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Rocks from subvolcanic silicic magmatism (SSM) are exposed intermittently over a wide area along the Sonora coast, NW Mexico. This region is characterized by syn-rift volcanism related to the development of the Pacific-North America transform plate boundary during the Late Miocene (~12.5-~6 Ma). The ca. 11 Ma U-Pb age obtained from the SSM in the study area makes this magmatism part of the first stages of this tectonic event. Variations in magnetic mineralogy and textural relationships of the SSM suggest that they are a hypabyssal suite emplaced at distinctive depths. The anisotropy of magnetic susceptibility used for the petrofabric analysis reveals that, generally, the magnetic lineation is subparallel to the magnetic foliation and both are subhorizontal, suggesting a lateral emplacement. Palaeomagnetic analysis of 31 sites on the intrusive bodies indicates clockwise vertical axis rotations up to 90° related to dextral strike-slip faults. The fieldwork observations, magnetic fabric orientation, and the distribution of the SSM suggest that the magmas were propagated in a transtensional setting ascending probably through tension gashes from a deep main magma body, and emplaced as sheet intrusions in the syn-rift volcanic sequence. We propose that the vertical axis rotations of the SSM bodies are part of a unique transtensional-type deformation system of the oblique-divergent plate boundary, which it gradually changed. The SSM rocks are distinguished from other plutonic rocks forming most of the basement of NW Mexico by their age, distribution, emplacement environment, and deformation style. Finally, this study provides compelling evidence of igneous plumbing system within rift zones and a valuable contribution to the history of magmatism and tectonic control related to the development of the oblique rifting in the Gulf of California plate boundary. [ABSTRACT FROM AUTHOR]

Abstract The lithic in the Cerro Colorado tephra deposits is dominated by clasts from shallow country rocks for a diatreme not well mixed, or, by a combination of material that reflects the relative proportions of country rock in a well-mixed diatreme. The clasts in a tephra ring bed might be ideal to use the paleomagnetic study to determine the emplacement temperature. We report emplacement temperature ranges of pyroclastic lithic fragments from the Cerro Colorado Tuff Ring in the El Pinacate Volcanic Field, Sonora, northwestern Mexico. Paleomagnetic analyses, particularly the thermoremanent magnetization procedure was carried out in 38 small cores, drilled from the 10 to 30 cm in size lithic fragments. It estimates the emplacement temperature ranges of the juvenile basaltic cauliflower bombs and not-juvenile diverse blocks produced in the phreatomagmatic eruption. Titanomagnetite and titanohematite are the main magnetic carriers, along with titanomaghemite. The 22 emplacement temperatures determined in this study are grouped in three temperature ranges, low (310–370 °C), medium (400–460 °C) and high (>500 °C). The low and medium temperature samples show two or more components, while the high temperature samples have, in general, only one component. This wide range of emplacement temperature could be related to the composite thermal history of the pyroclastic material incorporated in the water saturated pyroclastic flow deposit. The comparison of the mean direction of the secondary magnetization component with the secular variation curve of the global model SHA.DIF.14, suggests an age of 3915 ± 59 yr. BP. Highlights • The clasts in a tephra ring bed might be ideal to use the paleomagnetic study to determine the emplacement temperature. • The emplacement temperature was determined from Cerro Colorado Tuff Ring in El Pinacate Volcanic Field, Sonora, NW Mexico. • The emplacement temperatures are grouped in three temperature ranges, related to the composite thermal history of the pyroclastic material. • An eruption age of 3915 ± 59 yr. BP was determined using the secondary component of magnetization with the global model curve SHA.DIF.14k. [ABSTRACT FROM AUTHOR]

Lithological, petromagnetic, paleomagnetic and magnetic fabric studies are employed to determine the flow direction and the location of the source of the, 2.44 to 2.21 Ma, El Castillo ignimbrite in the central-east Mexico. Based on the increasing matrix to pumice ratio and decreasing pumice size, the ignimbrite field is divided into the northwestern, central and south-southeastern sectors. Lithological comparisons among the three sectors reveal that the ignimbrite had flowed from NW to SE, and the source is in the NW part of the study area. Thermomagnetic results concur with the increasing matrix proportions from the proximal to the distal sector. The coercivity and magnetization ratios of the hysteresis parameters are lower in the SE sector than in the NW and central sectors. The dominant flow direction inferred through magnetic fabrics, at most sites, is NW to SE, which coincides with the direction inferred from lithological comparisons. However, at some sites magnetic fabrics demonstrate flow towards ENE or other various directions. The paleomagnetic analysis and field observations reveal that these anomalous directions are a consequence of anticlockwise block rotation and tilting due to normal and lateral faulting in the region. [ABSTRACT FROM AUTHOR]

The within-flow vertical variation of anisotropy of the magnetic susceptibility (AMS) of three basaltic flow profiles from the Xitle volcano were investigated in relation to the lava flow-induced shear strain. Rock magnetic properties and opaque microscopy studies have shown that the magnetic mineralogy is dominated by Ti-poor magnetite with subtle vertical variations in grain size distribution: PSD grains dominate in a thin bottommost zone, and from base to top from PSD-MD to PSD-SD grains are found. The vertical variation of AMS principal direction patterns permitted identification of two to three main lava zones, some subdivided into subzones. The lower zone is very similar in all profiles with the magnetic foliation dipping toward the flow source, whereas the upper zone has magnetic foliation dipping toward the flow direction or alternates between dipping against and toward the flow direction. The K 1 (maximum AMS axis) directions tend to be mostly parallel to the flow direction in both zones. The middle zone shows AMS axes diverging among profiles. We present heterogeneous strain ellipse distribution models for different flow velocities assuming similar viscosity to explain the AMS directions and related parameters of each zone. Irregular vertical foliations and transverse to flow lineation of a few samples at the bottommost and topmost part of profiles suggest SD inverse fabric, levels of intense friction, or degassing effects in AMS orientations. [ABSTRACT FROM AUTHOR]

Paleomagnetism is used to decipher the emplacement setting of the Rajmahal traps. Sampling was restricted to flows with known age. Three magnetic minerals are identified, titanomaghemite, Ti-poor magnetite, and possibly Mg–Al-rich titanomagnetite. Detailed analysis of thermomagnetic curves revealed that the emplacement temperature for Rajmahal traps was lower than 600 °C. Two magnetization components are distinguished. The low coercivity component, probably chemical remanent magnetization, is erased in fields up to 3.1 mT. In contrast, the high coercivity thermal remanent magnetization is more stable, sometimes throughout the demagnetization. The paleomagnetic results show that the sites have suffered up to 11° of clockwise and 17° of counter-clockwise rotation. As there is no field evidence of faulting and pervasive deformation, we propose that this rotation is probably due to penecontemporaneous joints or faults. These structures could have acted as conduits for fluids that increased the cooling rate. • Paleomagnetic analysis of Rajmahal Traps. • Penecontemporaneous faulting with lateral movement. • Fast cooling rates and low emplacement temperature. [ABSTRACT FROM AUTHOR]

Ash flow tuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 ± 0.2 Ma (the youngest age obtained so far in this area) and 29.2 ± 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8°N; Long = 180.5°E; Kappa = 142; A95 = 6.3°), calculated for the period 28–31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 ± 4° compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12–9 Ma). Palaeointensity estimates, obtained with the Thellier–Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations. [ABSTRACT FROM AUTHOR]

Abstract: Paleomagnetic and magnetic fabric data for the 4.8 Ma San Gaspar ignimbrite, one of the largest in western Mexico, are used to investigate the source vents, emplacement mechanism and tectonics. Rock magnetic properties from distant parts of the ignimbrite are similar, suggesting relatively homogeneous mineralogy of the unit. Isothermal remanence and continuous susceptibility-temperature experiments point to low to medium-Ti titanomagnetites as the main magnetic carriers. Hysteresis ratio parameters of most samples fall in the pseudo-single-domain grain size region; wasp-waisted hysteresis loops were identified corresponding to high H cr / H c values. Stepwise thermal and alternating field demagnetization shows that secondary components are completely removed below 20 mT or 400 °C. Thereafter, the characteristic component is isolated with small within-site dispersion of mean direction (α 95 <10° at 7 out of 10 sites). The within-site consistency of directional features of magnetic fabric supports that the ignimbrite welded and cooled in situ and was probably emplaced in a NW-SE rift zone from a single eruption center located south or east of a younger Pleistocene caldera in the area. Paleomagnetic, rock magnetic and ore microscopy data support the hypothesis that the extensive and widespread ignimbrite deposits in western Mexico correspond to a major explosive volcanic phase in the Pliocene. [Copyright &y& Elsevier]

Presents absolute palaeointensity results from the Trans-Mexican Volcanic Belt and discusses their implications for the late Miocene geomagnetic field strength. Selection of samples from a collection of Central Mexico Miocene volcanic units sampled for magnetostratigraph; Palaeointensity estimates resulting from the Thellier palaeointensity experiments.

Abstract: We present archeointensity data carried out on pieces of incense burners from the ancient Maya city of Palenque, Chiapas, Mexico, covering much of the Mesoamerican Classic period, from A.D. 400 to A.D. 850. We worked on pieces from 24 incense burners encompassing the five Classic ceramic phases of Palenque: Motiepa (A.D. 400–500), Cascadas (A.D. 500–600), Otulum (A.D. 600–700), Murcielagos (A.D. 700–770), and Balunté (A.D. 770–850). All the samples come from highly elaborate, flanged pedestal of incense burners that are undoubtedly assigned to a ceramic phase by means of their iconographic, morphological and stylistic analyses. Archeointensity measurements were performed with the Thellier–Thellier''s method on pre-selected samples by means of their magnetic properties. We obtained archeointensities of very good technical quality from 19 of 24 pieces, allowing the determination of a precise mean value for each ceramic phase, between and . The firing temperatures of ceramics were estimated with Mössbauer spectroscopy between 700°C and 1000°C. These values ensure that a full thermo-remanent magnetization was acquired during the original heating. Our results suggest a relative stability of the field intensity during more than 400 years in this area. The abundance of archeological material in Mesoamerica contrasts with the small amount of archeomagnetic data available that are, in addition, of uneven quality. Thus, it is not possible to establish a trend of intensity variations in Mesoamerica, even using the global databases and secular variation predictions from global models. In this context, our high technical quality data represent a strong constraint for the Mesoamerican secular variation curve during the first millennium AD. The corresponding Virtual Axial Dipole Moments (VADM) are substantially smaller than the ones predicted by the last global geomagnetic models CALS3k.4, suggesting the need for additional data to develop a regional model and a reference curve for Mesoamerica. [Copyright &y& Elsevier]