MESOZOIC TECTONICS AND METAMORPHISM IN THE PEQUOP MOUNTAINS AND WOOD HILLS REGION, NORTHEAST NEVADA - IMPLICATIONS FOR THE ARCHITECTURE ANDEVOLUTION OF THE SEVIER OROGEN
Pa. Camilleri et Kr. Chamberlain, MESOZOIC TECTONICS AND METAMORPHISM IN THE PEQUOP MOUNTAINS AND WOOD HILLS REGION, NORTHEAST NEVADA - IMPLICATIONS FOR THE ARCHITECTURE ANDEVOLUTION OF THE SEVIER OROGEN, Geological Society of America bulletin, 109(1), 1997, pp. 74-94
The Pequop Mountains-Wood Hills-East Humboldt Range region, northeast
Nevada, exposes a nearly continuous cross section of Precambrian to Me
sozoic strata representing middle to upper crustal levels of the Mesoz
oic hinterland of the Sevier orogen. These rocks preserve the transiti
on from unmetamorphosed Mesozoic upper crust to partially melted middl
e crust. Integration of new structural, metamorphic, and U-Pb thermoch
ronologic data from the Wood Hills and Pequop Mountains, coupled with
a regional tectonic reconstruction, reveals substantial Cretaceous met
amorphism, contraction, and extension in the Sevier hinterland in nort
heast Nevada. We report two phases of contraction not previously recog
nized that are accommodated by top-to-the-southeast thrust faults, the
Windermere and Independence thrusts. Contraction was succeeded by two
phases of extension along west-rooted normal faults, the Late Cretace
ous Pequop fault and Tertiary Mary's River fault system. The earliest
phase of thrust faulting resulted in as much as 30 km of crustal thick
ening and an estimated minimum of 69 km of shortening along an inferre
d fault called the Windermere thrust, The timing of this thrusting eve
nt is bracketed between Late Jurassic (ca. 153 Ma) and Late Cretaceous
(84 Ma), Relaxation of crustal isotherms following and perhaps during
thrusting resulted in Barrovian-style metamorphism of footwall rocks,
and partial melting of metapelite at deep levels. Peak metamorphism w
as attained ca. 84 Ma, and by this time hinterland crustal thickening
had reached a maximum. During 84-75 Ma another minor pulse of shorteni
ng and thickening along the Independence thrust was followed by partia
l exhumation of the metamorphic rocks and as much as 10 km of crustal
thinning along the Pequop fault. Thus the interval from 84 to 75 Ma in
northeast Nevada marks a fundamental, and apparently permanent, chang
e from horizontal contraction to extension in the upper to middle crus
t in the hinterland. Final exhumation of the metamorphic rocks was acc
omplished by the Tertiary Mary's River fault system. Our data indicate
that much of the metamorphism and some of the contraction in the Sevi
er hinterland in northeast Nevada, which was previously thought to be
largely Late Jurassic, is actually Cretaceous in age. Furthermore, the
data indicate that widespread metamorphism of the middle crust is a b
yproduct of tectonic burial, and that hinterland and foreland thrust f
aulting were coeval, suggesting that thrust faults in the Sevier oroge
n do not form a simple foreland younging sequence.