Geology Reports⌕ Search

Geology topics

K. F. Fox

Publications and source records attributed to K. F. Fox.

7 recordsLinked to original sources
USGS1993

Seismic reflection profiling across Tertiary extensional structures in the eastern Amargosa Desert, southern Nevada, Basin and Range province

Outcrops, shallow well control, and coincident geophysical surveys are used to interpret a seismic reflection profile in the Amargosa Desert, within the Basin and Range province, of southern Nevada. The east-west-trending, 27-km-long seismic line crosses all or parts of three Tertiary subbasins, revealing that basin growth occurred by progressive shifts of basin-bounding faults. The reflection line images Tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults. A shallow (less than 100 to 200 m deep), laterally continuous, flat-lying, low-frequency reflector, interpreted as a Tertiary basalt flow, suggests that little vertical deformation has occurred within the easternmost of the small Tertiary basins since the eruption of the flow about 10 million years ago. Moderately dipping reflections within the pre-Tertiary bedrock may image Mesozoic thrust faults. The reflection data indicate that, whereas the top of the reflective lower crust shallows to the west, possibly in the direction of increasing crustal extension, the Moho is relatively flat between 30 and 33 km deep. Apparent bright-spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust, not for active magma chambers. Doming of the lower crust resembles that observed elsewhere in the Basin and Range province and is consistent with ductile flow in the lower crust.

Geological Society of America Bulletin↗
USGS1975

K-Ar age of the Similkameen Batholith and Kruger Alkalic Complex, Washington and British Columbia

Twelve K-Ar age determinations from the Similkameen batholith and the contiguous Kruger Alkalic Complex, including seven newly reported here, range from 69.9 m.y. to 177.2 m.y. Ages of coexisting hornblende and biotite show discordancies ranging from about 72 m.y. to 106 m.y., with hornblende consistently showing the greater analytical age. The hornblendes, with one exception, show increasing analytical age with increasing potassium content and approximately fit a 191.0-m.y. 40 Ar (radiogenic )/K isochron. The isochron shows a negative intersection with the 40 Ar (radiogenic) axis, indicating partial argon loss. The large negative deviation from the isochron of one low-potassium sample suggests that the isochron is biased to older ages. If so, the Similkameen and Kruger rocks probably crystallized between roughly 177 m.y. and 191 m.y. ago. The discordancies between hornblende and coexisting biotite are attributed to argon loss during one or possibly more episodes of thermal metamorphism, the most recent of which was probably between about 50 and 70 m.y. ago. This bracket is based on the age of apparently unmetamorphosed sedimentary deposits containing detritus from the Similkameen and Kruger bodies and the analytical age of the youngest biotite.

British Columbia, Washington↗