Cracks of the World: Global Strike-Slip Fault Systems and Giant Resource Accumulations

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Evidence is mounting that the Earth is encircled by subtle necklaces of interconnecting, generally latitude-parallel faults. Many major mineral and energy resource accumulations are located within or near the deeply penetrating fractures of these “cracks of the world.” Future exploration for large petroleum occurrences should emphasize the definition, regional distribution, and specific characteristics of the global crack system. Specific drill targets can be predicted by understanding the local structural setting and fluid flow pathways in lateral, as well as vertical conduits, detectable through patterns in the local geochemistry and geophysics.
The faults in the cracks of the world fracture system typically move in transcurrent (strike-slip) motions that are tied to plate tectonics. One of the dynamic driving forces in plate tectonics derives from revolutions about the Earth’s rotational axis. Familiar plate tectonic driving mechanisms, such as mantle convective overturn or gravitational trench-pull, become second-order driving forces that are subordinate to the Earth’s spin axis. The scale of the kinematic reference frame thus shifts from crustal plate motions to motions between spheres (that is, lithosphere-asthenosphere differential rotations).
At a more local scale, introduction of magma and hydrothermal fluids into the global “crack system” commonly is coincident with kinematic activity in the faults. Indeed, analysis of mineral and chemical fractionation patterns produced during sequential introductions of the hot fluids offers new tools for kinematic and dynamic analysis of the global-scale fracture system. Particularly important are lateral compositional patterns in the mineral zone artifacts of hydrothermal plumes. These lateral patterns reflect motion related to the strike-slip kinematics and inject a new laterality and conceptual opportunity into exploration for commodities deposited by the ascending hydrothermal plumes. The global scale and interconnected nature of the strike-slip fault system in both continental and oceanic crustal materials first became apparent from a regional geotectonic study of Mexico.
The Mexico “Mega-Shear” System
A recent tectonic synthesis of Mexico ore deposits and tectonics has implications for worldwide giant petroleum accumulations and resulted from the incorporation of new constraints related to the regional geographic distribution of crustal oxidation states. Oxidation state is an indicator of oxygen fugacity-essentially the amount of oxygen available for reaction in the Earth’s crust. It is as fundamental an Earth property as magnetics or gravity and can be measured directly by the ferric/ferrous ratio in rock or inferred from the whole-rock mineralogy or commodity (element) present. Regional crustal oxidation state patterns shown on the oxidation state map of Mexico (Figure 1) were based on ferric/ferrous ratios, mineral assemblages, and geochemistry from about 2900 plutons and mineral systems. 1
Petroleum accumulations of all sizes throughout the globe correlate with source and reservoir rocks of low oxidation state where ferric-ferrous ratios are equal to or less than 0.6. In the Mexico region, over 500 additional oil and gas field occurrences were used to constrain crustal oxidation state. Petroleum occurrences can regionally coexist with other‚ commodities (such as diamond, gold, and tin, antimony, mercury, lithium, and tantalum) that require low oxidation states for their stability throughout the source-transport-deposition process. Consequently, maps of the regional crustal oxidation state in any particular area are useful as a regional exploration tool for petroleum and many metallic commodities.
The Mexico oxidation state map produced a striking zig-zag pattern (Figure 1). When this is combined with an oxidation state map for the western United States, a southeastward offset of the inferred Cambrian craton edge is apparent. It extends for some 3500 km from Cajon Pass west of Los Angeles, CA, to Guatemala City, Guatemala. The southeastward offset is accomplished on west-northwest-striking fault elements that form a giant, country-wide shear system referred to as the Mexico “mega-shear”. The well-known “Texas zone” forms the northernmost structural element of the shear system and the Motagua/Polochic fault system forms the southernmost element. Fault elements within the shear system are defined by sharply telescoped oxidation state gradients, where at least two levels of oxidation state are crossed in very short distances, on the order of 30-50 km. A similar pattern was found for the inferred Cambrian craton edge, which comprises offset segments of north-northeast-striking zones of telescoped oxidation states.
The overall pattern confirms the “Sonoran mega-shear” concept originally proposed by Anderson and Silver (1979). The sense of displacement of the inferred Cambrian margin is along an approximately N50W trend, sub-parallel to the trace of the proposed mega-shear. Individual offsets, however, occur along east-west- to west-northwest-striking, apparently deep-seated fault zones that traverse the entire country of Mexico and adjacent areas. If the 3500-km offset is restored and the Gulf of Mexico is closed, Mexico and northern Central America form a southward-pointed mega-peninsula that fits neatly to the coast of northwestern South America, west of Columbia and Ecuador. This reconstruction elegantly removes the long-known “Bullard-fit problem.”
West-northwest-striking fault offsets are also apparent on the gravity map of the Gulf of Mexico (Sandwell and Smith, 1995) on a northeast-trending regional high that has similar characteristics to incipient mid-ocean rifts. For this reason, we believe this north-northeast-trending gravity high was a mid-ocean ridge during the original opening of the Gulf of Mexico. Numerous west-northwest trending transform faults offset the ridge crest along trends similar to those in the present Gulf of California (Figure 1).
The mega-shear system is not confined to the country of Mexico and adjacent regions. Individual fault elements in the Mexico mega-shear extend outward into the Pacific Basin, where they link with the Pacific oceanic fracture system between 18°N and 42°N. A similar, even more dramatic connection is achieved when the Mexico mega-shear system is extended to the east-southeast, where it links, structural element for structural element, with the central Atlantic fracture system between the equator and a latitude of 18°N (Figure 2). In both the Pacific and Atlantic ocean basins, the oceanic ridge system displays an apparent left offset of some 3500 km, in accord with the offset on the Mexico mega-shear system.
The specific offsets in the Atlantic Basin and their presumed Mexican analogs are particularly indicative of a linkage. At the southern end of the Atlantic transform/ ‚ shear system, large apparent left-lateral offsets of the Atlantic mid-ocean ridge along the Romanche fracture system match well with large offsets of the inferred Cambrian craton edge along the Motagua/Polochic/Cayman trough fault system from its initial position in the Chortis block of Nicaragua-Honduras. This large offset is matched by several minor 50- to 100-km offsets in both the central Atlantic and Mexico mega-shear. About two-thirds of the way northward into both systems, another large offset occurs at the Guinea fracture zones in the Atlantic and the Monterrey-Parras fracture system in north-central Mexico. A series of smaller offsets occurs until the northernmost offset of about 150 km (Barracuda fracture in the Atlantic and the central portion of the Texas zone in southwestern Arizona and southeastern California).
The Mexico

source: 
Houston Geological Society
releasedate: 
Sunday, April 6, 2003
subcategory: 
Oil and Gas