The title of one of the presented papers - "Terranes as Terrains" - aptly summarizes the essence of the study. Freely translated, this refers to an attempt to determine whether certain kinds of geo-structural units (terranes) have distinctive and diagnostic landscape features (terrains) that facilitate their identification and add to understanding their origin, significance, and history. Three goals prompted this investigation:

1. To define and describe, qualitatively and quantitatively, the distinguishing geomorphic characteristics of known (previously mapped) terranes,
2. To determine whether individual terranes in a multiple assemblage have geomorphic signatures sufficiently different to permit their recognition and separation in aerial/space imagery, and to better delineate boundaries between terranes based on terrain differences,
3. To assess geomorphic information about terranes as clues to modes of emplacement and structural imprints.

The word "terrane" originally referred to "a crustal block, usually bounded by faults, with a geologic history distinct from histories of juxtaposed blocks of differing characteristics". With the advent and acceptance of plate tectonic theory, this concept was modified by new insights into certain crustal blocks that came to be called "tectonostratigraphic", "accreted", or "suspect" terranes. In this view, terranes are considered to be diverse segments of crust that originated and developed often far from their present locations and over time were transported on a moving plate that converged against another plate, eventually colliding with a continental margin (along or within this second plate). There, they were obducted (shoved onto) against the margin rather than incompletely subducting (diving under) the second plate, thereby becoming accreted to that plate. To give an example: masses such as microcontinents embedded in oceanic crust enroute to a subduction zone will be unable to follow that crust downward and will be "scraped off" along thrust faults to emplace along the growing edge of the continental crust on the other plate. Or, a variant: if an island arc complex (e.g., Java, p. 17-2) on the upper plate above a subduction zone were squeezed between two converging plates as a continent on the lower plate eventually reached the zone, much of the arc would be caught "in the crunch" to be driven on and welded to (accreted) the encroaching continent. The usual criteria for terrane recognition are centered on stratigraphic, structural, and paleomagnetic discontinuities between adjacent terranes. (For a quick review of relevant concepts, read pp. 507ff and other parts of Chapter 18 in the introductory text: Physical Geology by B.J. Skinner and S.C. Porter, 1994, J. Wiley & Sons, Inc.)

Whereas in the thinking about growth of continents prior to the accreted terrane concepts, complex geo-crustal units in the continents were largely explained as consequences of fold-squeezing and fault transport during mountain building at geosynclinal sites, without much geographic displacement. Now, the new view recognizes distant site origins and relocations owing to long movements on the "conveyor belts" of diverging plates. Continents themselves thus grow by progressive accretion of numerous plates over time onto early cratonic nuclei. Since this concept first gained favor in the 1970s, hundreds of terranes have now be recognized on all continents, in the sense that older ideas about mapped geologic units have been re-interpreted to fit the terrane model. As an illustration of the terrane addition/continental growth version, consider this map that portrays a collage of the major terranes emplaced along the western margin of North America.

The Klamath Mountains of northwestern California-Southwestern Oregon were the type locality for development of the concept of accreted terranes, owing to pioneering mapping and interpretation by Irwin, Blake, and others. Early, somewhat generalized, maps of the two adjacent areas display the first interpretations of these assemblages of terranes (the upper map that follows shows the terranes in northern California and part of the Oregon Klamaths; the lower map continues coverage in Oregon to the north and west) :

Each of the named terranes is considered a unit which, after travelling some distance, "docked" against the paleo-North American continent over some specific period. Most of the inner (continentward) units are progressively older than those outside, although thrust faulting can sometimes carry a younger unit over part of older ones. Each terrane maintains some internal structural integrity and consists of series of stratigraphic units (formations), many not having counterparts in other nearby terranes (those that do were likely deposited across boundaries after terrane emplacements).

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