Terrane analysis

Many orogens are composed of a collage of fault-bounded blocks that preserve geologic histories unrelated to those of adjacent blocks. These units are known as terrones and may range in size from a few hundreds to thousands of square kilometers. Terranes usually are classified into groups according to whether they are native or exotic to their adjacent continental cratons (e.g. Section 11.5.5). Exotic (or allochthonous) terranes are those that have moved relative to adjacent bodies and, in some cases, have traveled very great distances. For example, paleomagnetic investigations have demonstrated that some terranes have a north-south component of motion of several thousand kilometers (Beck, 1980; Ward et al., 1997) and have undergone rotations of up to 60° (Cox, 1980; Butler et al., 1989). The boundaries of terranes may be normal, reverse, or strike-slip

Fold-and-thrust belt

(Passive Asian continental margin)

H Pleistocene-Holocene foreland

Miocene-Pleistocene shallow sediments Paleogene shallow sediments

Accretionary prism

(Deep marine slope-trench sequences) Miocene slate and turbidites Underthrust Eurasian continent

Pre-Cenozoic Metamorphic basement

Accreted Luzon arc-forearc ^

I Accreted North Luzon — Trough and volcanic arc

Taiwan Strait

Figure 10.29 Tectonic map of the Taiwan arc-continent collision (images provided by C.-Y. Huang and modified from Huang C.-Y. et al., 2000, with permission from Elsevier).

Figure 10.29 Tectonic map of the Taiwan arc-continent collision (images provided by C.-Y. Huang and modified from Huang C.-Y. et al., 2000, with permission from Elsevier).

(a) Intra-oceanic subduction (15-11 Ma to Recent)

Accretionary wedge

(Hengchun Ridge) arc-prism Manila boundary

Trench

North Luzon Arc

Accretionary wedge

(Hengchun Ridge) arc-prism Manila boundary

Trench

North Luzon Arc

Eurasian Continent

North Luzon Trough

Eurasian Continent

(b) Initial arc-continent collision (5 Ma to Recent)

Fold-and-thrust belt Accretionary wedge

Western Foothills- arc-prism

Hsuehshan Range Central Range boundary (Kaoping Slope) (Hengchun Ridge) / North Luzon Arc

Deformation Lishan-Laonung

North Luzon Trough

Western Foothills- arc-prism

Hsuehshan Range Central Range boundary (Kaoping Slope) (Hengchun Ridge) / North Luzon Arc

Deformation Lishan-Laonung

Intra-arc basin

Eurasian Continent

Philippine Sea Plate

Intra-arc basin

Eurasian Continent

Philippine Sea Plate

Advanced arc-continent collision (2 Ma to Recent)

Fold-and-thrust belt Accretionary wedge

Hsuehshan Range Central Range Clockwise rotation

Western Foothills ,. u _ , (Slate.belt) (MetamorPhic belt) ,---------

Coastal Range

Coastal Range

Philippine Sea Plate

(d) Arc collapse/subduction (1-2 Ma to Recent)

Fold-and-thrust belt Accretionary wedge

Hsuehshan Range Central Range Chihshui

(d) Arc collapse/subduction (1-2 Ma to Recent)

Fold-and-thrust belt Accretionary wedge

Hsuehshan Range Central Range Chihshui

Figure 10.30 Tectonic evolution of the Taiwan arc-continent collision (images provided by C.-Y. Huang and modified from Huang C.-Y. et al., 2000, with permission from Elsevier). (a) Intra-oceanic subduction. (b) Initial arc-continent collision. (c) Advanced arc-continent collision. (d) Arc collapse, subsidence, and subduction. LV, Longitudinal Valley, HTR, Huatung Ridge.

Figure 10.30 Tectonic evolution of the Taiwan arc-continent collision (images provided by C.-Y. Huang and modified from Huang C.-Y. et al., 2000, with permission from Elsevier). (a) Intra-oceanic subduction. (b) Initial arc-continent collision. (c) Advanced arc-continent collision. (d) Arc collapse, subsidence, and subduction. LV, Longitudinal Valley, HTR, Huatung Ridge.

faults; occasionally they may preserve thin ophiolites, blueschist, or highly deformed flysch. Terranes are 'suspect' if there is doubt about their paleogeography with respect to adjacent terranes or to the continental margin (Coney et al., 1980; Howell, 1989).

The identification and analysis of terranes is one of the most useful approaches to determining the long-term evolution of orogens, the mechanisms of continental growth, and the origin of the constituent components of continental lithosphere. Terrane recognition is based on contrasts in detailed stratigraphic and structural histories, although in many cases these have been destroyed or modified by younger events. Similarly the original nature of the bounding faults of many terranes may be obscured by metamorphism, igneous activity, or deformation. Consequently, in order to determine whether the geologic histories of adjacent terranes are compatible with their present spatial relationships, very detailed and comprehensive structural, geochemical and isotopic investigations are necessary (e.g. Keppie & Dostal, 2001; Vaughan et al., 2005). In practice several criteria are used to distinguish the identity of terranes, including contrasts in the following:

1 the provenance, stratigraphy, and sedimentary history;

2 petrogenetic affinity and the history of magmatism and metamorphism;

3 the nature, history, and style of deformation;

4 paleontology and paleoenvironments;

5 paleopole position and paleodeclination.

The rock associations that make up terranes tend to be similar among orogens. Consequently, investigators have grouped them into several general types (Jones et al., 1983; Vaughan et al., 2005):

1 Turbidite terranes characterized by thick piles of land-derived sediment that are transported offshore by density currents and deposited in a deep marine environment. The sequences commonly are siliciclastic and may also be calcareous. Most of these terranes have been metamorphosed and imbricated by thrust faulting during or after accretion; some may preserve a crystalline basement. Three main varieties occur:

(a) turbidites forming part of an accretionary prism in a forearc setting (Section 9.7)

with a large proportion of basaltic rock;

(b) turbidites forming part of an accretionary prism in a forearc setting with a minor proportion of basaltic rock;

(c) turbidites that escaped being incorporated into an accretionary prism.

2 Tectonic and sedimentary mélange terranes consisting of a heterogeneous assembly of altered basalt and serpentinite, chert, limestone, graywacke, shale, and metamorphic rock fragments (including blueschist) in a finegrained, highly deformed, and cleaved mudstone matrix. These terranes commonly are associated with flysch, turbidite terranes, and collision-subduction zone assemblages (Section 9.7), and may occur along the boundaries between other terranes.

3 Magmatic terranes, which may be predominantly mafic or felsic according to the environment in which they form. Mafic varieties commonly include ophiolites, pillow basalts associated with pelagic and volcanogenic sediment, subaerial flood basalts, sheeted dikes, and plutonic complexes. This category may represent rock generated by seafloor spreading, LIP formation (Section 7.4.1), arc volcanism, ocean islands, and fragments of basement derived from backarc and forearc basins. In some cases oceanic fragments are associated with overlying sedimentary sequences charting travel from deep sea to continental margin environments. Felsic varieties commonly include calc-alkaline plutonic rock and dispersed fragments of old continental crust.

4 Nonturbiditic clastic, carbonate, or evaporite sedimentary terranes, which fall into two categories:

(a) well-bedded, shallow marine fluvial or terrestrial sequences; such as those deposited on continental margins and shallow basins;

(b) massive limestones, such as those scraped off the tops of seamounts as they become incorporated into accretionary prisms.

5 Composite terranes, which consist of a collage of two or more terranes of any variety that amalgamated prior to accretion onto a continent. Examples of this type of

Successor basins or overlap sequences

Provenance linking

Successor basins or overlap sequences

Provenance linking

Pluton stitching

Figure 10.31 Geologic relationships that help establish the timing of terrane amalgamation and accretion (redrawn from Jones et al., 1983).

Pluton stitching

Figure 10.31 Geologic relationships that help establish the timing of terrane amalgamation and accretion (redrawn from Jones et al., 1983).

terrane include the Intermontane and Insular Superterranes of the Canadian Cordillera (Fig. 10.33a) and Avalonia (Figs 10.34; 11.24b).

The chronological sequence of terrane accretion onto a continent can be determined from geologic events that postdate accretion and link adjacent terranes (Fig. 10.31). These include the deposition of sediments across terranes boundaries (Fig. 10.31a), the appearance of sediments derived from an adjacent terrane (Fig. 10.31b), and the "stitching" together of terranes by plutonic activity (Fig. 10.31c).

Following the identification of the terranes that comprise an orogen, a variety of analytical tools may be applied to determine whether they are exotic or native to the adjacent cratons. In addition to paleomag-netic, structural, and paleontologic studies; these include the application of isotope geochemistry and geochronology to determine the thermal evolution, provenance history, and crustal sources of the terranes. The most commonly used provenance techniques include the dating and geochemical characterization of zircon using U, Pb, and Hf isotope compositions (e.g. Gehrels, 2002; Hervé et al., 2003; Griffin et al., 2004). Zircon is a highly refractory mineral that commonly occurs in granitoids and sedimentary rock and may preserve isotopic evidence of multiple phases of igneous and metamorphic growth. Comparisons of the age spectra from detrital zircon populations collected from sedimentary and metasedimentary rocks are especially usefUl for determining provenance history (e.g. Sections 3.3, 11.1). Analyses of the composition and petrologic evolution of xenoliths carried to the surface from great depths provide another important means of probing the deep roots of terranes to determine their age, sources, and tectonic evolution (e.g. Section 11.3.3).

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