The dynamic history of the Trans-Mexican Volcanic Belt and the Mexico subduction zone

The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale. Methods of data filtering, display, and interpretation have also advanced, especially with the availability of low-cost, high-performance personal computers and color raster graphics. The magnetic method is the primary exploration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining targets through 3D inversion. These new applications have increased the method’s utility in all realms of exploration — in the search for minerals, oil and gas, geothermal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, mapping impact structures, and engineering and environmental studies.

/pdf/the-historical-development-of-the-magnetic-method-in-2o9psruy4m.pdf

The historical development of the magnetic method in exploration

Satellite telemetry studies of 20 adult and sub- adult white sharks (360-530 cm estimated total length (TL)) in the eastern North Pacific during 1999-2005 re- vealed long distance seasonal migrations from the coast of California to an offshore focal area 2,500 km west of the Baja Peninsula, as well as the Hawaii Islands. Three tags were recovered allowing detailed behavioral analyses, including one shark's migration cycle from the coast to the offshore focal area and back. While near pinniped rook- eries in autumn and winter, sharks avoided the surface and used water to 50 m depth, consistent with a silhouette- based hunting strategy. Offshore migrations were initiated during November-March and followed periods of decreasing pinniped abundance. Migrations were highly directed, taking 23 ± 5 days to reach the offshore focal area along similar paths among sharks and years, defining a migration corridor. Sharks exhibited a broad depth distri- bution (0-644 m) in the offshore focal area, and remained there for up to 167 days during spring and summer, though primary productivity and fishery data suggest that forage resources are scarcer there than in other regions of the eastern North Pacific. Archival data from one shark re- vealed intensive oscillatory movements while in the off- shore focal area, a behavior that may be related to foraging or mating. Sharks traveling to Hawaii remained near the islands up to 122 days, potentially feeding on pelagic fishes and marine mammals that concentrate around the islands.

/pdf/migration-and-habitat-of-white-sharks-carcharodon-carcharias-32m90022ww.pdf

Migration and habitat of white sharks (Carcharodon carcharias) in the eastern Pacific Ocean

Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1–8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001–2007 when in 2001–2002 a large slow slip event (SSE) had occurred in the Guerrero-Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S-waves. More than 100 strong NVT bursts were recorded in the narrow band of ~40 × 150 km^2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001–2002 and 2006 SSE compared with that for the “quiet” period of 2003–2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes.

/pdf/nonvolcanic-tremor-observed-in-the-mexican-subduction-zone-59zilldl6e.pdf

Nonvolcanic tremor observed in the Mexican subduction zone

SUMMARY Regional spherical coordinate observations of the Earth’s crustal magnetic field components are becoming increasingly available from shipborne, airborne, and satellite surveys. In assessing the geological significance of these data, theoretical anomalous magnetic fields from geologic models in spherical coordinates need to be evaluated. This study explicitly develops the elegant Gauss‐Legendre quadrature formulation for numerically modelling the complete magnetic effects (i.e. potential, vector and tensor gradient fields) of the spherical prism. We also use these results to demonstrate the magnetic effects for the crustal prism and to investigate the crustal magnetic effects at satellite altitudes for a large region of the Middle East centred on Iran.

Spherical prism magnetic effects by Gauss–Legendre quadrature integration

Data presented on the magnetic anomaly map in the continental areas have largely been derived from published map sources. However, these maps for the most part have been digitized and subjected to various processing steps in preparation for compositing. The marine areas of the map consist mostly of digital data track, except in the Gulf of Mexico, Caribbean Sea and the continental shelf areas of Alaska where the data were obtained from digitized maps.

Magnetic anomaly map of North America

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Magnetic anomaly map of North America