Geology topics

T.W. Stern

Publications and source records attributed to T.W. Stern.

3 recordsLinked to original sources

USGS1967

Oligocene or younger thrust faulting in the Ruby Mountains, northeastern Nevada

A klippe of unmetamorphosed Devonian carbonate rocks rests on the Harrison Pass intrusive body south of Toyn Creek in the central Ruby Mountains , Elko County, Nevada . This klippe and other klippen of Carboniferous strata - first mapped by R. P. Sharp (1942) - that rest on lower Paleozoic strata are believed to represent a once-continuous thrust sheet that developed after emplacement of the intrusive body. This intrusion of coarse-grained granodiorite to quartz monzonite is exposed over an area of about 45 square miles. Potassium-argon and lead-alpha age determinations on four samples of the intrusive body establish Oligocene or younger age for the thrusting. Potassium-argon age determinations on biotites from the four samples range from 29 to 36 m.y. with a possible analytical error of ±10 percent. Lead-alpha age determinations (all with a possible analytical error of ±10 m.y.) on zircon showed 40 m.y. for three of the samples and 30 m.y. for the fourth. Lead-alpha and potassium-argon dates on a fifth sample collected by R. R. Coats also fall within these ranges. The agreement of the radiometric dates indicates a lack of thermal activity subsequent to the emplacement of the intrusion and establishes a maximum age of Oligocene for the thrust faulting . The present distribution of thermally metamorphosed Paleozoic rocks in the Ruby Mountains seems to preclude the possibility that the thrust plate originated within the Ruby Mountains . Sharp suggested a western source with displacement of from 7 to 10 miles.

Nevada↗

USGS1966

Geochronology of the St. Kevin granite and neighboring precambrian rocks, northern Sawatch Range, Colorado

Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths ( St . Kevin Granite and granite of Cross Creek), and various minor intrusive rocks . A whole- rock Rb-Sr isochron age on the St . Kevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the St . Kevin and numerous other rocks are either about the same as the St . Kevin whole- rock age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor Precambrian intrusive masses that are probably younger than St . Kevin Granite yield mica ages within analytical error of the St . Kevin age, indicating that these rocks can be younger than the granite by only a few tens of millions of years. The mica ages, both Rb-Sr and K-Ar, are thought to be minimal, but a K-Ar age of 2020 m.y. on horn-blende probably reflects excess argon. Mica ages from all rocks known geologically to be older than St . Kevin Granite are low and are interpreted as heating ages reflecting intrusion of the granite , in some cases modified further by heating during Laramide time. In this area, Precambrian intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the St . Kevin Granite and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from Colorado although weak regional metamorphism may also have been a factor.

Coloradao↗

USGS1961

Ages of batholithic intrusions of northern and central Chile

Their stratigraphic setting demonstrates that three batholithic intrusions in northern and central Chile are respectively pre-Jurassic, Jurassic, and Cretaceous. Radiometric age determinations, by the lead-alpha (Larsen) method, indicate ages of 265 ± 30, 120 ± 15 to 125 ± 15, and 95 ± 10 to 130 ± 15 million years respectively for samples from the batholiths whose stratigraphic setting has been established or is inferred. With one exception, the radiometric ages are consistent with the stratigraphy. The exceptional determination of 130 ± 15 million years was for a sample of granite that intrudes Cretaceous rocks. Three other samples of Cretaceous granitic rocks were within the range of 95 ± 10 to 105 ± 10 million years. Angular unconformities whose ages are correlative with the intrusions suggest that the three batholiths can be related respectively to Hercynian, Late Jurassic, and middle Cretaceous orogenies.

Geological Society of America Bulletin↗