Lithospheric velocity structures under the southern New England Orogen: Evidence for underplating at the Tasman Sea margin

Wide‐angle reflection and refraction seismic studies indicate that two velocity models are appropriate for the structure of the crust and upper mantle under the New England Batholith, eastern Australia. Under the southern part of the batholith crustal velocities increase gradually from 5.5–6.03 km/s at the surface to 6.45 km/s near a distinct Moho at 34–35 km depth, suggesting that rocks of granitic composition make up a large part of the crust. Under the northern part there is a (? mafic) sill‐like feature at 21–24 km depth with a velocity maximum of 6.7 km/s. The crustal velocities are significantly less than those of the Lachlan and Thomson Orogens underlying the Bowen‐Gunnedah‐Sydney Basin system just to the west of the New England Orogen (typically > 6.3 km/s at > 10 km depth) and of terranes in the northern New England Orogen. In areas with similar granite geochemistry and inferred to have similar evolutionary process (for example the Peninsular Ranges Batholith, southwestern USA), there are again contrasts with the New England Batholith with high velocity mafic rocks (inferred ophiolite basement) at quite shallow (10 km) depths. Under the southern New England Orogen an upper mantle velocity of 7.7 km/s is determined, the lowest upper mantle velocity yet interpreted for any region of continental Australia. This upper mantle velocity persists down to ∼45 km depth and then increases to 8.08 km/s at 60 km. This again contrasts with upper mantle velocities under the adjacent Lachlan and Thomson Orogens (typically > 8.04–8.15 km/s) and with the batholiths in southwestern USA (∼8.2 km/s). Low velocity material in the mantle under New England could be the residue from geochemical differentiation within the mantle during I‐type plutonism, but our preferred interpretation is that it indicates underplated material emplaced during the formation of the Tasman Sea Basin and the subsequent uplift of eastern Australia.