Offset Axis Research Articles

Changes in the axial configuration of the East Pacific Rise near 6°S during the past 2 m.y.

The axis of the East Pacific Rise (EPR) in the area of detailed survey, 5°−7°S, is generally marked by a topographic block about 300 m high and 15–20 km wide. This axial block trends 14° through much of the survey area, but at 5.8°S it changes to a regional trend of 22° which continues to the north. A 200-m-high step (downdropped to the west) lies subparallel to and 20 km west of that part of the axial block which trends 14° and dies out to the north where the axial trend changes. Large fault blocks 1000 m high and 6–10 km wide occur 175 km west of the axis in the survey area, and a topographic depression of similar dimensions occurs 80 km east of the rise axis. Identification of magnetic anomalies here on the magnetic equator is complicated by the electrojet effect which produces a large and irregular diurnal variation in the earth's magnetic field. The few identifications possible indicate a whole spreading rate of 153 mm/yr for the EPR at 6° S. Mapped anomalies reveal a trend of 42° for the Gilsa anomaly, a 20° deviation from the present regional axis trend, and show the present topographic axis to be 10 km west of a central position between the flanking Brunhes/Matuyama anomalies. At about 6.5°S the rise axis is offset 55 km right laterally along a fracture zone which is poorly defined bathymetrically. A 1000-γ magnetic anomaly trends 108° along the fracture zone and coincides with its active portion. Both the bathymetric and the magnetic expressions of the fracture zone terminate a few tens of kilometers outside the offset axes, suggesting a change in the process that formed the fracture zone a few hundred thousand years ago. These data, combined with the areal distribution of earthquake epicenters, suggest that (1) during the past 1.7 m.y. the trend of the spreading axis has changed from highly oblique to more orthogonal to the spreading direction, (2) during the past 0.69 m.y. the spreading axis has shifted 10 km to the west, and (3) at present, that portion of the rise axis trending 14° is unstable and is in the process of reorienting to the regional trend of 22°. The westward shift of the rise axis may have occasioned the change in the morphologic and magnetic signatures of the 6.5°S fracture zone.

Evaporite Basin Configuration--Structural versus Sedimentary Interpretation: ABSTRACT

This paper presents different interpretations of the structural and/or stratigraphic criteria which are the bases for understanding evaporite-basin sedimentation and its subsequent role in oil exploration. The gross profile and configuration of Devonian Famennian, Givetian, and Eifelian evaporite basins in North America have induced the a priori conclusion that these lenses and wedges are the result of differential sedimentary tectonics with contemporaneous evaporite-carbonate deposition. Such basins are viewed as culminating in uniform subsidence and conformable deposition of superjacent successions--generally of open-marine and nearshore facies. Although this hypothesis deserves equal consideration with any other as a basis for interpreting evaporite-basin sedimentation, its preponderant acceptance without benefit of local and regional detailed stratigraphic studies hardly endows it with any real validity. In testing, this hypothesis should be examined seriously by the following considerations: (1) Walther's Law of Correlation of Facies demands that an unconformity must be recognized between the evaporites and the overlying transgressive redbed, sand, and/or carbonate succession; (2) related to Walther's Law--contemporaneous carbonate-evaporite deposition generally is impossible; the concept of cyclical superposition is much more rational; and (3) the offset axes of superjacent successions demand End_Page 550------------------------------ interdepositional tectonics and not subsidence contemporary with sedimentation. The above three points support the validity of the concept that the Devonian Famennian, Givetian, and Eifelian evaporite-basin configurations are not simply the result of differential sedimentary tectonics but are mainly eroded structural remnants. Supporting this concept are detailed stratigraphic subdivisions suggesting that these basins or lenses are only remnants of originally much more expansive evaporite depositional regions. These basins are analogous to structural basins in contemporaneous open-marine sedimentary provinces. Post-depositional epeirogenic movement and vast areal erosion have left thick broad lenses or wedges of evaporite sections--regionally defined as evaporite provinces of deposition. In searching for hydrocarbon accumulations among structural and stratigraphic traps within the evaporite complex, one has to consider these critical factors--the supra-evaporite basin unconformities and their tectonic origin. End_of_Article - Last_Page 551------------