PALEOMAGNETIC CONSTRAINTS ON THE STRATIGRAPHIC AND TECTONIC DEVELOPMENT OF THE BLACK MTNS. ACCOMMODATION ZONE IN THE GRASSHOPPER JUNCTION-WESTERN CERBAT MTN. REGION, AZ

Paleomagnetic data were used to assess both the stratigraphic development and rotation history of the limbs of a regionally extensive, anticlinal accommodation zone in the Grasshopper Junction (GHJ)-western Cerbat Mtn. (WCM) area. Several ash flow tuffs occur within the region at or near the base of the mid-Tertiary volcanic section, immediately above or within meters of a regional erosion surface cut into Precambrian crystalline rocks. In the (GHJ) area, tuffs in this stratigraphic position are sanidine-dominant, slightly to moderately welded ash flows and related air-fall, mineralogically similar to the 18.5 Peach Springs Tuff (PST). The basal tuff in the eastern GHJ area is relatively thin, comprising a single cooling unit. Sites in this area have stable characteristic magnetic directions (ChRM) with normal polarity lying near the anomalous NE-directed reference direction for the PST. In the westernmost GHJ area, however, much thicker sections of this tuff with multiple cooling units have normal-polarity ChRM directions that lie along a trend connecting the PST reference direction and the average mid-Miocene field direction. Alternative explanations for these data include (1) sampling of rapid secular variation during PST and possibly other potentially related eruptions, and (2) varying remagnetization by local silicic stocks. In the western foothills of the WCM, a moderately to highly welded tuff exists at the base of the Tertiary section which is mineralogically similar to the plagioclase-dominate Cook Cyn. tuff (CCT) exposed near Kingman, AZ. However, anomalously shallow reversed ChRM directions within the tuff of the WCM. are unlike our preliminary, normal-polarity CCT reference direction suggesting that it constitutes a previously unrecognized tuff. Paleomagnetic results also confirm presence of the reversed-polarity Tuff of Bridge Spring and define a steep, normal-polarity reference for the 16.3 Ma Highway tuff exposed in the eastern GHJ area. Time-averaged data in the GHJ area restore to expected mid-Miocene orientations with untilting of the limbs of a major extensional anticline and suggest little vertical-axis rotation during deformation. Tilt magnitudes within a silicic stock in the southern GHJ area are less than those within surrounding units suggesting intrusion during extension.