Geophysical Expression and Tectonic Setting

In document Basement uplift and basin subsidence in Central Australia (Page 42-45)


2.1 Geophysical Expression and Tectonic Setting

As an example of the crustal structure in central Australia a section is examined where the gravity anomalies are most conspicuous, that is across the northern margin of the Amadeus Basin, the Arunta basement, and the Ngalia Basin ( Figs 2-01 and 2-02 ).

A 30-50 km wide thrust belt at the northern margin of the Amadeus Basin extends east- west for about 700 km ( Fig. 2-01). It is an unusual feature because, although it contains major thrusts and nappes typical of plate collision boundaries, it was almost certainly formed in an intracratonic setting. Activation of the thrust belt was preceded by widespread development of interconnected intracratonic basins over a 600 Ma period ( Shaw et al. in prep (a).). Unlike most orogenic belts synorogenic magmatism is negligible. During re-activation of the thrust belt in the Late Devonian to Carboniferous during the Alice Springs Orogeny, granulites were uplifted along a moderately north-dipping, crustal-scale shear zone, which was previously active in the mid-Proterozoic (Shaw et al. 1984). Large amplitude gravity and teleseismic residual anomalies indicate a lack of isostatic balance, yet the crustal structure produced

substantially in the Late Devonian to Early Carboniferous has remained relatively stable. A lack of isostatic balance can be concluded from the large magnitude of the gravity anomalies (up to 200 mgal peak to trough amplitude for Bouguer anomalies; over a region with moderate topography). The major gravity gradient and the rapid change in teleseismic arrival times are both centred on the main megashear in the thrust belt, referred to as the Redbank Deformed Zone ( R DZ ) (Lambeck et al. 1988). A difference in P-wave travel time anomalies of up to nearly 1.5 s has been documented across the thrust belt with a minimum recorded in the basin and the maximum in the basement north of the granulites ( Lambeck et al. 1988).

The thrust belt is made up of several hinterland-dipping, south-verging thrust sheets (Shaw et al. 1971; Forman and Shaw, 1973; Maijoribanks, 1976 ). A detachment zone, localised in a salt horizon, formed during the Alice Spring Orogeny when the sedimentary

cover slid away from the rising basement and formed para-allochthonous sheets. Box folds formed in the sequence above the main detachment zone. As basement uplift continued, basement thrusts folded earlier formed basement cored nappes and broke through the arching cover sequence.

The thrust belt is dominated by three major geological structures ( Figs 2-03, 2-04, 2-05, 2-06 ):

(1) the Redbank Deformed Zone ( RD Z), (2) the Amadeus Basin Homocline ( A H ) and (3) the Ormiston Nappe and thrust zone ( O N ).

The RDZ is about 7 to 10 km wide and is made up of narrow, anastomosing mylonite zones that dip northwards from 30° to 65°, with an average surface dip of 45°. It separates a southern terrane, composed mainly of granidc rocks metamorphosed to amphibolite facies, from a northern terrane dominated in outcrop by mafic and subordinate felsic rocks

metamorphosed to granulite facies ( Fig. 2-07 ). The two terranes differ not only in gross composition and metamorphic grade, but also in structural history and regional stratigraphy (Maijoribanks, 1976; Shaw et al. 1984; Shaw and Black, in press ). The sense of shear across the zone is north over south ( Shaw et al. 1984; Shaw and Black, in press; Tessyier et al. in

1988 ). A series of fault blocks across the southern terrane show evidence of increasing metamorphic grade northwards due to progressive uplift. Most of the change in grade, however, is concentrated at the northern margin of the RDZ. Maijoribanks (1976) suggested that the thrust at the base of the Razorback Nappe (RN in Fig. 3-1) originally rooted in the RDZ.

The AH is an abrupt upturn in strata at the northern margin of the Amadeus Basin ( Fig. 2-06) which folds earlier formed thrusts and decollements and is locally expressed as a zone of

reverse faults. It has a strike length in excess of 200 km. Immediately south of the homocline the basin reaches depths of more than 10 km. The homocline formed at the time of basement uplift in the Late Devonian to Early Carboniferous when thick molasse-like sandstones and conglomerates were deposited in front of the rising basement ( Jones, 1972 ). The homocline has rotated one nappe klippe into a sub-vertical position ( e.g. Razorback Nappe of

Maijoribanks, 1976 ) so much of the uplift across the homocline post-dates thrusting. Upturning at the homocline edge has exposed multiple detachment zones at the base of the sedimentary sequence. The most prominent sub-vertical detachment zone has been traced ( as part of the present study) within the Bitter Springs Formation along the length of the

homocline. The detachment zones probably formed as the basement rose and the basement cored nappes moved southwards, whereas the AH is a dominately younger structure which folds the detachment zones as well as the southernmost basement-cored nappes. The main fault controlling the development of the homocline is immediately south of the preserved basin margin. It separates upturned beds in the north from steep south-dipping beds to the south. The homocline is a complex, composite feature made up of both younger fault zones, which formed at the time of bending and originated in the basement, and older bedding-parallel

detachment zones in the cover. The ages of these features probably overlap. Early erosion and uplift along the Amadeus Homocline is suggested by low temperatures, relative to stratigraphic level, inferred from low conodont CAI values (1-1.5 in the Horn Valley Siltstone ) which Gorter ( 1984 ) relates to thinning of the synorogenic sediments ( Pertnjara Group ).

The ON ( Figs. 2-04, 05, 3-16 and reverse fault north of Ormiston Gorge in 2-07 ) (Maijoribanks, 1976) lies between the RDZ and the AH in the east, but cuts up section to merge with the AH in the west. The fault dips at 50° for much of its length and separates a region of high-grade migmatitic rocks to the north from a lower grade sequence of granitic

gneisses unconformably overlain by metasediments to the south. To the east the thrust zone merges with the Charles River Fault (Fig. 3-16). In its westwards strike continuation the thrust zone appears to overide the decollements in the lowest carbonate unit in the cover sequence ( Bitter Springs Formation ) and to place the basal sandstone unit ( Heavitree

Quartzite ), with basement attached, in contact with units much higher up in the sequence ( e.g. Pertatakaka Formation ). It is the rise of the basement at the ON late in the thrusting sequence that has probably folded and overturned older klippen and partly contributed to the severe upturning of strata in the western section of the Amadeus Basin Homocline ( e.g. Mount Razorback area, Fig. 2-07 ; station 4 in Redbank line, Fig. 2-04). However, overturned beds related to faulting within the homocline also occur in the east (e.g. between Ellery Creek (Fig. 2-5) and the Hugh River (Sections 4 & 5, Fig 2-1) (Figs 3-19 & 3-20) and Fig.2-05 ).

In document Basement uplift and basin subsidence in Central Australia (Page 42-45)