Structural rotation and volcanic source implications of magnetic data from Eocene volcanic rocks, SW Idaho.

Abstract

Eocene crystal-rich ash-flow tuffs in the southwest part of the Challis volcanic field in Idaho yield a biotite K-Ar isochron date near 45.38+ or -0.19 Ma. This age determination is in good agreement with a previous result [1] from this locality, near Poison Creek, which corrected for modern decay constants is near 44.7+ or -0.8 Ma. AF and thermal demagnetization experiments on 113 oriented cores from sixteen sites in these Challis ash-flow tuffs provide a tilt-corrected mean paleomagnetic direction with I = +63.6 degrees , D = 019.4 degrees , alpha sub 95 = 6.2 degrees , N = 15, k = 39.4. The corresponding paleomagnetic pole 74.9 degrees N, 313.3 degrees E, A sub 95 = 8.9 degrees lies clockwise 34.0 degrees + or -9.8 degrees relative to a synthetic 40 Ma reference pole for North America [2]. The observed inclination 63.6 degrees is close to that expected 61.8 degrees for this locality and age. Therefore, a clockwise tectonic rotation of about 34 degrees is inferred to have affected these rocks in the interval 40 to 15 Ma, approximately, accompanying basin-and-range regional extension. AMS analyses of these ash-flow tuffs yield a predominant E-W trend for the maximum susceptibility axis. Assuming that this axis reflects the primary flow fabric of the rock, and that this direction was also rotated by the amount indicated in the paleomagnetic results, a flow source along a trend N55 degrees E-S55 degrees W is calculated. A source region northeast of the study area, in the main region of development of the Challis volcanic field is consistent with these results.

4 samples from this reference