loads

Western Mexico is characterized by several large-scale tectonic depressions which have been interpreted in terms of active continental rifting, tectonic transpression and coastal sinistral lateral transport of terranes. In this paper we report results of a paleomagnetic study of 148 samples from 22 sites in the Neogene volcanics from the eastern sector of the Chapala graben (western end of the E-W Chapala-Tula fault zone). Characteristic remanent magnetization directions have been isolated after detailed thermal demagnetization for sixteen sites. Six sites present reverse polarities and ten sites present normal polarities. Two sites show a low-latitude VGP and are considered as transitional. The normal and reverse polarity directions are almost antipodal. The overall characteristic pole position for the normal and reverse polarity sites, N = 16, pLAT = 74 N, Plong = 160 E, K = 25 and A95 = 7.6 , lies to the left of the Neogene segment of the North American or northern Mexico apparent polar wander path, which suggest the occurrence of a counterclockwise rotation of -15.5 7.4 to -16.5 6.9. Results are interpreted in terms of counterclockwise vertical-axis rotation associated with regional transtension and left-lateral shear as a result of oblique subduction of the Cocos plate along the Middle American trench. We suggest that the Chapala graben developed early in the Miocene, in a left-lateral strike-slip environment within the regional E-W Chapala-Tula fault zone. Results support recent studies that propose relative motion for southern Mexico along the volcanic arc and trench-parallel strike-slip faulting. " 1994.",,,,,,"10.1016/0040-1951(94)90107-4",,,"http://hdl.handle.net/20.500.12104/43468","http://www.scopus.com/inward/record.url?eid=2-s2.0-0028601852&partnerID=40&md5=eef018aa6e3579184e246baa767f7ca9",,,,,,"01-abr",,"Tectonophysics",,"6

Alkaline lavas in the volcanic front of the Western Mexican Volcanic Belt: Geology and petrology of th Ayutla and Tapalpa volcanic fields

The Plio-Quaternary Ayutla and Tapalpa volcanic fields in the volcanic front of the western Mexican Volcanic Belt (WMVB) contain a wide variety of alkaline volcanic rocks, rather than only calc-alkaline rocks as found in many continental arcs. There are three principal rock series in this region: an intraplate alkaline series (alkali basalts and hawaiites), a potassic series (lamprophyres and trachylavas), and a calc-alkaline series. Phlogopite-clinopyroxenite and hornblende-gabbro cumulate xenoliths from an augite minette lava flow have orthocumulate textures. The phlogopite-clinopyroxenite xenoliths also contain apatite and titanomagnetite and probably formed by accumulation of minerals fractionated from an augite minette more primitive than the host. The intraplate alkaline series is probably generated by decompression melting of asthenospheric mantle as a result of corner flow in the mantle wedge beneath the arc. Alkaline magmas may be common in the WMVB as a result of prior metasomatism (during Tertiary Sierra Madre Occidental magmatism) of the Mexican sub-arc mantle. Generation of the more evolved andesites and dacites of the calc-alkaline series is due to either combined assimilation and fractional crystallization (AFC) or magma mixing. The preponderance of alkaline and hydrous lavas in this region demonstrates that these lava types are the norm, rather than the exception in western Mexico, and occur in regions that are not necessarily associated with active rifting

Paleomagnetic study of the eastern sector of chapala lake and implications for the tectonics of west-central mexico

Western Mexico is characterized by several large-scale tectonic depressions which have been interpreted in terms of active continental rifting, tectonic transpression and coastal sinistral lateral transport of terranes. In this paper we report results of a paleomagnetic study of 148 samples from 22 sites in the Neogene volcanics from the eastern sector of the Chapala graben (western end of the E-W Chapala-Tula fault zone). Characteristic remanent magnetization directions have been isolated after detailed thermal demagnetization for sixteen sites. Six sites present reverse polarities and ten sites present normal polarities. Two sites show a low-latitude VGP and are considered as transitional. The normal and reverse polarity directions are almost antipodal. The overall characteristic pole position for the normal and reverse polarity sites, N = 16, pLAT = 74°N, Plong = 160°E, K = 25 and A95 = 7.6°, lies to the left of the Neogene segment of the North American or northern Mexico apparent polar wander path, which suggest the occurrence of a counterclockwise rotation of -15.5 ± 7.4 to -16.5 ± 6.9°. Results are interpreted in terms of counterclockwise vertical-axis rotation associated with regional transtension and left-lateral shear as a result of oblique subduction of the Cocos plate along the Middle American trench. We suggest that the Chapala graben developed early in the Miocene, in a left-lateral strike-slip environment within the regional E-W Chapala-Tula fault zone. Results support recent studies that propose relative motion for southern Mexico along the volcanic arc and trench-parallel strike-slip faulting. © 1994