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| Scientific Investigations Map 2884 |

## Precambrian Crystalline Basement Map of Idaho—An Interpretation of         Aeromagnetic Anomalies

#### By P.K. Sims, Karen Lund, and E. Anderson

| 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).

**Version 1.0**
Posted April 2005 |
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U.S. Department of the Interior | U.S. Geological Survey      URL: https://pubsdata.usgs.gov/pubs/sim/2005/2884/Questions or Assistance: USGS Web ContactPage Last Modified: Thr Dec  1 17:02 EDT 2016 |
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