|Spreading center type:||Large-scale extinct MOR|
|Time of cessation:||Between 77 and 121 Ma, during the Cretaceous Normal Superchron, with individual interpretations given more precise estimates, as summarized in Table 1B.|
|Subsequent active spreading center:||Probably the Wharton Basin Ridge, however it is possible that a number of reorganizations took place.|
It is generally agreed that the seafloor offshore the southern West Australian coastline formed as a result of Mesozoic rifting between the Indian and Australian plates, before which time India is believed to have been joined to the Australian plate within the Gondwanan supercontinent assembly (Markl, 1974; Ali and Aitchison, 2014; Gibbons et al., 2012; Whittaker et al., 2013). However, the basins within this region are structurally complex and contain a number of continental fragments that suggest that there was a multi-stage rifting history. Identified magnetic anomalies suggest probable idge-jumps, although there have been several different interpretations of the anomalies (for example, Markl, 1974; Mihut and Müller, 1998; Williams et al., 2013). The Perth Abyssal Plain (Perth AP), was recently confirmed to contain continental fragments at the Gulden Draak and Batavia Knolls (Williams, Whittaker and Müller 2013; Whittaker et al. 2013). Together with updated magnetic anomaly identifications in the basin (Williams et al., 2013) has strongly suggested that at least one extinct spreading center is located on the Perth AP.
Markl (1974) reported Jurassic to Cretaceous magnetic anomalies (M11 – M0) on the Perth AP, that are consistent with ages determined at deep sea drilling sites 256 and 257 (which are of equivalent age, but separated by 1000 km) and site 259. Markl (1974) noted the difficulty in identifying the position of the extinct spreading axis but suggested that it may have been to the west of the Dirk Hartog Ridge, having possibly formed at ca. 77 Ma. This placement was endorsed by later studies by Royer and Sandwell (1989) and Powell et al. (1988).
Mihut and Müller (1998) place the former axis of spreading on the Perth AP in a similar position on the basis of a linear negative free-air gravity anomaly that is subparallel to the assumed position of conjugate margins at the time of spreading, as inferred by the present-day West Australian coastline and by a fault scarp to the west. Both Markl's (1974) and Mihut and Müller’s (1998) placements result in significant asymmetry within the basin, which is difficult to account for with the usual patterns of asymmetric spreading. It is likely that for this reason (Müller et al., 1998) introduced a second extinct axis in the central Perth AP within their kinematic reconstruction of the region, despite limited evidence to support this placement.
Song et al. (2001) suggested that the extinct axis of spreading may have been close to the Dirk Hartog Ridge, with the central segments coincident with the elevated volcanic ridges that form this structure. Their proposed placement requires that four right-stepping segments make up the ridge axis between the Lost Dutchman Ridge to the north and the Naturaliste Plateau to the south. Recent studies (Gibbons et al. 2013; Whittaker et al. 2013), have also placed the extinct spreading center in the region of the Dirk Hartog Ridge on the basis of its position relative to the assumed position of the Indian and Australian conjugate margins at the time of the significant plate reorganization and the proposed ridge jump to the west of the Batavia and Gulden Draak Knolls.
The morphology of the Dirk Hartog Ridge is complex and does not resemblance other well-known extinct ridges. However, numerous deep troughs and narrow, linear ridges alongside the ridge could reflect an initial formation as a spreading center that was later modified by volcanism, and/or, subsequent deformation.
Williams et al. (2013) recently proposed that symmetric M-sequence anomalies could be identified much closer to the West Australian coastline and suggested that this was the sight of a ridge jump to the west during the Cretaceous Normal Superchron. The proposed extinct spreading center does not have a linear bathymetric or gravity feature associated with it and is therefore difficulty to locate precisely. However, the possibility of the basin having formed through two episodes of spreading, with two significant ridge reorganizations could solve some of the remaining problems for interpretation of the basins evolution.
Whittaker et al. (2013) proposed that the Kerguelen Plume may have played a role in modifying the location of the Indian Ocean spreading system throughout its evolution and tmay be implicated in the migration of the ridge to the west of the continental fragments at the Batavia and Gulden Draak Knolls, in the mid-Cretaceous. The jump represented a major change in regional tectonics during the Indian Ocean development (Matthews et al. 2012; Gibbons et al. 2013), with a dramatic change in the spreading direction between India and Australia.
We consider the several alternative placements that have been proposed for the extinct ridge axial segments. We digitized the prospective placements of Mihut and Müller (1997), Williams et al. (2013), Markl (1974) and two placements around the Dirk Hartog Ridge to assess whether any of these could be seen to have a characteristic ‘extinct-ridge’ signature.
Perth AP Option 1 - Mihut and Müller (1998)
Perth AP Option 2 - Fracture zones adjacent to Dirk Hartog Ridge
Perth AP Option 3 - Williams et al. (2013)
Perth AP Option 4 - Markl (1974)
Dirk Hartog Ridge, Whittaker et al., (2013)
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