Musgrave Block

Australia: The Land Where Time Began

Musgrave Block

The Musgrave Block crustal structure of the present has been found to date from the Petermann Ranges Orogeny, as determined from teleseismic studies of travel time (Lambeck & Burgess, 1992; op.cit.). Upward movement of the subeclogitic rocks to the positions they occupy at the present in the Bates Sheet is accounted for by overthrusting along the Woodroffe Thrust, though the mechanism of their descent to 40 km is uncertain. The author has previously depicted the Woodroffe Thrust as steepening at depth, following Lambeck & Burgess, invoking underthrusting of the footwall block (Stewart, 1997; op.cit. Fig.17). The hanging wall mid-crustal granulites were left stranded much shallower than 40 km. It has been concluded that substantial thickening of the crust was involved in the Petermann Ranges Orogeny when the regional extent of subeclogitic rocks immediately to the east of Bates was recognised, and a possible solution was presented (Scrimgeour & Close, in press (at time of writing): op. cit.)

At about 550 Ma, crustal compression and thickening of the crust depressed the mid-crustal granulites, of 1.2 Ga age, to subeclogitic-facies depths of about 40 km, according to one possible event sequence (Source 1, Fig. 18b). The subeclogitic rocks from the lower crust were transported onto the rocks of the amphibolite-facies by the east-northeast overthrusting along the Woodroffe Thrust (source 1, Fig. 18c). Along the Wintiginna and Lindsay Lineaments, this was accompanied by high-angle reverse faulting.

On the Mann Fault movement directions have proven to be problematic, and it been considered to be a thrust (Lambeck & Burgess, 1992: op. cit. p.17). According to the author, the hanging-wall block above the Woodroffe Thrust is depicted as upper crust in their figure 11, which conflicts with their textural reference to this block (pp. 11-12) as having a lower crustal velocity (7 km/s). To the south of the Mann Fault, the lower crust is displaced downwards relative to Woodroffe Thrust hanging wall block-i.e. nett movement on the Mann Fault is normal. In the south of the Bates region, the Mount Aloysius Fault is a splay or en echelon offshoot of the Mann Fault to the east. Along the Mount Aloysius-Mann Fault a wedge of crust was left as the highest part of the region by normal, or transtensional, faulting (Source 1, Fig. 18d). In the lower portion of this wedge the subeclogitic rocks of the lower crust are preserved in the Bates as the hanging wall of the Woodroffe Thrust.

2 thrusting episodes are involved in an alternative scheme that was based on modelling of the Alps and the Himalayas (Butler, 1986: Journal of the Geological Society of London, 143, 857-873). A slab of crust is depressed to subeclogitic-facies depths by underthrusting along the proto-Mann Fault (low-angle). The subeclogitic rocks were elevated to the position of the present, and caused the steepening of the Mann Fault to its present attitude by subsequent initiation of the Woodroffe Thrust and renewed contraction (Source 1, Fig. 18g).

There are significant differences between the 2 models - their sense of movement on the Mann Fault, normal or reverse, and just south of the Bates region, their depiction of the moho. Stewart suggests a detailed study of the Mann Fault and its adjoining rocks could be tested to determine their sense of shear, and across this part of the Musgrave Block, a deep seismic survey.

Metamorphic rocks, granites, some of which have been metamorphosed, intrusions of mafic-ultramafic (Giles Complex), and mafic dykes comprise the Musgrave Block. There is a range of metamorphic facies from greenschist to subeclogite. The block is cut, and the crust is penetrated, by major low to high angle faults that are east-striking, the largest of which is the Woodroffe Thrust. To the north of it there are felsic gneisses and deformed granite that have amphibolite mineral assemblages that have been dated to 1600-1550 Ma. To the south of it there are subordinate mafic and volcanic rocks, as well as sedimentary rocks of shallow-water origin that were deposited between about 1580-1300 Ma, that had been metamorphosed to granulite facies at about 1200 Ma. The granulites were succeeded by outliers of the Giles Complex, large volumes of granite masses, that have been dated to about 1190 Ma, and 3 generations of mafic dykes. In the Bates area the relationship of the 2 regions before they were juxtaposed is not known.

Compression of the Australian Plate, that was north-south, resulted in the Woodroffe Thrust, that dips gently south, during the Petermann Ranges Orogeny at 550-530 Ma (Lambeck & Burgess, 1992; Australian Journal of Earth Sciences, 39, 1-19). There are granulite facies rocks on both sides of the Mount Aloysious Fault, that dips steeply to the south and is normal, crosses the Bates area in the south. Along the northern edge of the Mount Aloysius Massif, coincident topographic and magnetic lineaments, and pressure estimates of 1000-1400 MPa, (equating to a 40 km depth) north of the fault and south of the fault, 300-500 MPa Clarke et al., (1995a: op. cit.).

During the Petermann Ranges Orogeny, regional metamorphism produced subeclogitic rocks, displaying mafic grains in the hanging-wall rocks of the Woodroffe Thrust that are surrounded by ubiquitous and spectacular garnet-bearing coronas.

As the overriding rocks travelled up-slope, the changing metamorphic conditions are indicated by mineral assemblages in the thrust zone, that is intensely deformed, that range from subeclogitic to greenschist.

See Source 1 for more detailed information

Situated in central South Australia, the Musgrave Block separates the North Australia Craton, that is dominantly of Palaeoproterozoic age, from the Gawler Craton in South Australia, of Late Archaean to Early Mesoproterozoic age. The early history of the Musgrave Block was probably linked to subduction development along the northern margin of the Gawler Craton, as suggested by geochemical and Nd isotopic data from felsic rocks in the Mann Ranges, dating to about 1.59-1.55 Ga. Negative anomalies of Nb, Ti and Y are among the characteristic geochemical patterns of these felsic rocks, and they are accompanied by steep light rare earth element patterns, and Nd isotopic compositions that are comparatively juvenile (Ɛ_Nd (1550) values from -1.2 to 0.9). These early Mesoproterozoic felsic rocks have been found to have geochemical and isotopic signatures that are similar to those of island arcs in which there are Ti-bearing minerals and garnet. The authors propose that the arc-like rocks in the Musgrave Block that date to 1.59-1.55 Ga indicate there was an active margin between the North Australian Craton and the South Australian Craton, suturing of the Australian continent subsequently occurring in the Early Mesoproterozoic. During the Early Mesoproterozoic, the presence of arclike magmatism in the Musgrave Block suggests there was probably a period of major crustal growth in Australia in the Proterozoic, with important implications for current Proterozoic reconstructions of Australia, as well as the fit of Australia in the Rodinia supercontinent (source 2).

Magmatic arc development involving similar processes to those described for Proterozoic terranes in Laurentia and the Baltic Shield, were involved in the geological evolution of the Musgrave Block (e.g. Hoffman, 1980; Lewry. 1981; Condie, 1982; Park, 1985; Dickin & McNutt, 1990; Snyder et al., 1996; Torsvic et al, 1996; Patchett & Chase, 2002; Steltenpohl et al., 2003). Minor crustal contamination, possibly by the subduction of sediment, is suggested by Nd data that indicate a juvenile signature.

The arc rocks of the Musgrave Block, and coeval tectothermal events, dated to about 1.58 Ga, affecting the North Australian Craton and the South Australian Craton, developed more than 200 My, later than the period of global crustal growth in other parts of the world, that have been well-characterised at about 2.0-1.8 Ga. It is suggested by these results that in Australia during the Proterozoic the major periods of growth of the crust occurred over a larger range of ages than occurred in other parts of the world.

See Source 2 for more detailed information

Source 3

The Musgrave Block is a plutonic rock from the Proterozoic with prominent granites and gneisses. Though an irregular skyline results from the varied composition of the hills, what is apparently a low relief summit surface can be detected. A clearer representative of this surface may be a a prominent high plain that cuts across rocks, that are metamorphic and sedimentary, in the Petermann Ranges. According to the author³ it is believed likely to be of considerable antiquity, based on its height above the extensive silcreted surface to the north and east that dates to the Miocene. The Everard Ranges to the south consist of granite rocks in the form of domes and bornhardts. In the Kulgera area to the east the bornhardts are from an earlier time than the Jurassic, as sedimentary remnants of the age standout as mesas among the bornhardts (Twidale, 1994). The western limit of the marine transgressions from the later Mesozoic, and therefore the exhumed surface, is not known in detail, as is the situation with the planation surface on the southwestern margin of the Eromanga Basin, which is to the south of the area being discussed. To the west the bevelled domes, which are roughly to the west of the Stuart highway may be either exhumed or epigene landforms from the Late Jurassic or Early Cretaceous.