Joseph M. Kruger

Raft model of crustal extension: Evidence from seismic reflection data in southeast Arizona

An archlike zone of seismic reflectivity, interpreted as an uplifted zone of ductilely deformed middle and lower crust, is imaged below the Pinaleno Mountains core complex in southeast Arizona. The top of the reflective zone coincides with the base of an inferred mid Tertiary detachment fault beneath the Safford basin but diverges from the detachment fault as an apparent mylonite front to form a culmination at ∼1.9 s (∼4 km) beneath the Pinalenlo Mountains. From this culmination, the zone of reflectivity dips to the southwest below the Eagle Pass detachment fault and flattens at ∼4.8 s (∼13.5 km) beneath the relatively unextended upper crust of the Galiuro Mountains. Most of the reflective fabric probably formed during mid-Tertiary extension, although some of it may be older. These data suggest that mylonite zones form not only as the continuation of detachment faults into the brittle-ductile transition, but also along a regional zone of decoupling between the middle and upper crust. Highly extended and relatively unextended domains in the Basin and Range may be separated by zones of discrete (simple) shear in the upper crust, but both are rafted above regional bulk pure shear in the middle and lower crust.

Microgravity and magnetic detection of mafic dikes, fissures, and lava tubes in basalt: potential barriers and fast-flow paths for contaminant migration within the eastern Snake River Plain, Idaho

Summary Microgravity and ground-based magnetic investigations of exposed fissures and underlying mafic dikes, as well as exposed lava tubes in Holocene basalts on the eastern Snake River Plain of Idaho demonstrate that gravity and magnetic anomalies created by these features can be detected in some cases. However, in many cases, in any single profile, these anomalies can also be explained by near-surface density, magnetic susceptibility, and magnetic remanence variations of the overlying and adjacent basalt. This suggests that microgravity and ground-based magnetic detection of subsurface fissures, lava tubes, and dikes that have no surface expression, but can influence the migration of contaminated groundwater, may be detected on several parallel profiles if the features are large enough and close enough to the surface. However, unique identification on a single profile may not be possible due to the relatively large lateral variability in basalt density, susceptibility, and remanence unrelated to the volcanic features of interest.

Validation of Sediment Accumulation Regions in Kansas, USA

Recognition of recurrent structural deformation and sedimentation in cratonic shelves is not new and these processes affect these areas worldwide (Merriam and Forster, submitted b). “Plains-type folds” have been described in the literature since the turn of the century. Structural reactivation continues to be documented in the northern Midcontinent, USA and refined in timing, spatial distribution, and causal relationships (Gerhard et al. 1982; Baars and Watney 1991; Berendson 1997; Watney et al. 1997; Merriam and Forster; submitted b). This chapter further documents km-scale reactivation features.