VOLCANIC AND TECTONIC GAP BETWEEN THE LASSEN AND SHASTA SEGMENTS OF THE CASCADES VOLCANIC ARC

The ~50 km gap in Quaternary volcanic vents between the Lassen and Shasta segments of the Cascades volcanic arc has been known for over 3 decades (Guffanti and Weaver, 1988, JGR 93-6). Our recent 1:24,000 geologic mapping in this region provides detailed evidence that the gap has been a significant E-W volcanic and tectonic discontinuity for at least 1.8 Ma. Young calc-alkaline volcanic rocks (280–38 ka) related to subduction of the Gorda South plate are restricted to south of ~41° latitude, where the physiographic expression of the arc is conspicuous. In this region, low-potassium olivine tholeiites (LKOTs; 1,600–24 ka) related to Basin and Range extension impinge on the calc-alkaline arc and flowed from south to north to termini near ~41° at the Pit River. Both calc-alkaline and LKOT vents are aligned NNW, parallel to regional normal faults. North of ~41°, however, for ~50 km there is no expression of the young calc-alkaline arc, and the Shasta segment is offset ~50 km to the west. This intermediate terrain consists of low-relief LKOTs (1,770–296 ka) that flowed from north to south, surrounding and partially burying >2 Ma calc-alkaline edifices that have eroded and degraded physiography. The termini of both sets of LKOTs near ~41° display hyaloclastites and pillow breccias, indicating contact with lakes or streams in the ancestral Pit River valley. South of ~41°, the arc is characterized by prominent, NNW-trending, predominantly normal faults separating grabens or half-grabens containing young volcanic rocks from horsts of older volcanic rock. North of ~41°, distributed NNW normal faulting affected the LKOTs but did not produce a graben-and-horst fabric involving the older calc-alkaline edifices. A topographic low has been present at latitude ~41° for at least 1.8 my, as witnessed by ⁴⁰Ar/³⁹Ar dates of 998 ka on silicic tuff in the thick diatomite deposits along the Pit River and 1,773 ka on an underlying basalt. This basalt in turn overlies gravels representing the ancestral course of the Pit River. The observed volcanic and tectonic features are consonant with a differing geometry of the subducting Gorda Plate north and south of ~41° latitude.