Horizontal and vertical tectonics

The origin of the unique dome-and-keel architecture of the Archean cratons (Section 11.3.4) is important for understanding the nature of Archean tectonics. In general, interpretations can be divided into contrasting views about the relative roles of vertical and horizontal displacements in producing this pattern. The Eastern Pilbara craton in western Australia illustrates how vertical and horizontal tectonic models have been applied to explain the dome-and-keel structural style. During this discussion, it is important to keep in mind that the crustal structure, as illustrated by the Pilbara example, is the product of multiple episodes of deformation, metamorphism, and pluton emplacement rather than a single tectonic episode.

Vertical tectonic models describe the diapiric rise of hot granitoid domes as the result of a partial convective overturning of the middle and upper crust. Collins et al. (1998) and Van Kranendonk et al. (2004) used strain patterns, a dome-side-up/greenstone-side-down sense of displacement in shear zones, and other features to link the formation of dome-and-keel structures to a sinking of the greenstones. The process begins with the emplacement of hot TTG suite (Section 11.3.2) granitoids into an older greenstone succession (Fig. 11.10a). Domes are initiated at felsic volcanic centers due to a laterally uneven emplacement of TTG magma. After a hiatus of several tens of millions of years, the emplacement of thick piles of basalt on top of less dense granitoids creates an inverted crustal density profile (Fig. 11.10b). The magmatism also buries the granitoids to mid-crustal depths where they partially melt due to the build up of radiogenic heat and, possibly, the advection of heat from mantle plume activity. Thermal softening and a reduction in mid-crustal viscosity facilitates the sinking of the greenstones, which then squeezes out the underlying partial melts into rising, high-amplitude granitoid domes (Fig. 11.10c). The convective overturning depresses geotherms in the greenstone tracts, resulting in local cooling and the preservation of kyanite-bearing metamorphic rocks, which equilibrate at moderate-low pressures (~600 MPa) and temperatures (500°C). This model explains the formation of the dome-and-keel structure without rigid plates or plate boundary forces and is similar to the sinking or sagduc-tion models proposed for the formation of dome-and-keel structures in the Dharwar craton of India (Chardon et al., 1996).

Horizontal tectonic models for the Eastern Pilbara propose that the greenstones were affected by one or more periods of horizontal contraction and extension (Blewitt, 2002). In these interpretations, the contraction results from episodes of Early Archean collision (Sections 10.4, 10.5) and terrane accretion (Section 10.6). Periods of horizontal extension result in the formation of crustal detachments and the emplacement of the granitoid domes. Kloppenburg et al. (2001) used observations of multiple cross cutting fabrics and unidirectional patterns of stretching lineations to suggest that the Mt. Edgar Dome initially formed as an extensional metamorphic core complex (Sections 7.3, 7.6.3, 7.6.6). An initial period of terrane collision and thrusting prior to 3.32 Ga thickens the Early Archean Warrawoona Greenstone Belt and buries granitoid basement to mid-crustal levels where it partially melts. Partial melting

(a) 3.515-3.430 Ga early crust formation

Duffer and Panorama Fms

Duffer and Panorama Fms

(b) 3.410-3.325 Ga volcanism and thermal incubation

(b) 3.410-3.325 Ga volcanism and thermal incubation

(c) 3.325-3.308 Ga convective overturn - Mt. Edgar Dome

(c) 3.325-3.308 Ga convective overturn - Mt. Edgar Dome

Fig. 11.10 Three-stage diapiric model of dome-and-keel provinces in the Eastern Pilbara craton (after Collins et al., 1998, with permission from Pergamon Press, Copyright Elsevier 1998; the age-spans of the stages are from Van Kranendonk et al., 2007).

', ' Kyanite-bearing rock

Fig. 11.10 Three-stage diapiric model of dome-and-keel provinces in the Eastern Pilbara craton (after Collins et al., 1998, with permission from Pergamon Press, Copyright Elsevier 1998; the age-spans of the stages are from Van Kranendonk et al., 2007).

facilitates the extensional collapse of the thickened crust, forming detachment faults (i.e. Mt Edgar shear Zone, Fig. 11.11a) similar to those found in Phanerozoic core complexes (e.g. Figs. 7.14b, 7.39c). The density inversion created by dense greenstones overlying buoyant, partially molten basement triggers the rise of granitoid domes at 3.31 Ga by solid state flow during extension (Fig. 11.11b). This extension is accommodated by displacement on the Mt. Edgar shear zone and by lateral strike-slip motion in a transfer zone within the gneissic basement. Normal-sense displacements drop greenstones down between the rising domes. Emplacement of the domes steepens the detachments and changes the geometry of the system so that its structure no longer resembles that of Phanerozoic core complexes (Fig. 11.11c). Steepening during periods of shortening provides an alternative explanation of the near vertical sides of the granitoid domes.

The application of both vertical and horizontal models to Archean cratons involves numerous uncer-

Marble Bar Belt

Coppin Gap Belt

Coongan Belt

Marble Bar Belt

Coongan Belt

SAL = Salgash Subgroup

DF = Duffer Formation

TT = Talga Talga Subgroup

= Active deformation

Mt Edaar Dolerite sills feed the

Basement unknown j

Layer parallel NE-SW extension and Emplacement of 3.46-3.43 Ga doming of the precursor to the Mt. Edgar Dome precursor to gneisses s -10 -15 "

Back-tilting and exhumation of the MESZ

Compressed geothermal gradient

Sinistral transfer movement

Flank of detachment

Back-tilting and exhumation of the MESZ

Compressed geothermal gradient

Sinistral transfer movement

Flank of detachment

Folding of the greenstones Discordant intrusion post-extensional

A WOONA

CORUNNA DOWNS A \ WA-BRAWOONA

MT. EDGAR GRANITOID COMPLEX

A WOONA

CORUNNA DOWNS A \ WA-BRAWOONA

Steepening of the back-tilted flank and batholith-up sense of shear

MT. EDGAR GRANITOID COMPLEX

Discordant intrusion

Discordant intrusion

Steepening of the back-tilted flank and batholith-up sense of shear

Fig. 11.11 Cartoon summarizing the tectonic and magmatic development of the Warrawoona Greenstone Belt (WGB) and Mt. Edgar Dome by horizontal extension (after Kloppenburg et al., 2001, with permission from Elsevier). (a) Pre 3.33 Ga gabbro/diorite and dolerite intrusions, NE-SW extension, and doming of the Mt. Edgar Granitoid Complex. (b) Differential extension on the Mt. Edgar Shear Zone (MESZ) and lateral motion along a transfer zone within the granitoid complex at 3.31 Ga. (c) Final localized normal displacements and steepening of the MESZ followed by discordant intrusions of post-extensional plutons.

tainties. Problems with diapiric models commonly include uncertainties surrounding the timing of convec-tive overturn and how an inverted density profile could be maintained or periodically established over a 750 million year history without thrust faulting (Van Kranendonk et al., 2004). How the stiff rheology of granitoids allows diapirism also is unclear. Problems with horizontal tectonic models may include a lack of evidence of large-scale tectonic duplication of the greenstones by thrusts in some areas and uncertainties surrounding how the formation of metamorphic core complexes could produce the distinctive ovoid patterns of the granitoids. Horizontal tectonic models also commonly encounter difficulty explaining the kinematics and horseshoe-shaped geometry of shear zones that border many granitoid domes (Marshak, 1999).

A comparison of the evolution of various Archean cratons has suggested that aspects of both horizontal and vertical tectonic processes occurred in different places and at different times. Hickman (2004) highlighted numerous tectonic and metamorphic differences between the Eastern and Western parts of the Pilbara craton prior to ~2.95 Ga. He showed that, unlike the more or less autochthonous dome-and-keel structure of the Eastern Pilbara, the Western Pilbara preserves a series of amalgamated terranes (Section 10.6.1) that are separated by a series of thrusts and strike-slip shear zones (Fig. 11.8) and involved episodes of horizontal compression that resemble a Phanerozoic style of plate tectonics. These differences suggest that both vertical and horizontal tectonics played an important role during the formation of the Pilbara craton.

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  • aleeza
    How are the horizontal tetonic movements different from the vertical movements?
    2 years ago
  • Goytiom Temesgen
    What are vertical and horizontal tectonics with sketch?
    11 months ago
  • ernesta
    What are the holizontal tectonic movement?
    10 months ago
  • HOB
    What is the result of horizontal tectonic movements?
    2 months ago
  • Outi
    What is the result of horizontal tectonic movement?
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