Article
Cenozoic extensional processes and tectonics in the northern Rocky Mountains
Northwest Geology
(2007)
Abstract
Extension has shaped the crust of the Northern
Rocky Mountains for more than a billion years.
Rifting produced the Belt basin shortly after 1.5
Ga and initiated the Paleozoic passive margin.
Mesozoic to early Cenozoic convergence pro-
duced the Cordilleran fold-and-thrust belt, the
uplifts of the Rocky Mountain foreland
(Laramide) and an unus
ually broad Challis arc
in the northern Rocky Mountains. The struc-
tural grain of the contractional belts, basement
features, and the rheology of the Idaho batho-
lith influenced the locus, kinematics and ge-
ometry of the >55 m.y. of extension that fol-
lowed.
Sparse data suggest that extension may have
begun in the Cretaceous during the "Sevier
orogeny", partially collapsing large culmina-
tions of the thrust belt near Salmon Idaho.
Widespread extension started during and after
middle Eocene time. The large amount of core
complex-related extension, starting within the
massive Eocene Challis-Sanpoil-Absaroka vol-
canic flare up, is highlighted by the unusual
presence of the paired Bitterroot and Anaconda
metamorphic core complexes.
Paleogene extension is the first of many kine-
matically distinct episodes of extension. Some
areas near the Eastern Snake River Plain pre-
serve as many as 6 different episodes of normal
faulting with extension directions ranging from
NE to NW to N. The normal faults with the
largest displacement typically parallel the curv-
ing contractional structures of the Late Meso-
zoic to Early Tertiary Sevier belt or formed
parallel to basement thrusts in the Rocky
Mountain foreland. This parallelism shows that
crustal inheritance has controlled the locus of
faults for more than 45 m.y. after the end of
shortening. Paleogene supradetachment basins
are localized in a fairly narrow N-S trending
belt between the Eastern Snake River Plain in
the south and the Lewis-and-Clark fault zone in
the north. Provenance and paleocurrents show
persistent southward flow along the axis of the
extended zone from much more highly ex-
tended areas in the core complexes to less ex-
tended regions of the Grant and Muddy-
Nicholia protobasins. Feldspathic and tuf-
faceous sediment accumulated within the su-
pradetachment basins during their translation
phase of development, and facies patterns re-
semble those of typical rift basins with steep
basin-bounding normal faults.
The youngest, currently active system of nor-
mal faults has a regular spacing and consistent
N to NW trends. More faults dip west than east
within the former fold-and-thrust belt. Faults
become more widely spaced westward in the
strong and isotropic Idaho batholith. Normal
faults east of the thrust belt have a range of
strikes and reactivate Laramide structures with
NE, NW and N and E trends. The Rocky
Mountain Basin-and-Range province narrows
northward and terminates near the Canadian
border. The spatial pattern and geometry of the
faults suggest clockwise rotation about an axis
near the north edge of the province, in agree-
ment with inversion of GPS data.
Active normal faults display additional com-
plexities near the Yellowstone hotspot and it is
clear that it influences the deformation in the
Basin-and-Range province. Normal faults
within the neotectonic, seismic, and topog-
raphic parabola centered on Yellowstone are
unusually active, whereas structures beneath
the Eastern Snake River Plain are unusually
quiescent because normal faults no longer ex-
tend across the Eastern Snake River Plain at the
surface (Anders et al., 1989). The 70° NW
plunge of the Yellowstone plume to 500-600
km (Yuan and Dueker, 2005) may be responsi-
ble for the asymmetry in extensional belts NW
and SE of the Eastern Snake River Plain
(Pierce and Morgan, 1992).
Earthquakes in the greater Yellowstone region
show domains of anomalous N-S extensional
strains north of the Eastern Snake River Plain
from the anomalous north-dipping Centennial
fault eastward. Flexure toward the Eastern
Snake River Plain plus SW subsidence away
from Yellowstone could produce a northeast-
ward migrating domain of E-W striking normal
faults NW of the Eastern Snake River Plain and
NNE-striking normal faults south of the East-
ern Snake River Plain. Lateral flow of lower
crustal rocks from beneath the Eastern Snake
River Plain might also contribute to this pat-
tern.
Keywords
- Cenozoic extension,
- Rocky Mountain,
- tectonic process
Disciplines
Publication Date
Summer January 1, 2007
Citation Information
Janecke, S. U., 2007, Cenozoic extensional processes and tectonics in the
northern Rocky Mountains: southwest Montana and eastern Idaho: Northwest
Geology, v. 36, p. 111-132.