Page 352 - Global Tectonics

OROGENIC BELTS 335

faults; occasionally they may preserve thin ophiolites, with a large proportion of basaltic rock;

blueschist, or highly deformed flysch. Terranes are (b) turbidites forming part of an accretionary
‘suspect’ if there is doubt about their paleogeography prism in a forearc setting with a minor
with respect to adjacent terranes or to the continental proportion of basaltic rock;
margin (Coney et al., 1980; Howell, 1989). (c) turbidites that escaped being incorporated
The identification and analysis of terranes is one of into an accretionary prism.

the most useful approaches to determining the long- 2 Tectonic and sedimentary mélange terranes
term evolution of orogens, the mechanisms of conti- consisting of a heterogeneous assembly of
nental growth, and the origin of the constituent altered basalt and serpentinite, chert, limestone,
components of continental lithosphere. Terrane recog- graywacke, shale, and metamorphic rock
nition is based on contrasts in detailed stratigraphic and fragments (including blueschist) in a fi ne-
structural histories, although in many cases these have grained, highly deformed, and cleaved

been destroyed or modified by younger events. Simi- mudstone matrix. These terranes commonly
larly the original nature of the bounding faults of many are associated with fl ysch, turbidite terranes,
terranes may be obscured by metamorphism, igneous and collision-subduction zone assemblages
activity, or deformation. Consequently, in order to (Section 9.7), and may occur along the
determine whether the geologic histories of adjacent boundaries between other terranes.
terranes are compatible with their present spatial rela-
3 Magmatic terranes, which may be
tionships, very detailed and comprehensive structural,

predominantly mafic or felsic according to the
geochemical and isotopic investigations are necessary
environment in which they form. Mafi c
(e.g. Keppie & Dostal, 2001; Vaughan et al., 2005).
varieties commonly include ophiolites, pillow
In practice several criteria are used to distinguish
basalts associated with pelagic and volcanogenic
the identity of terranes, including contrasts in the
sediment, subaerial fl ood basalts, sheeted dikes,
following:
and plutonic complexes. This category may
1 the provenance, stratigraphy, and sedimentary represent rock generated by seafl oor spreading,
history; LIP formation (Section 7.4.1), arc volcanism,
ocean islands, and fragments of basement
2 petrogenetic affinity and the history of

derived from backarc and forearc basins. In
magmatism and metamorphism;
some cases oceanic fragments are associated
3 the nature, history, and style of deformation;
with overlying sedimentary sequences charting
4 paleontology and paleoenvironments; travel from deep sea to continental margin
5 paleopole position and paleodeclination. environments. Felsic varieties commonly
include calc-alkaline plutonic rock and dispersed
The rock associations that make up terranes tend to fragments of old continental crust.
be similar among orogens. Consequently, investigators
4 Nonturbiditic clastic, carbonate, or evaporite
have grouped them into several general types (Jones
sedimentary terranes, which fall into two
et al., 1983; Vaughan et al., 2005):
categories:
1 Turbidite terranes characterized by thick piles (a) well-bedded, shallow marine fl uvial or
terrestrial sequences; such as those
of land-derived sediment that are transported
offshore by density currents and deposited in a deposited on continental margins and
shallow basins;
deep marine environment. The sequences
commonly are siliciclastic and may also be (b) massive limestones, such as those scraped
calcareous. Most of these terranes have been off the tops of seamounts as they
metamorphosed and imbricated by thrust become incorporated into accretionary
faulting during or after accretion; some may prisms.
preserve a crystalline basement. Three main 5 Composite terranes, which consist of a
varieties occur: collage of two or more terranes of any
(a) turbidites forming part of an accretionary variety that amalgamated prior to accretion
prism in a forearc setting (Section 9.7) onto a continent. Examples of this type of