Twenty new heat flow measurements in southern Mexico are presented. These measurements document a very broad zone of low heat flow between the coastline and the Trans‐Mexico volcanic belt. At the Trans‐Mexico volcanic belt, heat flow increases to high values and remains high into central Mexico. Heat flow values in the Trans‐Mexico volcanic belt and northward are in excess of 80 mW m−2 (1.9 μcal/cm2 s) and typical of values observed in the high heat flow regions of the western United States. The average heat flow in the Sierra Madre del Sur is 26 mW m−2 (0.6 μcal/cm2 s), in the Sierra Madre Oriental is 89 mW m−2 (2.1 μcal/cm2 s), and in the Trans‐Mexico volcanic belt is 91 mW m−2 (2.2 μcal/cm2 s). A surface heat flow profile was constructed perpendicular to the trench, and the data were matched with a simple thermal subduction model. The resulting final model parameter set suggests that the angle of subduction is low and that convective heat transfer in the back arc region is probable. For data south of the Trans‐Mexico volcanic belt, reduced heat flow values (upper crustal radioactive effect removed) are less than 15 mW m−2 (0.4 μcal/cm2 s). Thus essentially the total mantle heat flow is being absorbed by the subducting block. The values are lower than the heat flow observed in the Sierra Nevada Mountains of California and support the hypothesis that the heat flow couple associated with the Sierra Nevada Basin and Range province is a relic subduction zone thermal pattern.
Se establecieron tres estaciones sismográficas nuevas, en la región norte del Golfo de California en México. Durante un perio-do representativo, en abril y mayo de 1969, la sismicidad estuvo concentrada en las fallas de Imperial, San Jacinto, Sierra Juárez y San Miguel, y la dispersión de epicentros fue mucho menor de lo que se conocía previamente. Un importante enjambre de temblores ocurrió en marzo de 1969, cerca de la Roca Consag en el norte del Golfo, cuyo estudio ha contribuido al conocimiento de la tectónica regional. En el norte del Golfo de California y en la depresión de Salton adyacente, podemos aproximar la estructura tectónica por una serie de seis fallas transformadas, conectadas con cinco centros de dispersión (segmentos de cresta) caracterizados por zonas geotérmicas activas, volcanes recientes, enjambres de temblores y depresiones topográficas submarinas. Las complejidades en el patrón de fallas pueden relacionarse con una disminución de la velocidad de dispersión en los segmentos de cresta hacia el norte. Se encuentran en construcción cinco nuevas estaciones sismológicas de alta calidad, en torno al Golfo de California, cuyo objeto es una comprensión más detallada del patrón de dispersión del fondo submarino en esta región de excepcional importancia. Contribución conjunta de la Universidad Nacional Autónoma de México (U.N.A.M.) (Instituto de Geofísica, Contribución No. 1042), el Instituto Tecnológico de California (División de Ciencias Geológicas, Contribución No. 1948) y la Universidad de California, San Diego (Instituto de Geofísica y Física Planetaria, Contribución No. 2 Instituto de Geofísica, U.N.A.M. 3 Sección de Geología, Comisión Federal de Electricidad, México. 4 Laboratorio Sismológico, Instituto Tecnológico de California, Pasadena. 5 Instituto de Geofísica y Física Planetaria, Universidad de California, San Diego.
A permanent seismographic array has been established around the Gulf of California. Solar-powered stations are operating at Caborca, Bahia de los Angeles, Guaymas, Topolobampo and La Paz. Trailer stations using local 60-cycle power are operating at San Felipe, Rio Hardy, Rancho Meling, and Ensenada. The array is unique in the following aspects: (1) It is the only array operating in close proximity to an active region of sea-floor spreading; (2) its sophisticated low-power design and use of solar energy made siting choices simpler so that local noise sources could be minimized; (3) it is the first accurately timed array to operate in northwest Mexico. The array has already provided important information on seismicity in the Gulf of California and has aided in several sonobuoy studies of swarms and aftershock sequences. As more data accumulate, many of the important questions concerning the seismicity, tectonics, and structure of the Gulf of California will be answered.
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