A regionally significant revision of Neogene tectonic history, northern Wassuk and Singatse Ranges, Lyon and Mineral Counties, Nevada

About 15–25% of the current dextral motion between the Pacific and North
America plates is accommodated across the Walker Lane belt, and it is likely that
similar accommodation dates back to at least 10 Ma when the eastern California
shear zone formed. The Wassuk Range-Singatse Range portion of the Walker Lane
belt holds the key to understanding the interrelationships between the early history of
this plate-boundary strain and Basin and Range extension.
Geologic relations reported here call for revisions in the magnitude and timing of
late Tertiary deformation as well as in the pattern of strain migration and the role of
magmatism in facilitating extension. Extension magnitude is revised downward from
150–200% to an estimated moderate amount of about 30–40% on the basis of a newly
discovered association of steep stratal dips in the Wassuk Range with folds rather
than tilt blocks and uniformly low to moderate dips in the southern Singatse Range.
The timing of main stratal tilting and extensional deformation is revised downward
from 15–13 Ma to 10–7.5 Ma for the following reasons: 1) 15–13 Ma andesite/dacite
rocks are generally concordant on, or intruded into, underlying tuffs, 2) andesite/
dacite intrusive masses lack the preferred orientation or dike-on-dike habit characteristic
of large-magnitude extension, 3) there is no widespread unconformity at the
ca. 12.5 Ma base of the Wassuk Group, and 4) there is a lack of syntectonic growthfault
fanning in 12.5–9 Ma sedimentary strata of the Coal Valley Formation of the
Wassuk Group. Moderate tilting of Tertiary strata is interpreted to be associated with
coeval strike-slip and normal faulting that define a transtensional tectonic setting, but
most steep tilts are associated with an added component of monoclinal folding at the
flanks of uplifted blocks of Mesozoic rocks and flexing of footwalls beneath convexupward
faults. Also, right- and left-slip faults with similar strikes are recognized, and
some of these are associated with a transtensional nappe structure. To explain this
complex and heterogeneous total strain field, we suggest that basal traction related to
extension-normal distributed shear or flow beneath a zone of partial decoupling was
superposed on regional transtension. The main 10–7.5 Ma transtensional deformation
is interpreted as associated with development of the eastern California shear
zone, greatly increasing the breadth of that deformation at latitude ~39° N within that
time interval and diminishing the amount of westward encroachment of extension
toward the Sierra Nevada. The northern Wassuk Range was the site of several
episodes of late Tertiary magmatism, any one of which could have caused thermal
softening and facilitation of deformation driven by far-field forces. There is no a-priori
reason to assume large-magnitude extension either was triggered by or accompanied
the 15–13 Ma andesite magmatism, as has been the practice in recent reports.