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

Eocene crystal-rich ash-flow tuffs in the southwest part of the Challis volcanic field in Idaho yield a biotite KAr isochron data near 45.38 ± 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 ± 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°, D = 019.4°, α95 = 6.2°, N = 15, k = 39.4. The corresponding paleomagnetic pole (74.9°N, 313.3°E, A95 = 8.9°) lies clockwise 34.0° ± 9.8° relative to a synthetic 40 Ma reference pole for North America [2]. The observed inclination (63.6°) is close to that expected (61.8°) for this locality and age. Therefore, a clockwise tectonic rotation of about 34° 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°E-S55°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.