Precambrian crystalline basement map of Idaho: An interpretation of aeromagnetic anomalies

Idaho lies within the northern sector of the U.S. Cordillera astride the
boundary between the Proterozoic continent (Laurentia) to the east and
the Permian to Jurassic accreted terranes to the west. The continental
basement is mostly covered by relatively undeformed Mesoproterozoic
metasedimentary rocks and intruded or covered by Phanerozoic igneous
rocks; accordingly, knowledge of the basement geology is poorly
constrained. Incremental knowledge gained since the pioneering studies
by W. Lindgren, C.P. Ross, A.L. Anderson, A. Hietanen, and others during
the early- and mid-1900's has greatly advanced our understanding of the
general geology of Idaho. However, knowledge of the basement geology
remains relatively poor, partly because of the remoteness of much of the
region plus the lack of a stimulus to decipher the complex assemblage of
high-grade gneisses and migmatite of central Idaho. The availability of
an updated aeromagnetic anomaly map of Idaho (North American Magnetic
Anomaly Group, 2002) provides a means to determine the regional
Precambrian geologic framework of the State. The combined geologic and
aeromagnetic data permit identification of previously unrecognized
crystalline basement terranes, assigned to Archean and Paleoproterozoic
ages, and the delineation of major shear zones, which are expressed in
the aeromagnetic data as linear negative anomalies (Finn and Sims,
2004). Limited geochronologic data on exposed crystalline basement
aided by isotopic studies of zircon inheritance, particularly Bickford
and others (1981) and Mueller and others (1995), provide much of the
geologic background for our interpretation of the basement geology. In
northwestern United States, inhomogeneities in the basement inherited
from Precambrian tectogenesis controlled many large-scale tectonic
features that developed during the Phanerozoic. Two basement
structures, in particular, provided zones of weakness that were
repeatedly rejuvenated: (1) northeast-trending ductile shear zones
developed on the northwest margin of the Archean Wyoming province during
the Paleoproterozoic Trans-Montana orogeny (Sims and others, 2004), and
(2) northwest-trending intra-continental faults of the Mesoproterozoic
Trans-Rocky Mountain strike-slip fault system (Sims, unpub. data, 2003).
In this report, geologic ages are reported in millions of years (Ma) and
generalized ages are given in billions of years (Ga). The subdivision
of Precambrian rocks used herein is the time classification recommended
by the International Union of Geological Sciences (Plumb, 1991).