THE SOUTHERNMOST CASCADES VOLCANIC ARC AND ITS RELATIONSHIP TO THE BASIN AND RANGE AND THE SIERRA NEVADA

We present three geologic maps that illustrate relationships of the southernmost Cascades Volcanic Arc to the Basin and Range and the Sierra Nevada. The maps cover 8,500 km² and extend 167 km north from 40° to 41° 22.5ʻ latitude. Our geologic mapping has been enhanced by many paleomagnetic determinations, precise chemical analyses and argon age determinations.

During the last 2 m.y., the southernmost Cascades Volcanic Arc reflects subduction of the Gorda plate and impingement of Basin and Range extension. Rocks of the volcanic arc are calc-alkaline (CA), whereas rocks related to Basin and Range extension are low-potassium olivine tholeiitic basalt (LKOT). Geochemistry, mineralogy and isotopes demonstrate that CA magmas are derived from melting of asthenosphere above the subducting Gorda plate, whereas LKOT magmas are derived from shallower depths in the mantle lithosphere of the North American plate.

From 40° to 40° 37.5ʻ, the Cascades Volcanic Arc is dominated by 6 major CA volcanic centers (voluminous, long-lived foci of intermediate to silicic volcanism) that overlie Sierra Nevada basement. The volcanic centers are nested among CA basalts and andesites and scattered LKOTs. Flows emplaced southward from the Yana Volcanic Center (3.4–2.4 Ma) towards the Sierra Nevada now dip north towards their vent area, indicating relative uplift of the Sierra Nevada. Prominent NNW-striking normal faults SSE of our maps are mostly buried by young volcanic rocks in the Lassen Volcanic Center (~825–0 ka) and the Caribou Volcanic Field (~425–25 ka) but are conspicuous again NNW of these centers.

From 40° 37.5ʻ to 41°, young CA volcanic rocks of the Cascades Volcanic Arc spatially overlap with extensive Basin and Range LKOTs that flowed northward towards the Pit River (at 41°). Vent locations for both magma types were controlled by NNW-striking normal faults, with vents of the two types as nearby as 1 km. For 50 km from 41° to 41° 22.5ʻ, there is no CA volcanism younger than ~2 Ma, despite the presence of many normal faults. Instead, the terrain is dominated by extensive LKOTs (1,770–36 ka) that enveloped >2 Ma CA edifices and flowed south towards the Pit River. The Pit River has thus occupied a topographic low for at least 1.8 m.y.. North of 41° 22.5ʻ, young CA volcanism is again abundant at Mount Shasta, Medicine Lake volcano and vicinity.