William R. Muehlberger

Caribbean–Americas plate boundary in Guatemala and southern Mexico as seen on Skylab IV orbital photography

Skylab photographs of Guatemala clearly show the prominent fault zones that have acted as the Caribbean-Americas plate boundary. The present boundary, the Cuilco-Chixoy-Polochic fault zone, can now be extended westward into Mexico where it apparently bifurcates into northwest-trending and southwest-trending forks. The southwest-trending fork separates regions of different seismicity and volcanism and is proposed as the present plate boundary. The eastern end of the Cuilco-Chixoy-Polochic fault zone abuts a pair of faults that trend N60°E and bound Lago de Izabal. These faults bound a graben that has the shape of a parallelogram. This shape is repeated along the eastern Motagua fault zone and offshore in the adjacent part of the Bartlett Trench. The wedge of the Caribbean plate between the Americas and Cocos plates (southern Guatemala, El Salvador, and western Honduras) is being fragmented by east-west extension. Individual fragments are being slightly rotated counterclockwise as a result of a northward component of motion of eastern Honduras.

Rotation of the Chortís block causes dextral slip on the Guayape fault

The Chortis block is undergoing deformation as a direct result of large-magnitude sinistral strike-slip faulting on the North America-Caribbean plate boundary. We have studied the Chortis block using geologic mapping, remote imagery, and earthquake seismology to evaluate the effect that large-magnitude strike-slip faulting has on an adjacent microcontinental block. In the eastern part of the Chortis block, recent geologic mapping has shown that the northeast striking Guayape fault is dextral instead of sinistral as previously assumed. Because this is a major fault that cuts most of the Central America isthmus, the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the Chortis block. The presence of several north trending rifts in the western part of the Chortis block has led previous workers to propose that the Chortis block is divided into smaller, independently rotating blocks. We use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the Chortis block. However, we have modified their model to account for dextral slip on the Guayape fault. We believe that rotation of the Chortis block causes dextral slip on the Guayape fault even though Central America is undergoing large sinistral displacement with respect to North America.

Quaternary faulting in Trans-Pecos Texas

Faults that displace Quaternary units can be observed in scarps in Trans-Pecos Texas and are restricted to two north-trending zones in contrast to late Tertiary faults that cover the region and strike north, northwest, and west. The western zone of Quaternary faults, near El Paso, is usually included as the southern part of the Rio Grande rift. The eastern zone of Quaternary faults extends for 300 km from southern New Mexico along the Salt Basin graben through Van Horn, Texas; its probable extensions and subparallel associates extend southward to Presidio, Texas. This belt of faults is parallel to the Rio Grande rift zone and should be considered a southeast extension of that zone. These fault zones die out southward into the edge of the Chihuahua tectonic belt, a region underlain by a thick Mesozoic carbonate and clastic section that in turn rests on a thick layer of evaporites. The evaporite zone may mask Cenozoic normal faulting and thus may define a zone of no data rather than a southern limit of basin-and-range or Rio Grande graben tectonics. All Quaternary and most Tertiary faults trend parallel to pre-existing structures. The map pattern of Quaternary scarps suggest a maximum extension oriented about S80°W; however, first-motion studies of the 1931 Valentine, Texas earthquake show a maximum elongation direction of S50° to 55° W. This difference may be unique to this one earthquake or may be due to the pre-existing lines of weakness that control the location of presently active faults.

Igneous evolution of a complex laccolith-caldera, the Solitario, Trans-Pecos Texas: Implications for calderas and subjacent plutons

The Solitario is a large, combination laccolith and caldera (herein termed “laccocaldera”), with a 16-km-diameter dome over which developed a 6×2 km caldera. This laccocaldera underwent a complex sequence of predoming sill, laccolith, and dike intrusion and concurrent volcanism; doming with emplacement of a main laccolith; ash-flow eruption and caldera collapse; intracaldera sedimentation and volcanism; and late intrusion. Detailed geologic mapping and 40Ar/39Ar dating reveal that the Solitario evolved over an interval of approximately 1 m.y. in three distinct pulses at 36.0, 35.4, and 35.0 Ma. The size, duration, and episodicity of Solitario magmatism are more typical of large ash-flow calderas than of most previously described laccoliths. Small volumes of magma intruded as abundant rhyolitic to trachytic sills and small laccoliths and extruded as lavas and tuffs during the first pulse at 36.0 Ma. Emplacement of the main laccolith, doming, ash-flow eruption, and caldera collapse occurred at 35.4 Ma during the most voluminous pulse. A complex sequence of debris-flow and debris-avalanche deposits, megabreccia, trachyte lava, and minor ash-flow tuff subsequently filled the caldera. The final magmatic pulse at 35.0 Ma consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture. Solitario rocks appear to be part of a broadly cogenetic, metaluminous suite. Peralkaline rhyolite lava domes were emplaced north and west of the Solitario at approximately 35.4 Ma, contemporaneous with laccolith emplacement and the main pulse in the Solitario. The spatial and temporal relation along with sparse geochemical data suggest that the peralkaline rhyolites are crustal melts related to the magmatic-thermal flux represented by the main pulse of Solitario magmatism. Current models of laccolith emplacement and evolution suggest a continuum from initial sill emplacement through growth of the main laccolith. Although the Solitario laccocaldera followed this sequence of events, our field and 40Ar/39Ar data demonstrate that it developed through repeated, episodic magma injections, separated by 0.4 to 0.6 m.y. intervals of little or no activity. This evolution requires a deep, long-lived magma source, well below the main laccolith. Laccoliths are commonly thought to be small, shallow features that are not representative of major, silicic magmatic systems such as calderas and batholiths. In contrast, we suggest that magma chambers beneath many ash-flow calderas are tabular, floored intrusions, including laccoliths. Evidence for this conclusion includes the following: (1) many large plutons are recognized to be laccoliths or at least tabular, (2) the Solitario and several larger calderas are known to have developed over laccoliths, and (3) magma chambers beneath calderas, which are as much as 80 km in diameter, cannot be as deep as they are wide or some would extend into the upper mantle. The Solitario formed during a tectonically neutral period following Laramide deformation and preceding Basin and Range extension. Therefore, space for the main laccolith was made by uplift of its roof and possibly subsidence of the floor, not by concurrent faulting. Laccolith-type injection is probably a common way that space is made for magma bodies of appreciable areal extent in the upper crust.

Geological implications of Middle Cambrian boulders from the Haymond Formation (Pennsylvanian) in the Marathon basin, west Texas

Fossiliferous Middle Cambrian boulders from a previously unknown boulder unit in the Haymond Formation of Pennsylvanian age in the southeastern part of the Marathon basin, Texas, contain trilobites, brachiopods, and mollusks indicative of the seaward margin of a late Middle Cambrian carbonate platform. The source area was to the southeast of present outcrops. Paleogeographic relocation of the source, about 100 to 200 km southeast of the present Marathon basin, provides the first clear evidence for the position of the seaward margin of the earliest Paleozoic carbonate platform along the southern sector of the United States. Metamorphic rocks of the “interior zone” of the late Paleozoic Ouachita orogen now lie well inboard of the seaward edge of the carbonate platform and are most likely allochthonous on a scale of tens of kilometres.

Earth observations during Space Shuttle mission STS-28 - 8-13 August 1989

An overview of the STS-28 earth observation is provided with attention given to meteorology, oceanographic phenomena, and human activity such as urban environments and related land uses. Environmental observations discussed include evidence of a monsoon, vegetation changes such as deforestation, and water pollution and eutrophication of major rivers. Particular attention is paid to atmospheric palls in both the Northern and Southern Hemispheres. Discussion of geological observations focuses on the Mount St. Helens volcano and the little-recognized landform type, the immense illuvial cone. Observation techniques include use of color infrared film and two types of polarization observations.

Deep Crustal Drilling, Texas Gulf Coast, United States of America

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the Gulf itself to the causes and effects of long-lasting circulation of thermobaric waters throughout the thick sedimentary section. The Gulf Coast is a “passive margin”, and plate tectonics theory indicates that the Gulf originated by rifting. From the little available information concerning deep structure and lithology, this theory is appropriate but not conclusively demonstrated. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; diagenesis and early depositional history of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance.

Earth observations during space shuttle mission STS‐33: November 23–27, 1989

An overview is presented of photographs, astronaut impressions, and photographic equipment and techniques coincident with the Discovery STS-33 mission of November 23-27, 1989. Particularly significant geographical photographs of southern Africa, Madagascar, northern Africa, Southeast Asia islands, Pacific islands, and Australia are shown and reviewed. Meteorological and oceanographic observations are summarized along with film anomalies that occurred during this mission.

Deep crustal drilling, Texas Gulf Coast

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long-lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville-age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita-Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto-South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben-fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.