Ninetyeast Ridge - active faulting relation to Indo-Australian Plate Deformation

Australia: The Land Where Time Began

Ninetyeast Ridge - Active Faulting, Relation to Indo-Australian Plate Deformation

Broad zones of deformation in the northeast Indian Ocean are crossed by the Ninetyeast Ridge (NER), which is about 4,500 km long, where the Indo-Australian plate is in the process of fracturing into 3 smaller plates (India, Capricorn, Australia) that are separated by diffuse boundaries, though their boundaries are poorly defined. Active faults have been imaged along the entire length of the NER by new multichannel seismic reflection profiles which show spatial changes in deformation style along the ridge. Transpressional movement along WNW-ESE faults along the northern section of the NER between 0^oN and 5^oN. At the western extent of the Wharton Basin earthquake swarm that occurred in April 2012 strike-slip was confirmed by observed patterns of faults. Nearly N-S compression is implied by deformation on thrust faults that are WNW-ESE-trending in the central NER, between 5^oS and 8^oS. Between 8^oS and 11.5^oS there is an abrupt change in fault style, with the southern NER (11^oS-27^oS) being characterised by modest extension. Though extension is dominant, narrow fault zones with strike-slip or compressional characteristics also have been found to occur in the southern NER. Active faulting at all sites is controlled by the reactivation of original normal faults formed by a spreading centre, which implies that deformation is opportunistic and focused along existing weakness zones, even in cases where the trend of the original fault is oblique to the direction of relative plate movement. Sager et al. say faulting that has been observed can be interpreted as India-Australia deformation in the northern section of the NER, and Capricorn-Australia in the southern part of the NER. The ridge is implied to be a tectonic boundary by the India-Capricorn boundary being directly adjacent to the northern part of the NER, and the combination of this juxtaposition and a different faulting style to the east of the NER.

It has been found that the vast Indo-Australian Plate in the central Indian Ocean is in the process of fragmenting into 3 smaller plates that are separated by diffuse boundaries (Royer & Gordon, 1997). Many details are still not known owing to the large extent a remote location of the area affected, even though this location is possibly the best documented distributed deformation within an oceanic plate. In the Central Indian Basin to the south of India and west of the Ninetyeast Ridge (NER) there are compressional faults and folds (Weissel et al., 1980; Petroy & Wiens, 1989; Bull & Scrutton, 1992; Krishna et al., 2001; Delescluse & Chamot-Rooke, 2007). In the Wharton Basin to the east of the NER, in contrast, deformation is mainly by strike-slip faulting (Petroy & Wiens, 1989; Stein et al., 1989; Deplus et al. 1998; Deplus, 2001). The NER is a long volcanic ridge about 4,500 km in extent in the middle of this deformed zone displaying active seismicity in some locations, though the nature of this deformation is unclear as a result of the sparseness of data in this region. Sager et al. suggest that if the NER is a passive weal on the oceanic lithosphere it may simply respond to tectonic forces, which are on a broad scale, with little change to the faulting pattern extending from adjacent basins. An alternative suggestion has been that the NER is a mechanical boundary because of the apparent change in deformation across it (e.g., Tinnon et al., 1995; Delescluse & Chamot-Rooke, 2007), therefore faulting may be complex. In this study Sager et al. used new multichannel seismic reflection profiles and date from multibeam bathymetry from the NER to understand the role it plays in the fracturing of the Indo-Australian Plate. Evidence was found of variable style active faulting along its entire length. In this report they document the faulting and explore implications for the location and style of diffuse plate boundaries.

Conclusions

Active faulting has been found by seismic reflection data analysis along the entire length of the about 4,500 km NER, though the intensity and style vary depending on the location relative to the diffuse plate boundaries and relative plate motion Euler poles. Recent deformation at all sites has reactivated normal faults formed at the spreading centre. It is implied by this finding, together with similar results from adjacent basins, that the fracturing of the Indo-Australian Plate mainly occurs by the reactivation faults that were formed at the time the lithosphere was forming at the spreading centre. It has been observed that fault direction corresponds to the original structure and not to the pattern of regional plate motion, which indicates the most important factor determining fault geometry is preexisting structure.

The dominant style of deformation in the northern NER, north of 1.5^oN, is transpression on faults that trend WNW-ESE. The direction of motion on strike-slip faults that has been observed is unexpected as earlier interpretation of earthquake focal mechanisms within the northern section of the NER had been interpreted to support strike-slip that was N-S, which would be consistent with the reactivation of ancient fracture zones in the Wharton Basin which is nearby. It appears that within the India-Australia diffuse plate boundary NW-SE compression reactivates original spreading centre normal faults in the NER, as well as fracture zones in the Wharton Basin along oblique Riedel shear directions.

Deformation shifts in the central NER, 6.7^oS-7.7^oS, to predominantly thrust faulting, which is similar to that observed in the Central Indian Basin, possibly with a small amount of strike-slip motion. The India-Australia relative motion is a simpler explanation as it agrees with the interpretations of sites to the north and south, though the deformation could possibly be the result of either India-Capricorn or India-Australia relative motion. Evidence of strike-slip motion is shown by a few faults, which suggests the tress pattern that is applied by regional plate motions has a component that is parallel to the original faults.

Between 7.7^oS and 11.3^oS deformation style abruptly changes again. Deformation in the southern section of the NER is modest and mainly expressed as normal faulting, though interrupted by zones of faulting that is strike-slip or compressional. At the southernmost end of the NER extension is explained as part of a zone of Capricorn-Australia divergence south of that Euler pole, whereas extension is explained as part of the India-Australia boundary at 11.3^oS, as are sited further to the north. The style of faulting is not clear but believed likely to be Capricorn-Australia convergence which is consistent with thrust fault mechanisms that have been observed, though in the gap between these areas low-resolution seismic data show active faulting.

It is indicated by seismic data from the NER that there is diffuse deformation that is widespread and often complex, sometimes having mixtures of different types of fault, which suggests interplay of regional stresses and structures that are preexisting. The interpretation of Sager et al. is that the northern NER shows India-Australia relative motion, whereas the southern NER responds to Capricorn-Australia relative motion. That deformation style changes can be abrupt is implied by India-Capricorn deformation that has been documented to the western edge of the NER. Sager et al. say this observation is in agreement with published findings that the northern NER is a break between deformation fields on either side. Faulting has been found to be more complex than predicted by existing plate models, though the observations of deformation can be understood by the framework of larger plate motions.

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