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Title:
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super 40 Ar/ super 39 Ar and K-Ar geochronology and tectonicsignificance of the Upper Cretaceous Adel Mountain Volcanics andspatially associated Tertiary igneous rocks, northwestern Montana
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Reference Number:
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1986
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ISSN:
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1044-9612
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Publication Year:
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2005
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GeoREF Number:
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2005-062998 (View Original Data File)
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Authors:
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Harlan, Stephen S; Snee, Lawrence W; Reynolds, Mitchell W; Mehnert,Harald H; Schmidt, R G; Sheriff, Steve D; Irving, Anthony J
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Descriptors:
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absolute age; Adel Mountain Volcanics; anticlinoria; Ar/Ar; Cenozoic;Cretaceous; dates; deformation; dikes; extension; feldspar group; foldand thrust belts; folds; framework silicates; igneous rocks;intrusions; K/Ar; lithostratigraphy; Mesozoic; Montana; monzonites;northwestern Montana; paleomagnetism; plagioclase; plutonic rocks;silicates; sills; tectonics; Tertiary; United States; UpperCretaceous; USGS
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Source:
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U. S. Geological Survey Professional Paper, Report: P 1696, 29 pp.,2005
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Abstract:
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We report new super 40 Ar/ super 39 Ar and K-Ar dates from theUpper Cretaceous Adel Mountain Volcanics of northwestern Montana andspatially related Tertiary igneous rocks. The Adel Mountain volcanicfield consists of about 900 square kilometers of lavas, associatedvolcaniclastic strata, and intrusions that lie astride the easternmostfolds of the Montana disturbed belt of the Cordilleran fold and thrustbelt. The Adel Mountain volcanic rocks have been intensely deformed byfolds and thrust faults along their southwestern margin but areessentially undeformed to the east. Prior to isotopic dating, the ageof the Adel Mountain Volcanics was the subject of debate, with ageassignments ranging from Late Cretaceous to early Tertiary. Isotopicdates reported here demonstrate that the Adel Mountain Volcanics areclearly Late Cretaceous and that the volcanic rocks were probablyemplaced during an approximately 2- to 3-million-year interval betweenabout 76 to 73 mega-annum Ma. The new dates from the Adel MountainVolcanics are significant in that they provide a more refined andreliable age for the Late Cretaceous cratonic paleomagnetic referencepole for North America. The dates from the Adel Mountain Volcanics, aswell as those from spatially related younger intrusions, also provideimportant constraints on the age of fold and thrust-belt deformationalong the eastern margin of the Montana disturbed belt. Syntectonicdeformation of the Adel Mountain Volcanics, as well as apparentfolding and faulting of Tertiary quartz monzonite sills, indicatesthat contractional deformation clearly spanned the Late Cretaceous andmay have extended to as young as the Paleocene/Eocene boundary atabout 55.5 Ma. Elsewhere, posttectonic field relationships indicatethat deformation may have ended prior to 60 Ma. Complexities in fieldrelationships with respect to folds and faults shown by the earlyTertiary intrusions as well as complications in the argon systematicsindicate that these interpretations must be considered preliminary.Further field work and structural studies and additionalhigh-precision geochronology are needed in order to place limits onthe cessation of contractional deformation in this part of Montana.Unambiguously posttectonic dikes 47.5 Ma that cut all deformed rocksand structures in the area indicate that disturbed belt deformationhad clearly ceased by the early middle Eocene prior to the onset ofwidespread crustal extension in this part of the northern Cordillera.
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Copyright:
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GeoRef, Copyright 2005, American Geological Institute.
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