Abstract The 50 km (31 mi) long Hat Creek fault, located along the western margin of the Modoc Plateau in northern California, is a geometrically complex segmented normal fault that offsets Pleistocene lavas by at least 570 m (1870 ft) of cumulative throw. Three subparallel, ∼NNW-trending sets of scarps (Rim, Intermediate, and Recent) reflect a progressive westward migration of surface rupture locations that offset progressively younger Pleistocene volcanic deposits during a ∼1 Myr fault history. The 50 km (31 mi) long Rim scarp comprises predominantly right-stepping segments with a maximum throw of ∼370 m (1214 ft) in ∼925 ka lavas. The 17.5 km (10.9 mi) long Intermediate scarp occurs 0.4 to 3.5 km (0.2–2.2 mi) west of the Rim, comprising left-stepping segments with a maximum throw of ∼177 m (581 ft). The 30.5 km (19 mi) long Recent scarp occurs several tens of meters west of the bases of older scarps, and is composed of left-stepping segments with a maximum throw of 56 m (184 ft). The northernmost segment of the Recent scarp offsets 53.5 ± 2 ka basaltic lavas, whereas the remaining segments offset 24 ± 6 ka basalt flows that erupted into Hat Creek Valley, indicating a youthful scarp system. Vertical propagation of the fault through young lavas produced fault-trace monoclines with amplitudes of up to 30 m (98 ft). The monoclines are commonly breached along their upper hinges by a vertical, dilational fault scarp. Shaking associated with repeated earthquakes progressively broke down these monoclines, causing disaggregation or partial to complete collapse. Fracture patterns and fault segment geometries and linkages were used to deduce the kinematic and stress history. The oldest segments of the Rim and Intermediate systems suggest initial NE-SW to ENE-WSW extension. Later Rim, Intermediate, and Recent segments responded to E-W extension, consistent with the previously documented stress state of the Cascades backarc. Complexity in Intermediate and Recent fault segments near a small shield volcano (Cinder Butte) suggests spatial variability in the stress field caused by a currently dormant magmatic system. Evidence for recent dextral-oblique kinematics along the Recent scarp, implying a slightly WNW-ESE extension, may reflect the transfer of dextral shear into the system from the Walker Lane Belt in western Nevada. Our interpretations require ∼45° of clockwise rotation of the horizontal principal stresses in the vicinity of the Hat Creek fault over the past ∼1 Myr, implying that significant complexity can develop in segmented normal fault systems over relatively short periods of geologic time.

We studied low prairie (Mima) mounds and ridges with sorted stone borders separated by broad rubbly soil intermounds in areas near Mount Shasta, northern California. An earlier study ascribed a purely physical origin for these soil features based on a four-stage conceptual model. Mounds were interpreted as periglacially produced clay domes formed in polygonal ground and stone perimeters as loose gravity accumulations in unexplained shallow trenches at dome peripheries. The model was widely cited to account for similar stone-bordered prairie mounds and rubbly soil intermounds in the Pacific Northwest. Our observations and measurements indicate, however, that these mounded landscapes are more complex, and that a polygenetic origin best explains them. We suggest that combined bioturbation, seasonal frost action, and erosion processes, with occasional eolian inputs, best account for the mounds, their well sorted stone borders, and the poorly sorted rubbly soil intermound pavements. We propose a transitional, eight-stage conceptual model to explain this complex landscape. The model may generally explain the origin of other similar strongly bioturbated, cold winter-impacted, erosion-prone mounded tracts in the Pacific Northwest.