U. S. Geological survey pamphlet introduction, tectonic definitions, acknowledgments



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INTRODUCTION
This pamphlet contains an introduction, tectonic definitions, acknowledgments, descriptions of post-accretion stratified rocks, and descriptions for tectonostratigraphic terranes, and references for the companion tectono-stratigraphic terrane and overlap assemblage map of Alaska. The companion map describes and depicts the tectono-stratigraphic terranes and major overlap assemblages, except for the Aleutian Islands. In Alaska the major overlap assemblages consist mainly of upper Mesozoic sedimentary rocks, and Upper Cretaceous and Cenozoic sedimentary and volcanic and plutonic rocks of Alaska that were deposited on, or intruded into adjacent terranes after tectonic juxtaposition or accretion.

A tectonostratigraphic terrane is define as a fault-bounded geologic entity or fragment that is characterized by a distinctive geologic history that differs markedly from that of neighboring terranes (Jones and others, 1983; Howell and others, 1985). Tectonostratigraphic terranes (hereafter referred to as "terranes") are mainly fault-bounded, stratigraphically coherent assemblages that formed before accretion to adjacent terranes or to a continental margin. A few terranes are fault-bounded structural complexes formed in subduction zones or in accretionary prisms. Terranes are bounded by faults or fault zones some of which are referred to as sutures. Some sutures are inferred beneath areas of overlap assemblages or from juxtaposition of contrasting geologic sequences. Paleontologic, stratigraphic, and paleomagnetic evidence suggests that some terranes were originally widely separated from one another, and (or) from cratonal North America by distances of up to thousands of kilometers. In contrast for other terranes, paleontologic, stratigraphic, and paleomagnetic evidence suggests that they are displaced from one another, or from another loci on the same continent by distances that are greater than the dimensions of terranes to

greater than hundreds of kilometers. Definitions of tectonic terms are discussed and defined below.

Inferred tectonic environments of terranes are defined below and depicted on the map. These environments are: (1) cratonal margin; (2) passive continental margin; (3) metamorphosed continental margin; (4) continental margin arc; (5) island arc; (6) turbidite basin; (7) oceanic crust, sea mount, and ophiolite; (8) accretionary wedge and subduction zone; and (9) metamorphic. For terranes with varying geologic history, the chosen tectonic environment indicates the dominant tectonic environment interpreted for the older sequences of rocks in the terrane. The tectonic environments inferred for plutonic rocks are described in relation to associated accretionary events. These environments are pre- and post-accretion.

Upper Mesozoic sedimentary rocks and Upper Cretaceous and Cenozoic sedimentary and volcanic rocks are also shown. These rocks are products of magmatism and sedimentation that occurred mainly after accretion of many terranes in the late Mesozoic or Cenozoic. Some of the Late Cretaceous and early Cenozoic igneous and sedimentary rock units, as well as fragments of terranes, are offset as much as 400 km by movement along post-accretion faults. Rock units that occur as overlap assemblages on two or more adjacent terranes provide minimum ages on the timing of accretion of terranes. For simplicity, the map does not depict some post-accretion, mainly Cenozoic age faults that occur within terranes, such as the Hines Creek and Tozitna faults. The time scale used in this report is that of Palmer (1983).

The basic outcrop pattern and distribution of tectonostratigraphic terranes for the map are from Beikman (1980), Jones and others (1987), Monger and Berg (1987), Dusel-Bacon (1991), Grantz and others (1991), Gehrels and Berg (1992), Moore and others (1992), Reed and others (written commun., 1993), cited references, and the authors and contributors. The outcrop pattern and age classification of Cenozoic and older igneous rocks is modified from T.P. Miller (written commun., 1991), D.A. Brew (written commun., 1989, 1990, 1991), Dusel-Bacon (1991), Moll-Stalcup (1990), and E.J. Moll-Stalcup (written commun., 1991).





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