INSIGHTS INTO SHALLOW EMPLACEMENT OF A MAFIC DIKE SYSTEM, FORTIFICATION HILL, NORTHWESTERN ARIZONA

The basalt of Fortification Hill (5.4-5.9 Ma; Feuerbach et al., 1991) consists of olivine basalt lava flows and scoria deposits located east of Lake Mead in northwestern Arizona. The feeder dikes for the basalts are well exposed along a strike length of 4.6 km south of Fortification Hill (FH). Precambrian metamorphic rocks and Tertiary plutonic and sedimentary rocks form the wall rocks for the dikes, which are subparallel to several N-trending faults in the area and discordant with the NNW-trending faults. Using the base of the lavas at FH as an approximation for the late Miocene paleosurface, the mafic dikes represent a subvolcanic plumbing system extending to a depth of 310 meters. Detailed mapping along strike documents complex dike-wall rock interactions at shallow crustal levels.

Over much of its length, the feeder system consists of a single discontinuous dike with individual segments up to 400 m in length, although 1 km south of FH, the dike system splits into three subparallel segments. Individual dike segments strike from N30°E to N15°W and are generally steeply dipping to the west (70-90°); however, in rare cases, the dip is much shallower (27°W). The thickest dike segment is 1.6 m wide, but most are about 1 m wide. In map view and in vertical exposures, the dikes show morphologies ranging from rectilinear to highly sinuous, and sporadic dikelets protrude up to 1.4 m from the main dike forming a branching pattern. Where observed, dike terminations are primarily bulbous with minor protrusions. Xenoliths of wall rock are ubiquitous in the dikes; some are inferred to represent bridges of wall rock that were detached and enclosed by intruding magma. Fault zones and joints in the surrounding wall rock influenced dike emplacement as evidenced by abrupt changes in dike attitude in localized areas. However, these structures did not alter the overall trend of the dike, suggesting that regional structures such as the N-trending Fortification Ridge fault played a major role in controlling dike emplacement.

The overall characteristics of the FH dikes are consistent with intrusion at shallow crustal levels along distinct fractures. Detailed documentation of the dike system at FH provides insight into the behavior of mafic magmas at a depth comparable to the proposed high-level nuclear waste repository at Yucca Mountain.