Abstract

Along the northwestern margin of the Basin and Range province, mid-Miocene to Pliocene volcanic rocks cover and obscure much of the earlier history of the region. In northeastern California, however, slip on the Surprise Valley fault has resulted in the uplift of the Warner Range, exposing >4 km of volcaniclastic and volcanic rocks as old as late Eocene. New geologic mapping, combined with geochemistry and geochronology of rocks in the Warner Range and surrounding region, documents a history of volcanism and extension from the Eocene to the present that provides insight into the evolution of this margin. Our work reveals that subduction-related arc volcanism began ca. 40 Ma and continued into the mid-Miocene, despite the nearby impingement of the Yellowstone hotspot and eruptions of flood basalts. Extensional normal faulting began in the mid- to late Miocene in relative isolation from other Basin and Range normal faults. Later Miocene and Pliocene volcanic rocks flowed into low-lying areas produced by mid-Miocene extension. These younger basalts are cut by normal faults, requiring a second episode of extension that began after 3 Ma. Our cross-section reconstructions indicate that 12%–15% extension has been accommodated across the Warner Range region, primarily along the Surprise Valley fault, which has accommodated 8 km of dip-slip motion. A similarly protracted or two-part history of extension has been observed elsewhere in the western Basin and Range. While relatively little extension has been accommodated in the Warner Range region, it continues to the present day. Thus, the Surprise Valley fault appears to have persisted as the westernmost boundary of Basin and Range extension since the mid-Miocene.

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