Early Miocene change in direction of least principal stress, southwestern United States: Conflicting inferences from dikes and metamorphic core‐detachment fault terranes

Abstract

Previous investigations have suggested that the direction of least principal stress in the western United States has shifted from northerly to easterly between 50 to 30 Ma. An effort is made to more tightly constrain the time of this shift using igneous dike‐host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 Ma. In such systems, the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock. These systems indicate that a northerly least principal stress orientation persisted more recently than previously believed, until probably after 30 Ma, possibly about 26 Ma, and conceivably as recently as 18 Ma in some areas. The data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift. However, the change could be diachronous—earlier in the southeast and later in the northwest. Major easterly trending magmatic and tectonic elements of Tertiary age in the Great Basin of Utah and Nevada are compatible with the dike data. However, numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes. This inconsistency is not clearly resolvable, but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by Mesozoic compression.