Page 386 - Global Tectonics

368 CHAPTER 11

(b)
(a)
Australia
Pilbara craton
Pilbara craton
X Paterson Orogen
X
Hamersley Basin
Hamersley Basin
Officer
Officer
Basin
Paterson Orogen
Basin
Port Hedland Capricorn
Collier
20 Collier
Capricorn
Basin
Basin
Pilbara
Newman Orogen
Orogen
25
Meekatharra X X
Yilgarn
30
Yilgarn craton
Kalgoorlie Yilgarn craton
PERTH
35
Perth
Perth
115 120 125
800
Albany – Fraser Orogen
Proterozoic/Phanerozoic basins
Proterozoic orogens nT Albany – Fraser Orogen
Archean cratons 800
Capricorn Orogen
(c) Officer Basin Collier Basin Hamersley
X Basin X
0 km 0 50
10
Yilgarn Craton Pilbara Craton 20 km
Moho 30
Fig. 11.17 (a) Tectonic map (after Hackney, 2004, with permission from Elsevier) and (b) magnetic anomaly
image emphasizing gradients in total magnetic intensity (after Kilgour & Hatch, 2002, with permission from
Geoscience Australia, image provided by M. Van Kranendonk, Geological Survey of Western Australia). Magnetic
anomaly image shows total magnetic intensity measured in nanoteslas (nT) compiled from airborne, marine and
land-based geophysical surveys. The distinctive magnetic anomaly patterns from the Pilbara and Yilgarn cratons

reflect different structural trends that resulted from Precambrian plate tectonic processes. (c) Interpretive cross-
section of the Capricorn Orogen showing the sutured cratons (after Cawood & Tyler, 2004, with permission from
Elsevier).
(Fig. 11.18) (Collins et al., 2004). In the modern Earth, recording conditions of ∼1.2 GPa and 650–700°C at
these unusual conditions are met in subduction– 1.9–1.88 Ga also occur in the Lapland Granulite Belt of
accretion complexes and at the sites of continental col- Finland (Tuisku & Huhma, 1998). The Aldan Shield in
lision where relatively cold crust is buried to subcrustal Siberia (Smelov & Beryozkin, 1993) and the Snowbird
depths. For these subducted rocks to return to the tectonic zone between the Rae and Hearne cratons of
Earth’s surface with preserved eclogite facies mineral Canada (Baldwin et al., 2003) preserve retrogressed
assemblages, they must be exhumed rapidly before tec- eclogites of 1.90 Ga. The absence of Archean eclogite
tonically depressed isotherms can re-equilibrate and facies rocks suggests that before Early Proterozoic times
overprint the assemblages with higher temperature either the conditions to produce such rocks did not
granulite facies minerals. exist, the processes to exhume them at a suffi cient rate
The oldest examples of in-situ eclogites (i.e. rocks to preserve eclogite-facies mineral assemblages did not
other than xenoliths) include 1.80 Ga and 2.00 Ga vari- exist, or all pre-existing examples have been obliterated
eties (<1.8 GPa, 750°C) from the North China craton by subsequent tectonic events.
and the Proterozoic orogens surrounding the Tanzanian The presence of ophiolitic assemblages in Precam-
craton (Zhao et al., 2001; Möller et al., 1995). Eclogites brian orogens provides another possible marker of tec-