Tectonic setting of the southern Cascade Range as interpreted from its magnetic and gravity fields

Abstract

We have compiled and analyzed aeromagnetic data from the southern Cascade Range and compared them with residual gravity data from the same region in order to investigate regional aspects of these young volcanic rocks and of basement structures beneath them. Various constant-level aeromagnetic surveys were mathematically continued upward to 4,571 m and numerically mosaicked into a single compilation extending from lat. 40°10′N to lat. 44°20′N. These data were reduced to the pole, upward continued an additional 10 km, and compared with a magnetic topographic model and with residual gravity data upward continued to the same level. Several intriguing regional features are suggested by these data. (1) The Trinity ophiolite complex that is exposed west of Mount Shasta probably dips at a shallow angle to the east and continues in the subsurface at least 10 km east of Mount Shasta. (2) Mount Shasta, Lassen Peak, and Medicine Lake volcanoes are located in a widespread magnetic low possibly caused by an upwarp of the Curie-temperature isotherm. (3) Crater Lake caldera is located at the intersection of various linear anomalies interpreted to be related to structure in basement rocks below the Cascade Range. (4) Three Sisters volcanoes and Newberry Crater are connected to each other by an arcuate magnetic source. (5) The High Cascades, from lat. 40°10′N to at least lat. 44°30′N, are marked by a residual gravity low which includes the Three Sisters volcanoes, Mount Shasta, Medicine Lake volcano, Mount McLoughlin, and Crater Lake. (We believe this gravity feature represents a major structural depression beneath the High Cascades.) (6) Except for Newberry Crater, every major volcano of the study area is located on the perimeter of a local gravitational low. We suggest that the gravity lows reflect subsidence of low-density volcanic material relative to denser country rock and that the major volcanoes have developed over structures at the perimeters of their respective depressions.