Eocene Stress Change in the Basin and Range and Colorado as Measured by Brittle Structures in Plutons

Owing to their moderate size, relative textural homogeneity, and isotopic age
constraints, and propensity to fracture systematically, plutons and their internal
joint-vein orientations and character offer a type of strain gauge that can be used to
determine the directions and relative magnitudes of sub-solidus maximum (σ1) and
minimum (σ3) principal regional stresses. These brittle structures, especially joints
and mineralized joints (hydrofractures), form both systematic and non-systematic
sets that record stresses that were present early in the history of the plutons and
that were preserved during subsequent unroofing, even over a regional extent; see
for instance the dominant ENE strike of J1 joints in central and southern Arizona indicative
of ENE greatest principal stress and NNW least principal stress (Rehrig and
Heidrick, 1972). A more recent regional study of numerous Laramide-age (ca. 75–60
Ma) stocks of central Colorado (Rehrig et al., 2007) reveals that orientations of mineralized
and barren structures are nearly identical to those found in Laramide-age
intrusions of southern Arizona, thus expanding the regional consistency of causative
paleo-stresses. Recent study of brittle structures in a number of Eocene-Oligocene
(40–30 Ma) plutons of the Rocky Mountain province of Colorado, and preliminary
study of similar age plutons in the Basin and Range province of Utah and Nevada
indicate that structure patterns are also similarly consistent over a large region but
indicative of a different stress regime.
The orientations of Laramide-age brittle structures over a multi-state area strike
ENE to EW and are believed to be the result of maximum compressive stress oriented
in the same direction. Approximately orthogonal to the attitude of mineralized joints
in Laramide plutons is a later unmineralized set of joints of different character believed
to be formed at shallower crustal levels. In sharp contrast, beginning in late Eocene
and persisting into the early Oligocene, brittle structures in plutons dominantly
strike NS to NNE and typically are curviplanar (sinuous), irregular in form, and variably
spaced. The overall character of fractures/joints of the Eocene-Oligocene plutons
may suggest shallower crustal emplacement and subdued compressive stresses
compared to that indicated by Laramide plutons. The strikes of brittle structures
formed over large ca. 108 km2 regions during both intrusive epochs are mirrored
by decameter-scale dikes and veins shown on published geologic maps from areas in
Colorado, Utah, and Nevada.
Our data to date help define the end of the classic Laramide Orogeny at about
40 Ma based on the regional consistency of brittle strain in the similarly aged plutons
over the CO, UT, AZ, and NV region. The marked change in regional principal
stress directions between Laramide and mid-Tertiary time is proposed to be linked
to a fundamental change in North American plate motions. Laramide compression
caused by the roughly EW motion of North America driven by opening of the Atlantic
Ocean and flat slab subduction along the west margin of the continent. The change to a significantly less intense, northerly bearing principal stress and major EW-trending
extension through Colorado and the Basin and Range may be due to a combination
of processes including opening of the Arctic Ocean (Gries, 1983) and/or EW extension
related to initial foundering of the subhorizontal Farallon slab and ingress of
asthenophere.
Key Words: pluton, Laramide, brittle structure, Sigma 1, extension, tension, orthogonal