Teleseismic travel-time anomalies and deep crustral structure in central Australia

Abstract

SUMMARY Teleseismic travel times have been recorded along two lines of instruments traversing the Amadeus Basin and southern Arunta Block of central Australia for the purpose of examining the deep crustal and upper-mantle structure underlying these features. Pronounced differential travel times, approaching 1.5 s, and with major azimuthal variation, have been recorded. Most of the useful observations originated from events to the north (Japan-Marianas) and to the east (Fiji-Tonga) at epicentral distances of 40-65d. A few observations of southerly azimuth events (Macquarie Ridge and South Sandwich Islands) were also recorded. The inferred structure beneath the basin and block exhibits major lateral variations, the essential aspects of which are: (1) a zone of relatively high-velocity material dipping steeply northwards such that the continuation of its southern edge to the surface coincides approximately with a mylonitic shear zone known as the Redbank Deformed Zone; (2) a steeply dipping zone of relatively low-velocity material immediately to the south of this shear zone and partly underlying the high-velocity zone. If the travel times are interpreted in terms of an undulating Moho, variations in Moho depth of 20 km occur over horizontal distances of less than 50 km with relatively shallow Moho depths occurring immediately to the north of the Redbank Deformed Zone. Such an interpretation is consistent with the gravity anomalies observed across the geological structures. The inferred structure is out of hydrostatic equilibrium and departures from local isostasy approach 150 MPa. These stress-differences must have persisted during the 300 Ma following the Alice Springs Orogeny that led to the present crustal structure. Evidence, in the form of the surface topography and seismicity, suggests that some stress relaxation has occurred and still is occurring but not at the shear zone itself. At the time of the orogeny the structure may have been close to isostatic equilibrium, but as erosion took place complete local rebound did not occur, either because horizontal compressive forces still operated or because the strength of the lithosphere increased with time.