In 1981, the U.S. Geological Survey conducted a seismic refraction survey of northeastern California designed to characterized the structure in four geologic provinces: the Klamath Mountains, Cascade Range, Modoc Plateau, an Basin and Range provinces. The survey consisted of north-south lines in the Klamath Mountains and Modoc Plateau provinces, northwest-southeast lines centered on Mount Shasta and Medicine Lake volcano, and an east-west line linking all the profiles. All lines except the east-west line ranged in length from 125 to 140 km, contained three shot points, and were recorded by 100 instruments. The east-west line was 260 km long, contained six shot points, and was recorded by 200 instruments. The Klamath and Modoc lines yielded the simplest models. The Klamath model is finely layered from the surface to at least 14-km depth, consisting of a series of high-velocity layers (6.1--6.7 km/s), ranging in thickness from 1 to 4 km, with alternating positive and negative velocity gradients. A layer with an unreversed velocity of 7.0 km/s extends from 14 km downward to an unknown depth. The Modoc model, in contrast, is thickly layered and has lower velocity at all depths down to 25 km. The uppermost layer, 4.5 km thick, consists of low-velocity material 2--4.5 km/s). Velocity beneath this layer is much higher 6.2 km/s) and increases slowly with depth. A small velocit step (to 6.4 km/s) is seen at 11 km, and a larger step (to 7.0 km/s) is seen at 25 km depth. Moho is probably 38--45 km deep under the Modoc Plateau, but its depth is unknown under the Klamath Mountains.

Models of the Shasta and Medicine Lake lines show special features including low velocity (less than 3.5 km/s) in the edifice of Mount Shata but high velocity (5.6 km/s) at shallow depth (1--2 km) under the summit of Medicine Lake colcano. The model for the east-west line consists of a western part similar to the Klamath model, and eastern part similar to the Modoc model, and laterally changing velocity structure in between, underlying the Cascade Range. This model was converted to a density model, and observed Bouguer gravity data were matched. A general decrease in Bouguer gravity values eastward may be explained by a general decrease in the density of crustal layers and does not require a change in crustal thickness. Beneath the 4.5-km-thick surficial layer, the velocity model for the Modoc Plateau is similar to that determined by other researchers for a refraction line in the Sierra Navada. It is unlike velocity models for rift areas, to which the Modoc Plateau has been likened by some authors. We theorize that beneath its surficial volcanic and sedimentary rocks, the Modoc Plateau is underalain by a basement of granitic and metamorphic rocks that are the roots of ancient magmatic arc(s). The fine layering in the Klamath model is consistent with the imbricate structure of the Klamath Mountains. Independent modeling of aeromagnetic data indicates that the base of the Trinity ultramafic sheet corresponds to a velocity step from 6.5 to 6.7 km/s at 7-kmk depth in our model. The 6.7 km/s layer beneath the Trinity ultramafic sheet apparently corresponds to rocks of the central metamorphic belt, mostly amphibole schists, which crop out west of the Trinity ultramafic sheet.

Deeper velocity layers can likewise be correlated to terranes that crop out farther west. In our geologic cross section of northeastern California, derived from our velocity-density model for the east-west line, the Klamath Mountains are underlain by folded and thrust-faulted slices of oceanic crust. The Modoc Plateau and westernmost Basin and Range province are underlain by a section of volcanic and sedimentary rocks overlying granitic and metamorphic rocks, all tilted westward between an inferred fault under Medicine Lake volcano and the Surprise Balley fault. In the Cascade Range, geologic units appear to be discontinuous, and structures include horsts, grabens, and a 10-km step downward to the east in the 7 km/s layer. The latter step may represent a fault, fold, intrusion, or a combination of any of the three. Apparently, the Cascade Range, a modern magnetic arc, is developed across the suture region between the stack of oceanic rock layers underlying the Klamath Mountains and the inferred roots of magnetic arc(s) underlying the Modoc Plateau.