New Mexico middle crustal cross sections: 1.65 Ga macroscopic geometry, 1.4 Ga thermal structure and continued problems in understanding crustal evolution
Williams, M.L., Karlstrom, K.E., Lanzirotti, A., Read, A.S., Bishop, J.L., Lombardi, C.E., Wingstead, M.B., 1999, New Mexico middle crustal cross sections: 1.65 Ga macroscopic geometry, 1.4 Ga thermal structure and continued problems in understanding crustal evolution, in: Rocky Mountain Geology: Lithospheric structure and evolution of the Rocky Mountains Part II, (special volume), University of Wyoming, K.E. Karlstrom (ed), v.34, no. 1, p. 53-56.
Recent detailed work in key regions along two north-south transects in
northern New Mexico highlights continued controversy about Proterozoic
tectonic evolution. Ductile deformation features (folds, ductile thrusts,
and associated foliations and lineations) are grouped into three deformation
generations. D1 includes cryptic bedding-parallel foliation
and fold nappes. D 2 involves north-verging, km- scale inclined
folds, the main shortening foliation, and D 2 structures that
further attenuate or reactivate F 2 folds. D 3 involves
east-west-trending open folds and domes and associated crenulation cleavage.
Although others can dominate locally, S 2 is the dominant regional
foliation that could possibly be imaged seismically. Map relationships
around ca. 1.65- and ca. 1.42-Ga plutons and porphyroblast- matrix studies
of dated minerals show that D 3 occurred at ca. 1.42. The age
of D 2 is more uncertain and could be 1.65 or 1.42 Ga. Metamorphic
studies also indicate multiple metamorphic events, M 1 -M 3
, that may relate to the deformational events. New geochronology indicates
that most metamorphic minerals grew (or were reset) at ca. 1.47-1.35 Ga.
U- Pb dates on metamorphic zircon, monazite, titanite, staurolite, garnet,
and tourmaline suggest regional metamorphism to 550-700 degrees C at 1.47-1.42
Ga. Metamorphic aureoles are present around plutons, but the highest grades
of metamorphism are in areas with no exposed 1.42-Ga plutons. Metamorphism
is interpreted to record a regional mantle-driven thermal event, the latter
parts of which correspond to a time of pluton emplacement. 40Ar/39Ar
dates record post-1.42- Ga cooling: the highest grade rocks yield the
youngest cooling ages, indicating slow cooling and gradual unroofing of
the 1.42- Ga thermal profile following 1.42-Ga metamorphism. Our preferred
model is that macroscopic geometries (D 1 -D2 )
were established by 1.65 Ga, and that regional amphibolite-grade metamorphism
and associated D 3 deformation at 1.47-1.42 Ga produced localized
high-strain domains and fabric reactivation at exposed levels. At deeper
levels, structures and assemblages may increasingly record 1.42-Ga reactivation.