John N. Davies

Volcanoes as Possible Indicators of Tectonic Stress Orientation—Aleutians and Alaska

A new method for obtaining from volcanic surface features the orientations of the principal tectonic stresses is applied to Aleutian and Alaskan volcanoes. The underlying concept for this method is that flank eruptions for polygenetic volcanoes can be regarded as the result of a large-scale natural magmafracturing experiment. The method essentially relies on the recognition of the preferred orientation of radial and parallel dike swarms, primarily using the distribution of monogenetic craters including flank volcanoes. Since dikes tend to propagate in a direction normal to the minimum principal stress (T-axis), the method primarily yields the direction of the maximum horizontal compression (MHC) of regional origin. The direction of the MHC may correspond to either the maximum (P-axis) or intermediate (B-axis) principal stress.The direction of MHC obtained at 20 volcanoes in the Aleutian arc coincides well with the direction of convergence between the Pacific and North American plates. This result provides evidence that in the island arc the inferred direction of MHC is parallel to the maximum principal tectonic stress. In the back-arc region, general E-W trends of MHC are obtained from seven volcanic fields on islands on the Bering Sea shelf and the mainland coast of Alaska. These volcanic fields consist mostly of clusters of monogenetic volcanoes of alkali basalt. In the back-arc region, the trends of MHC may correspond to an E-W intermediate, a vertical maximum, and a N-S minimum principal stress.Implications for the tectonics of island arcs and back-arc regions are: (1) volcanic belts of some island arcs, including the Aleutian arc, are under compressional deviatoric stress in the direction of plate convergence. It is improbable that such arcs would split along the volcanic axis to form actively spreading marginal basins. (2) This compressional stress at the arc, probably generated by underthrusting, appears to be transmitted across the entire arc structure, but is apparently replaced within several hundred kilometers by a stress system characterized by horizontal extension (tensional deviatoric stress) in the back-arc region. (3) The volcanoes associated with these two stress systems differ in type (polygenetic vs. monogenetic) and in the chemistry of their magmas (andesitic vs. basaltic). These differences and the regional differences in orientation of the principal tectonic stresses suggest that the back-arc stress system has its own source at considerable depth beneath the crust.

Crustal structure of the active margin, south central Alaska: An interpretation of seismic refraction data from the Trans‐Alaska crustal transect

Seismic refraction and wide-angle reflection data from the U.S. Geological Surveys Trans-Alaska Crustal Transect are used to investigate the upper crustal structure of southcentral Alaska and to develop a model using two-dimensional asymptotic ray theory. The data considered here are from the N-S 126-km Cordova Peak refraction profile, which was positioned to cross the Chugach and Prince William terranes. These two tectonostratigraphic terranes form part of a large accretionary complex related to the Alaska subduction zone. Interpretation of data from the Cordova Peak profile indicates systematically higher velocities in Chugach terrane rocks as compared with Prince William terrane rocks at comparable depths. Unconsolidated sediment and glacial ice overlie Chugach terrane rocks Of unusually high compressional velocities (4.7–7.0 km/s) in the upper 10 km of crust. A northward thickening metasedimentary and metavolcanic flysch sequence, having an average velocity of 6.0 km/s, is underlain by metavolcanic mafic basement rocks having an average velocity of 6.6 km/s. The upper 10 km of the Prince William terrane have compressional velocities ranging from 3.7 to 6.2 km/s and are correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks and underlain by mafic to ultramafic Prince William terrane basement rocks. The upper crustal velocity structures of the Chugach and Prince William terranes are distinct to at least 10 km depth, which suggests that the Contact fault zone is a boundary which extends to at least 10 km. Midcrustal layers beneath the Chugach terrane contain two velocity reversals which may indicate the presence of off-scraped oceanic sediment, overpressurized zones, shear zones, or some combination of these possibilities. Data from energy sampling midcrustal layers beneath the Prince William terrane do not provide similar evidence for low-high velocity zones in that area.