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

Geological evolution of the Caribbean region; A plate-tectonic perspective

Spreading along the Cocos-Nazca plate boundary since the breakup of the Farallon plate in the Miocene has resulted in the formation of the Panama basin and a complex interaction of plates in and near northwestern South America. Current plate boundaries have been defined, and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes, considering only well-located events, and through focal mechanism determinations. The existence of relict plate boundaries, bathymetric features, and the Panamanian isthmus has affected the subduction process of the Nazca plate beneath South America and determined the present-day configuration of the subducting lithospheric plate. There is no single triple junction separating the Caribbean, South American, and Nazca plates. Instead, the Panamanian isthmus and surrounding areas are accommodating east-west compression (and a lesser degree of north-south compression) along a series of thrust faults striking NW to NE, and the Andean ranges of Ecuador, Colombia, and Venezeula are moving as a block NNE relative to the rest of the South American plates, along a system of faults following the front of the Eastern Cordillera. The subducted portions of the Panama basin and old Farallon plate have become segmented into three pieces recognized in this study. From north to south, they are (1) a ‘Bucaramanga’ segment continuous with the Caribbean seafloor northwest of Colombia, (2) a ‘Cauca’ segment continuous with oceanic crust (Nazca plate) currently being subducted beneath South America at the Colombia-Ecuador trench, and (3) an ‘Ecuador’ segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern Peru. The segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted.

Subduction of the Eastern Panama Basin and seismotectonics of northwestern South America

This paper documents congruence in geographical patterns of speciation for four clades of birds having taxa endemic to the same areas within the Neotropics. Two genera, Pionopsitta parrots and Selenidera toucans, corroborate a well known biogeographic disjunction in which taxa endemic to southern Central America and the Choco region of northwestern South America are the sister-group to a radiation within the Amazon basin. These two genera, along with two lineages within the toucan genus Pteroglossus, also document a pattern of historical interrelationships for four well known areas of endemism within Amazonia: Guyanan + (Belem-Para + (Inambari + Napo)). These generalized historical patterns are interpreted to have arisen via fragmentation (vicariance) of a widespread ancestral biota. A review of the paleogeographic evidence suggests that these vicariance events could have originated as a result of several different mechanisms operating at various times during the Cenozoic. The inference that diversification of the Neotropical biota is primarily the result of the most recent of these possible vicariance events, namely isolation within Quaternary forest refugia, is unwarranted, given present data. These patterns of historical congruence are also interpreted as direct evidence against the hypothesis that diversification of the forest biota was a consequence of parapatric differentiation along recently established ecological gradients.

/pdf/patterns-and-processes-of-diversification-speciation-and-2vxi31gv96.pdf

Patterns and processes of diversification: speciation and historical congruence in some neotropical birds.

Basement rocks of parts of eastern Panama include tholeiitic pillow basalt and diabase overlain by sedimentary rocks typical of deep oceanic environments. Both paleontologic and stratigraphic evidence indicate that some of these rocks are of Late Cretaceous age or older. Regional Bouguer anomalies over the basement terrane exceed +120 mgal, indicating that eastern Panama is a raised block of oceanic crust. Age relations in the Caribbean region apparently preclude an “Atlantic” or single intra-Caribbean origin for eastern Panama and the southern Caribbean basin, but multiple intra-Caribbean origins of the basaltic basement rocks are permitted by the age data. An in situ origin of the oceanic basement of Panama at a position which later became a Cenozoic island arc is likewise consistent with the available geologic and geophysical data. Seismic horizon B″ in the Caribbean is correlative with or overlain by deep-sea sedimentary rocks of Coniacian to Campanian age. This horizon forms the top of the apparent basement of eastern Panama and, thus, the eastern isthmus is a horstlike block that has been elevated or obducted a minimum of 6 km since Coniacian-Maastrichtian time, partly caused by northeastward underflow of the Pacific plate but possibly related to southwestward movement of the Caribbean plate beneath the isthmus.

Oceanic Crust Forms Basement of Eastern Panamá

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

Negative Bouguer anomalies (−80 mgals) near the Pacific coast of southern Colombia define the position of the Tertiary Bolivar trough. Values increase eastward to a huge positive anomaly (+75 mgals) over Mesozoic “eugeosynclinal” rocks of the western Andes. This anomaly is part of the West Colombian gravity high, which extends from Panama into western Ecuador and is caused by shallow mafic crust. Bouguer anomalies are strongly negative (−220 mgals) over pre-Mesozoic(?) metamorphic rocks, Mesozoic(?) granitic bodies, and Tertiary to Holocene volcanic rocks of the central Andes between Pasto and Ipiales. The steep gravity gradient between the West Colombian gravity high and the negative anomaly of the central Andes represents the transition between mafic crust to the west and continental crust to the east. This zone parallels the Romeral-Cauca megashear system. East of the Andes, Bouguer anomalies range from −50 to −120 mgals over a Mesozoic-Tertiary basin of the Putumayo district, indicating that the crust there is thinner or denser than it is beneath the central Andes.

Models derived from gravity data suggest that the crust is about 45 km thick under the south-central Colombian Andes. If this is correct, the crust must thicken southward along the strike of the Andes, as thicknesses of 70 km have been reported in the Andes of southern Peru, Bolivia, and northern Chile by Lomnitz (1962) and James (1971a). Such differing crustal thicknesses may reflect different intensities of tectonic activity, greater crustal thickness indicating more intense or rapid growth of the volcano-plutonic arc or foreshortening of an existing crustal section.

Trans-Andean Geophysical Profile, Southern Colombia

This report represents part of a larger study undertaken to interpret the gross features of the earth9s crust by aeromagnetic methods. The larger survey covers a 100-mile-wide strip along a great circle arc from Washington, D.C., to San Francisco, California. The area considered extends from about 200 miles east of the Rocky Mountains to 150 miles west of the coast of the Pacific Ocean. Inferences on gross structure and lithology are made from the magnetic patterns and trends shown on the map. At the continental margin, the magnetic data strongly suggest that the oceanic crust becomes much more deeply buried as the continent is approached. A large magnetic anomaly extending northwestward along the center of the Great Valley is probably caused by mafic intrusive rocks. Broad wavelength magnetic anomalies over the Sierra Nevada are consistent with results obtained from gravity and seismic observations, indicating the batholith is underlain by gabbroic rocks. The Basin and Range province is characterized by an east-west and northeasterly magnetic grain, whereas the structural grain is nearly north-south. A complex belt of circular and elliptical anomalies extends southwesterly from the Utah line south of Wendover to a point beyond Ely, Nevada. These anomalies are due to magnetic basement rocks and small intrusive bodies that extend diagonally across several ranges. From the Wasatch fault block to the Rocky Mountain area the predominant grain is conspicuously east-west. The trend becomes more northwesterly as one progresses from north to south. The aeromagnetic data, supported by geologic mapping, suggest the existence of a regional pattern of en echelon northeast-trending shear zones across the Rocky Mountains and Denver basin. East-west magnetic lineaments, some as long as 400 miles, may represent fundamental fractures or fracture zones, most of which originate deep in the earth9s crust. These fracture zones may be genetically related to the system of east-west strike slip faults which have been discovered beneath the oceans by various oceanographic institutions.

Aeromagnetic Investigation of Crustal Structure for a Strip Across the Western United States

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Oceanic Crust Forms Basement of Eastern Panamá

Magnetic anomaly map of North America

Trans-Andean Geophysical Profile, Southern Colombia

Aeromagnetic Investigation of Crustal Structure for a Strip Across the Western United States