NEO Geophysics

Geophysical studies have provided potential answers to several important questions concerning the development of the New England Orogen and the nat ure of the crust in this region. Among these uncertainties are how the New England terranes and associated deformation relate to the older, early- mid Paleozoic Lachlan and Thomson orogens to the south and west, as well as the nature of faults separating major tectonostratigraphic terranes wit hin the orogen and whether some of these terranes may be exotic to continental Australia (eastern Gondwana). Both active and passive seismic studi es, as well as gravity and magnetics, have contributed to the understanding of the geologic history of this region and its present-day structure.

Seismic studies

The Australian Bureau of Mineral Resources (now the Australian Geological Survey Organization) conducted a series of active source seismic experiments in the mid-late 1980s and early 1990s, consisting of several predominantly east-west profiles across the orogen at a range of latitudes (Figure 4.1). Interpretations of this data suggest that the New England Orogen is doubly vergent, with decollement surfaces and numerous thrust faults dipping both east and west. In addition, parts of the New England Orogen have apparently been thrust beneath what is interpreted to be the Lachlan Orogen, while other slices have been thrust above it.

Upper crustal structure

Seismic profiles identified as BMR89.B01-B02, BMR84.14-16 and BMR86.17, and BMR91.G01 trend east-west and were respectively shot in the northern, middle, and southern portions of the New England Orogen (Figure 4.1).

Figure 4.1. Map showing the locations of BMR seismic lines in the northern, middle, and southern portions of the New England Orogen (from Korsch et al., 1997)

These profiles show some variations in upper crustal structure along the strike of the orogen. In particular, only east-dipping faults are observed on the northernmost profile, BMR89.B01-B02, while both east- and west-dipping thrusts are present in the middle and southern profiles (Figure 4.2). In addition, the contact between the Lachlan and Thomson orogens and the New England Orogen changes in character from north to south. In particular, the northern profile seems to show terranes of the New England Orogen displaced westward onto the Thomson Orogen.

Figure 4.2. BMR seismic data from profiles BMR89.B01 and BMR89.B02, collected in the northern New England Orogen. Subsurface crustal faults and sutures in this portion of the New England Orogen are uniformly east-dipping (from Korsch et al., 1997)

In contrast, the middle profile shows arc and forearc strata of the New England Orogen thrust over the Lachlan Orogen, while accretionary wedge strata of the NEO have apparently been thrust beneath the Lachlan Orogen (Figure 4.3). This observation implies that the boundary between the accretionary wedge and forearc strata is a weak detachment surface, perhaps a pre-existing fault. The southern profile shows a similar structure to that apparent in the middle profile, with part of the NEO thrust over the Lachlan Orogen and with a small volume of material in the Tamworth belt (forearc basin strata) detached from the undergoing thrust plate and deformed by backthrusting.

Figure 4.3. BMR seismic data from profile BMR91.G01, collected in the southern New England Orogen. Structures within the crust of this portion of the NEO dip gently both east and west (from Korsch et al., 1997)

Korsch et al. (1997) interpreted these changes as reflecting significant differences in the character of the crust along the length of the NEO and suggested that the crust may actually be divided into distinct zones that have behaved differently through time. These authors further interpreted the west-dipping contacts between the Lachlan/Thomson and New England orogens observed on seismic profiles as the original sutures between these orogens and the east-dipping contacts as detachments between terranes within the New England Orogen (Figure 4.4).

Figure 4.4. Summary of BMR seismic line interpretation. The crustal structure of the New England Orogen changes considerably along strike; in the north, structures are only east-dipping, whereas, in the central and southern regions, structures dip both east and west. In addition, portions of the New England Orogen have apparently been detached and thrust either above or beneath the adjacent Lachlan Orogen (from Korsch et al., 1997).

Both gravity and magnetic anomalies have helped to reveal the presence of large-scale oroclinal folds within parts of the southern New England Orogen (Figure 4.5). Magnetic anomalies have also been associated with major faults that separate tectonostratigraphic terranes, and the anomalies apparently result from serpentinites, possibly related to ophiolite sequences, and basalt bodies present along the fault surfaces (Ramsay and Stanley, 1976).

Figure 4.5. Gravity (upper or left) and Magnetic (lower or right) anomaly maps of Australia. Gravity units are in mGals, and magnetic anomalies are in nT. Patterns of gravity and magnetic anomalies in the southern New England Orogen reflect oroclinal folding. In addition, linear gravity trends in the southern NEO may represent buried arc strata (images from Australian Geodynamics Cooperative Research Center (AGCRC) website).

 

Large-scale crustal and lithospheric structure

Finlayson and Collins (1993) conducted a study that involved collection of seismic data along a north-south (roughly parallel to the orogen) profile across the southern New England Orogen (south of the Clarence-Moreton basin) (Figure 2.4). In addition to providing some structural information, this study yielded a representative compressional wave, or p-wave, velocity (Vp) structure for the crust of the southern NEO (Figure 4.7). The average Vp for this small area is around 6.2 km/s, with an upper mantle Vp of 7.7 km/s just beneath the moho, at a depth of 34 km.

Figure 4.6. P-wave velocity as a function of depth based on seismic reflection data from one north-south line in the southern New England Orogen (blue line). The purple line shows the average p-wave velocity calculated for eastern Australia using receiver functions. This average velocity is considerably higher than the average velocity of the more local New England profile, perhaps because of the high percentage of granites in the southern New England crust (data based on Finlayson and Collins, 1993; Chevrot and van der Hilst, 2000).

Another study of the Australian crust utilized receiver functions and found an average p-wave velocity of 6.6 km/s and a Vp/Vs of around 1.73 for eastern Australia (Chevrot and van der Hilst, 2000; Figure 4.7). This larger average Vp may be due to a higher percentage of granitic material in the crust of the southern New England Orogen relative to eastern Australia as a whole. These p-wave velocity values are also in contrast to those found by another study in the northern portion of the New England Orogen. In particular, Leven (1980) found a Vp of 6.06 km/s at 2 km depth that increased to 6.37 km/s by 4 km. The 34 km moho depth found by Finalyson and Collins (1993) is consistent with 30-35 km values obtained by other authors (Korsch et al., 1997; Chevrot and van der Hilst, 2000) (Figure 4.8).

Figure 4.7. Contour map of Australian crustal thicknesses, in km. The average thickness beneath the New England Orogen is around 30-35 km. The crust in this area may have been thicker during or shortly following late Paleozoic to early Mesozoic orogenesis but could have reasonably been thinned during Mesozoic to Cenozoic extensional events and magmatism (image from Australian Geodynamics Cooperative Research Center (AGCRC) website).

The 7.7 km/s upper mantle p-wave velocity of the NEO is unusually low. Finlayson and Collins (1993) suggest that this anomaly may be a result of late Mesozoic to Cenozoic magmatic activity associated with opening of the Tasman and Coral Sea basins. Alternatively, the anomalous velocity could be a result of geochemical differentiation within the mantle during the development of late Permian to Triassic I-type plutons that are widely present in the southern New England (see geochemistry).

Figure 4.8. Map showing locations of broadband stations used for tomographic imaging of Australian continent (from Simons et al., 1999).

Seismic tomography and the mantle

A series of broadband seismic stations were deployed in dense arrays throughout the Australian continent in 1993 as part of a project called SKIPPY (Figure 4.8). Multiple teleseismic events were recorded at these stations, primarily from earthquakes along the Pacific rim. Zielhuis & van der Hilst (1996) used data from 25 of these stations and approximately 350 earthquakes to construct a three-dimensional shear wave model of eastern Australian lithosphere and mantle to a depth of approximately 400 km, excluding the crust. The authors tested the resolution of their model and concluded that they could reliably image features larger than 250 km laterally and 50 km vertically. Their results showed generally low velocities in the upper mantle along a 400-600 km wide zone along the eastern margin of Australia. However, this trend is interrupted by the presence of a high velocity zone in the vicinity of the southern New England Orogen at around 350-400 km depth (Figure 4.9). The authors noted the correlation between the general area of low velocities and sites of Cenozoic volcanism that may be related to opening of the Coral and Tasman Seas, implying the velocity decrease is due to anomalously hot mantle. Because part of the New England Orogen does not exhibit low mantle velocities, they suggested that this area may have resisted the thermal event or erosion that resulted in the decreased velocities in the mantle beneath most of eastern Australia.

Figure 4.9. Tomographic images of Australian continent showing p-wave velocity residuals at a variety of depths (from Rob van der Hilst's website, at http://quake.mit.edu/hilstgroup/robspage/).

In addition to identifying this potentially thermally resistant portion of the eastern Australian mantle, Zielhuis and van der Hilst (1996) noted the occurrence of shear-wave splitting in records from stations located in the vicinity of the New England Orogen. This heterogeneous distribution of shear wave velocities may reflect fabrics developed during tectonic activity that resulted in the formation of the New England Orogen.

 

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