STRUCTURAL ROTATION AND VOLCANIC SOURCE IMPLICATIONS OF MAGNETIC DATAFROM EOCENE VOLCANIC-ROCKS, SW IDAHO

Eocene crystal-rich ash-flow tuffs in the southwest part of the Challi s volcanic field in Idaho yield a biotite K-Ar isochron date near 45.3 8 +/- 0.19 Ma. This age determination is in good agreement with a prev ious result [1] from this locality, near Poison Creek, which corrected for modem decay constants is near 44.7 +/- 0.8 Ma. AF and thermal dem agnetization experiments on 113 oriented cores from sixteen sites in t hese Challis ash-flow tuffs provide a tilt-corrected mean paleomagneti c direction with I = +63.6 degrees, D = 019.4 degrees, alpha(95) = 6.2 degrees, N = 15, k = 39.4. The corresponding paleomagnetic pole (74.9 degrees N, 313.3 degrees E, A(95) = 8.9 degrees) lies clockwise 34.0 degrees +/- 9.8 degrees relative to a synthetic 40 Ma reference pole f or 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, acco mpanying basin-and-range regional extension. AMS analyses of these ash -flow tuffs yield a predominant E-W trend for the maximum susceptibili ty axis. Assuming that this axis reflects the primary flow fabric of t he rock, and that this direction was also rotated by the amount indica ted in the paleomagnetic results, a flow source along a trend N55 degr ees E-S55 degrees W is calculated. A source region northeast of the st udy area, in the main region of development of the Challis volcanic fi eld is consistent with these results. (C) 1998 Elsevier Science B.V. A ll rights reserved.