The Southern Rocky Mountains first rose during the Laramide Orogeny (ca. 75-45 Ma), but today's mountains and adjacent Great Plains owe their current height to later epeirogenic surface uplift. When ...and why epeirogeny affected the region are controversial. Sedimentation histories in two central Colorado basins, the South Park-High Park and Denver basins, shifted at 56-54 Ma from an orogenic to an epeirogenic pattern, suggesting central Colorado experienced epeirogeny at that time. To interrogate that hypothesis, we analyzed thermal histories for seven samples from central Colorado's Arkansas Hills and High Park using thermochronometers with closure temperatures below ∼180°C, enabling us to track sample exhumation from ∼5-7 km depth. Three samples are from the Cretaceous Whitehorn pluton, and four are Precambrian granitoids. All zircon and titanite (U-Th)/He dates (ZHe and THe) and one apatite fission-track (AFT) date are similar to the 67 Ma pluton emplacement age. Whitehorn dates using the lower-temperature apatite (U-Th)/He (AHe) thermochronometer are 55-41 Ma. These data require two exhumation episodes, one ca. 67-60 Ma, the second beginning at 54-46 Ma. The pluton reached the surface by 37 Ma, based on the age of volcanic tuff filling a pluton-cutting paleovalley. The Precambrian samples do not further refine this thermal history owing to the comparatively higher He closure temperature of their more radiation-damaged apatite. Laramide crustal shortening caused 67-60 Ma exhumation. Arkansas Hills shortening ended before 67 Ma, so shortening could not have caused the exhumation event that began 54-46 Ma; thermochronology supports the Eocene epeirogeny hypothesis. Epeirogeny affected >2.0 × 104 km2, from the Sawatch Range to the Denver Basin. We attribute epeirogeny to an Eocene mantle drip that likely triggered subsequent drips, causing younger exhumation events in adjacent areas.
A concise history of India Metcalf, Barbara D; Metcalf, Thomas R; Ansari, Sarah
Journal of Imperial and Commonwealth history,
05/2004, Letnik:
32, Številka:
2
Book Review
This paper describes six Hutterite children from five families who appear to have been affected by the same syndrome that was described in two brothers by Bowen and Conradi 1. Our additional cases ...confirm that the major features of the syndrome include porportionate intrauterine growth retardation, microcephaly, micrognathia, a prominent nose, rocker-bottom feet, joint limitation, and failure to thrive, with death within the first year of life. Bowen-Conradi syndrome is an autosomal recessive trait and pedigree records show that all six families now known are related to each other through two couples born in the late 1700s but that there are additional earlier possible sources of the responsible gene. The differential diagnosis of this syndrome is discussed.
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