Walker, E.L., Kattenhorn, S.A. (2008)
Slip history and evolution of the Hat Creek fault, northern California.
Eos, Transactions of the American Geophysical Union 89(52), Fall Meeting Supplement, Abstract T21B-1976.
Normal faults commonly exhibit unique surface features in basalt such as vertical scarps and fault-trace monoclines that provide clues to the fault evolution. The Hat Creek fault, 25 km north of Lassen volcano in northern California, is a segmented fault system within Pleistocene and younger basalts. The fault is located along the western boundary of the Modoc plateau in the extended backarc of the Cascades. The fault geometry tells of a varied extensional history that likely reflects a complex interplay between tectonic and magmatic influences. In response, the northern portion of the fault system migrated progressively westward, abandoning older scarps in its wake, whereas the southern portion continues to utilize Pleistocene slip surfaces. This spatial evolution has created three distinct scarps. From oldest (easternmost) to youngest (westernmost), they are informally identified as: the rim (max. throw of 352 m), the pali (max. throw of 174 m), and the active scarp (max. throw of 65 m). The rim is oriented N-S, consistent with the regional E-W extension direction, and consists of 7 predominantly right-stepping segments (NNW oriented) that are physically linked through lower ramp breaches. This geometry implies a clockwise rotation of the stress field after the segments developed, with linkage driven by right-lateral oblique motion. Throw profiles along the rim illustrate mechanical interactions and partitioning of displacement between adjacent segments. The pali is a relatively younger fault plane located up to 3.3 km west of the northern portion of the rim. The pali is oriented NW-SE and consists of 5 left-stepping segments that are physically connected through upper ramp breaches, also consistent with right-lateral oblique motion. The pali likely nucleated along its central segment, where throw is maximized, in response to a magmatic perturbation of the stress field (causing a local NE-SW extension), possibly related to dike injection that culminated in the creation of the nearby volcanic edifice, Cinder Butte (38 ± 7 ka). The association with Cinder Butte is most apparent along the northernmost segment of the pali, which curves around the edifice. Southward propagation of the pali eventually resulted in linkage with the rim, which is still active south of the linkage point. The development of the active scarp was prefaced by the eruption of the 24 ± 6 ka Hat Creek basalts (about 50 m thick) in the hanging wall valley of the Hat Creek fault. These lavas pooled against the pali in the north and the rim in the south. Post-eruption slip along the fault forced the upper fault tip vertically through the Hat Creek basalt. Vertical growth was incremental, presumably over multiple earthquake events, resulting in monoclinal folding of the surface above the buried fault tip. The fault eventually pierced the surface, breaching the monocline and forming the vertical active scarp. The active scarp consists of 7 left-stepping segments in various stages of linkage (mostly unlinked, but 2 with upper ramp breaches) that trace the pali in the north and the rim in the south. This geometry, combined with left-stepping fractures along unbreached portions of the monocline upper hinge, suggest a recent component of right-lateral motion where the active scarp traces the pali, consistent with the contemporary regional E-W extension. The monocline shows variable states of disaggregation that point to the ongoing effects of earthquake activity. Although there have been no historic events along the fault, the length of the active scarp suggests possible M6.5 events.
External link: AGU database
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