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Volcanic signature of Basin and Range extension on the shrinking Cascade arc, Klamath Falls‐Keno area, Oregon

Detailed geologic mapping of the Klamath Falls‐Keno area revealed the complex relationship between subduction, crustal extension, and magmatic composition of the southern Oregon Cascade volcanic arc. Volcanism in the study area at ~7–4 Ma consisted of calc‐alkaline basaltic andesite and andesite lava flowing over a relatively flat landscape. Local angular unconformities are evidence that Basin and Range extension began at by at least ~4 Ma and continues today with fault blocks tilting at a long‐term rate of ~2°/Ma to 3°/Ma. Minimum NW‐SE extension is ~1.5 km over ~28 km (~5%). High‐alumina olivine tholeiite (HAOT) or low‐K, low‐Ti transitional high‐alumina olivine tholeiite (LKLT) erupted within and adjacent to the back edge of the calc‐alkaline arc as the edge receded westward at a rate of ~10 km/Ma at 2.7–0.45 Ma. The volcanic front migrated east much slower than the back arc migrated west: ~0 km/Ma for 6–0.4 Ma calc‐alkaline rocks; ~0.7 km/Ma, if ~6 Ma HAOT‐LKLT is included; and ~1 km/Ma, if highly differentiated 17–30 Ma volcanic rocks of the early Western Cascades are included. Declining convergence probably decreased asthenospheric corner flow, decreasing width of calc‐alkaline and HAOT‐LKLT volcanism and the associated heat flow anomaly, the margins of which focused on Basin and Range extension and leakage of HAOT‐LKLT magma to the surface. This declining corner flow combined with steepening slab dip shifted the back arc west. Compensation of extension by volcanic intrusion and extrusion allowed growth of imposing range‐front fault scarps only behind the trailing edge of the shrinking arc.