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The Contaya arch is an elongated topographic high that divides the Huallaga, Maranon and Ucayali basins in the Peruvian Amazonian plain. Its position well into the foreland basin and well inland from the main Andean thrust belt has proven to be enigmatic. Although it is often considered to be a single structural feature with a single origin, we show here that the Contaya arch is composed of distinct structures with different structural styles and different geologic histories: The main structures include the Contamana high, Contaya high, and Moa divisor. The Contamana high is limited by high angle reverse faults with a NW-SE orientation parallel to the Andean fold and thrust belt. This structure formed in the Tertiary, most probably in the Miocene after deposition of the Pozo Formation sands and shales. To the east of this is the Contaya high. This structure originally formed during the Triassic-Jurassic and was later reactivated in the Tertiary. The easternmost structure, the Moa divisor, separates the Ucayali basin from the Acre basin in Brazil. It is bounded by a high-angle, thick-skinned reverse fault. It appears to be a reactivated normal fault that formed in the Paleozoic during rifting and deposition of the Mitu Formation. From 10 to 4 Ma the subducted Nazca ridge was located beneath the Peruvian fold and thrust belt in the area where the Contamana high, Contaya high and Moa divisor are located. We suggest that the uplift of the Moa Divisor and the Contamana high as NW-SE oriented structures bounded by high-angle, thickskinned reverse faults and the reactivation of the Contaya high during the Miocene is related to the subduction of the Nazca ridge from 10 Ma to present.

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No description available.

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Pocatello Valley in southeastern Idaho and northern Utah is a structural and topographic basin bounded on all sides by mountains composed of Paleozoic platform carbonates and elastics. In the late Pleistocene it contained pluvial Lake Utaho, which, prior to 1981, was considered to have been an arm of Lake Bonneville. This study corroborates the finding of Currey (1981) that the two lakes were separate. The Quaternary deposits examined in this study are divided into two broad groups: those that were deposited prior to the last pluvial lake cycle, and those that were deposited during and after the pluvial lake maximum, (since approximately 16 ka) when the area was occupied by Lake Utaho and Lake Bonneville. Pediment gravels, alluvial fans, piedmont colluvium, and talus comprise the older group: the younger deposits include stream channel deposits, lacustrine sediments, and loess. Quantitative front sinuosity height ratios) geomorphological techniques (mountain ratios and valley floor width-valley indicate that the bounding ranges on the east and west margins of Pocatello Valley are slightly to moderately active tectonically. Precise surveying of the Lake Utaho highstand shoreline revealed significant deviations from the smooth, isostatically rebounded shoreline elevation curve of Crittenden (1963). The greatest deflections occur where the sinuous shoreline crosses the more linear inferred range front faults at the base of Samaria Mountain, on the east margin of the valley. The deflections (as much as 6.4 m over a horizontal distance of 900 m) suggest that movement has occurred along the range front faults since the shorelines were created approximately 16 ka, but no fault scarps were formed. A buried colluvium estimated to be 95 ka + 15 ka that was exposed draped toward in a over the trench at the range the inferred fault, valley. There are front is monoclinally and dips as much as 49° no fractures in the colluvium, which suggests that, although relative movement between the mountain and valley blocks has occurred, the displacement has only warped the colluvium. This further suggests that any earthquakes accompanying the movements must have been below the magnitude threshold (ML 6.2-6.3) necessary for surface rupture.