Payne, S.J., King, R.W., Kattenhorn, S.A., McCaffrey, R. (2008)
Accommodation of right-lateral shear along the northwest boundary of the Snake River Plain, Idaho
Eos, Transactions of the American Geophysical Union 89(52), Fall Meeting Supplement, Abstract G21A-0661.
The northwest boundary of the Snake River Plain (SRP) is a transition from range-bounding normal faults in the Centennial tectonic belt (CTB) to the topographically low and volcanic-dominated province of the SRP. Within the CTB, the northern and central segments of three prominent NW-trending normal faults are seismically active, but their activity decreases southward toward the SRP. Deformation in the SRP is associated with infrequent small magnitude (M<2) microearthquakes and NW-trending volcanic rift zones that result from basalt dike intrusion since 6 to 4 Ma. GPS results from 1994-2008 indicate that the CTB extends via normal faulting at a rate that is an order of magnitude greater than the extension rate in SRP. Right-lateral shear in a NE-trending transition region, what we call the Centennial Shear Zone (CSZ), is a geometric consequence of the different strain rates. The inferred slip rate across the CSZ increases from ~1 mm/yr in the southwest to ~2 mm/yr in the northeast. The shear may extend as far as the Yellowstone Plateau at an even faster rate. Estimated slip rates within the CSZ suggest up to 2 km of lateral offset in a million years. In the CSZ, investigators have identified only NE-trending normal faults but without significant strike-slip offset on them. The most recent and larger fault offsets (15-110 ka) are associated with the NW-trending, range-bounding normal faults. To assess how right-lateral shear is accommodated, we use block models to invert GPS horizontal velocities with earthquake slip vectors and fault slip rates for relative block motions. The predicted slip along the block faults is evaluated with geologic observations such as fault linkages within segment boundaries. We explore two possible alternatives for how right-lateral shear may be accommodated; 1) strike-slip earthquakes on NE-trending faults, or 2) oblique slip along the NW-trending faults, which is indicative of a regional transtensional environment.
External link: AGU database
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