Bruce Bryant

The significance of fission-track ages of apatite in relation to the tectonic history of the Front and Sawatch Ranges, Colorado

Fission-track ages of apatite in the Mount Evans area of the Front Range show that the Late Cretaceous 100 °C isotherm, which was 4 km below the sea floor, is now at an altitude of 3.5 km. Assuming a geothermal gradient of 25 °C per kilometre, the projected level of the latest Cretaceous sea floor, represented by the Fox Hills Sandstone, is at an altitude of 7.5 km, about 3.2 km above the top of Mount Evans, and the structural relief from that projection of the Fox Hills on the crest of the Front Range uplift to the deepest part of the Denver Basin to the east is 6.5 km. Ages of apatite in Precambrian rock along the Elkhorn fault on the west margin of the Front Range uplift show that in the area of maximum stratigraphic displacement the fault must have a very low dip and that the east margin of the Paleocene South Park basin must have been nearby. Ages in the Sawatch Range suggest that significant uplift occurred during or after Eocene time. Ages on the east side of the range reflect development of the Arkansas Valley graben and concomitant uplift of the east margin of the modern Sawatch Range in Miocene time.

Evidence for Strike-Slip Faulting Along the Brevard Zone in North Carolina

The Brevard zone is a narrow belt of steeply dipping low-grade metamorphic rocks known to extend for at least 375 miles from near Montgomery, Alabama, northeast to Mount Airy, North Carolina. It probably continues northeastward into Virginia, but its position there can only be inferred. The Brevard zone separates the Blue Ridge geologic belt on the northwest from the Inner Piedmont belt on the southeast, but it is only locally coincident with the physiographic boundary between the Blue Ridge and Piedmont. The Brevard zone has previously been interpreted both as a narrow syncline of younger rocks infolded into a terrane of metamorphic and plutonic rocks and as a belt of retrogressively metamorphosed rocks along an overthrust fault (the Martic or Brevard overthrust). Detailed mapping and petrographic studies southeast of the Grandfather Mountain window in North Carolina show that there the Brevard is a fault zone, and rocks along this fault zone display pervasive structural and metamorphic effects related to it across an outcrop width of as much as 5 miles. Latest movement along the zone occurred after major thrusting along the Linville Falls fault, which bounds the Grandfather Mountain window, and before emplacement of an unaltered diabase dike believed to be Late Triassic. Reconnaissance field and petrographic studies along the zone in North Carolina, South Carolina, and northern Georgia support the conclusion that the Brevard is a fault zone. The linearity of the zone, the presence of blastomylonite and phyllonite within it, the horizontal cataclastic lineation associated with it, and the contrast in rocks across it suggest that the Brevard is a zone of strike-slip faulting of great magnitude. Right-lateral displacement of at least 135 miles is believed to have occurred in late Paleozoic or Early Triassic time or both.

Potassium-Argon Ages Bearing on the Igneous and Tectonic History of the Elk Mountains and Vicinity, Colorado: A Preliminary Report

K-Ar ages for epizonal plutonic rocks together with field studies indicate that uplift of the Sawatch Range began at least 72 m.y. ago. Vertical uplift of the Sawatch Range was followed or accompanied by gravity sliding of sedimentary rocks along the Elk Range thrust fault. The greatest volume of exposed intrusive rocks in the Elk Mountains, Ruby Range, and West Elk Mountains consists of granodioritic rocks of Oligocene age which are younger than the Elk Range thrust. Miocene and Pliocene(?) mafic dikes were emplaced after most of the structural features in the area had developed. The Treasure Mountain dome near Marble was formed by emplacement of a unique soda granite pluton about 12.5 m.y. ago. Discordant K-Ar and Rb-Sr ages for biotite from the Twin Lakes stock in the Sawatch Range suggest that this large pluton is of Late Cretaceous or early Tertiary age and lost argon during Oligocene plutonism, or is of Eocene age and represents a separate event not yet supported by other radiometric dates from the area.

ORIGIN OF SOME INTERMITTENT PONDS ON QUARTZITE RIDGES IN WESTERN NORTH CAROLINA

Several intermittent ponds and closed depressions as much as 200 feet wide occur on the crests of ridges in gently dipping Cambrian(?) quartzites in the southeastern foothills of the Blue Ridge Mountains near Morganton, North Carolina. The unconsolidated fill and debris in the ponds consists of clayey sand and saprolite with accessory minerals that could have been derived entirely from the quartzite. The pond water contains appreciable quantities of dissolved silica and with the aid of organic substances could have formed the depressions by solution since the beginning of the Pleistocene.