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[1] This paper presents results from 13 paleomagnetic sites from an area west of Mexico City and 7 sites from an area of dispersed monogenetic volcanism in the state of San Luis Potosi, accompanied by seven 40 Ar/ 39 Ar radiometric dates. An analysis of secular variation and time-averaged paleomagnetic field in the Trans-Mexican Volcanic Belt (TMVB), using compiled data both newly obtained and from the literature, is presented. Interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected. The selected data include 187 sites of late Pliocene-Holocene age. The mean direction among these sites is Dec = 358.8°, Inc = 31.6°, a 95 = 2.0°, k = 29. This direction does not overlap the expected geocentric axial dipole (GAD) but is consistent with a GAD plus a 5% quadrupole. The virtual geomagnetic pole scatter of this group of sites (12.7°, with lower and upper 95% confidence limits of 11.9°and 14.1°) is consistent with the value expected from Model G (13.6°).

The San Quintin Volcanic Field (SQVF) is unique for the Baja California peninsula as the only known location of intraplate‐type mafic alkalic volcanism and the only known source of peridotitic and granulitic xenoliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # > 64)(Mg # = 100 × Mg/(Mg + (0.85 × FeTotal))), which carried occasional small xenoliths. As the SQVF evolved with time, differentiated magmas (Mg # < 64) became increasingly common, but primitive magmas, virtually devoid of xenoliths and unusually rich in olivine phenocrysts, dominanted at the youngest cones. Abundances of incompatible elements declined during evolution of the SQVF, implying a temporal increase in the extent of partial melting in the mantle, or progressive exhaustion of these elements in the source. Samples from two cones, Mazo and Ceniza, show relatively low Ce/Pb, εNd, and 206Pb/204Pb and high 87Sr/86Sr, which we interpret as evidence for crustal contamination of these magmas. Small isotopic variations for the other cones are collectively interpreted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and differentiated rocks, supporting fractional crystallization as the mechanism for deriving the latter from the former. Most differentiated rocks can be successfully modeled by fractional crystallization of olivine, plagioclase, clinopyroxene, and spinel from primitive parents. The largest and most abundant xenoliths were carried by differentiated magmas, indicating that fractional crystallization took place within the mantle, below the level of peridotite entrainment, and reflecting the importance of fractionation‐elevated volatile contents for driving these differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate‐type mafic alkalic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and trends of rising Al2O3 and falling CaO with decreasing incompatible element abundances are all consistent with origins for the SQVF primitive magmas by progressive partial melting of spinel lherzolite at unusually shallow levels in the mantle.

ResumenLa falla San Marcos (FSM) es un lineamiento estructural regional con más de 300 km de largo, rumbo WNW y que se inclina hacia el NNE, separando el bloque de Coahuila del Cinturón Plegado de Coahuila en el noreste de México. La FSM documentan su actividad intermitente por lo menos desde el Jurásico Tardío hasta el Plioceno-cuaternario. Las evidencias estructurales más antiguas reconocidas en este trabajo documentan actividad de la FSM durante tiempos pre-Titoniano y Neocomiano, sugiriendo que la FSM acomodó principalmente extensión de la corteza en dirección NNE. Esta extensión contribuyó al crecimiento de la cuenca de Sabinas; con lo anterior, se pone en duda la existencia de grandes desplazamientos laterales a través de la FSM por lo menos para estos tiempos.Se han reconocido al menos cuatro eventos de reactivación de la FSM. El primero fue con componente normal en el Neocomiano y causó el depósito de la Formación San Marcos. El segundo evento de reactivación fue inverso en el Paleógeno y debió incluir a fallas menores asociadas a la FSM. Se interpreta que el segundo evento de reactivación está representado por la ocurrencia de: a) plegamiento tipo drape y transporte tectónico menor hacia el sur-suroeste sobre la traza principal de la FSM, b) ocurrencia de relaciones perpendiculares entre los ejes de pliegues en la parte suroeste de la cuenca de Sabinas, c) por el levantamiento de rocas más antiguas progresivamente hacia el noreste dentro del Valle San Marcos (VSM) y d) por la existencia de direcciones perpendiculares de transporte tectónico determinadas para diferentes estructuras en el VSM (e.g. fallas en el sector oeste del VSM registran transporte hacia el oeste y fallas en el sector suroeste registran transporte hacia el sur-suroeste). Las fallas menores asociadas a la FSM presentan orientaciones desde E-W hasta cercanamente N-S como las fallas El Caballo y El Almagre expuestas al oeste de Coahuila y sureste de Chihuahua. Este evento de reactivación inverso de la FSM es tardío con respecto a una fase anterior de despegues (duplicación de la secuencia por fallas) en localidades de la plataforma de Coahuila y la cuenca de Sabinas. La importancia y escala de los despegues debe ser explorado con mayor detalle en futuros trabajos.La tercera reactivación es normal con componente lateral izquierda (Mioceno tardío-Plioceno temprano) y, la cuarta y última, predominantemente normal (Plioceno -cuaternario). Estas reactivaciones fueron reconocidas sobre la traza de la M E X IC ANA A .C . SO C I E

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Rincon de Parangueo is a Quaternary maar that had a perennial lake until the 1980s. The lake was gradually desiccated as a consequence of drawdown in the Salamanca-Valle de Santiago regional aquifer and now functions as a playa-lake. In contrast with the features observed in other crater-lakes in the region (La Alberca, Cíntora, and San Nicolás), which also dried up at the same time, the bottom of the Rincón crater displays clear evidence of active deformation associated with mass movement of lake sediments towards the depocenter inside the crater. The most conspicuous topographic feature is a 10 -12 m high scarp parallel to the former lake coast. The scarp is produced by an annular shaped normal fault system, down towards the depocenter. Evidence of active mass movement is observed along the topographic scarp. Rotational slides associated with rollover anticlines and local grabens produced by antithetic faults are common on the eastern and northern parts of the scarp. Planar slides with open folds at their base occur at western part of the lake basin.Evaporites (trona, thermonatrite, eitelite, halite, and silvite) are abundant in the playa-lake sediments. Their presence makes us believe that a mass removal process is acting as a consequence of salt dissolution and infiltration of the brine towards the aquifer. This process, probably in conjunction with lake sediment compaction and/or diatreme subsidence, may explain the significantly higher fault displacement rate observed inside the crater ( ≈ 50 cm/year ) in comparison with active faults elsewhere in the Salamanca-Valle de Santiago aquifer ( ≈ 6 cm/year ).Keywords: drawdown, maar, evaporite, stromatolite, landslide. Resumen

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