Moho depth variations of Zealandia from gravity data inversion and implications for continental breakup

Abstract

The thickness of the Earth's crust, along with the relief and characteristics of the Moho discontinuity, serves as a critical constraint for numerous geoscientific investigations. By removing the gravitational effects of topography, seawater, sediment, and thermo-gravitational effects from the lithosphere in Zealandia and its adjacent regions, we isolate the gravity anomaly associated with the Moho discontinuity to compute its depth. We determined the intercept of the linear relationship between Moho depth derived from the global crust model CRUST1.0 and corresponding Moho gravity anomalies as an estimate for mean Moho depth. A laterally variable crust-mantle density contrast was initially obtained using Bouguer plate formula based on both CRUST1.0-derived Moho depths and observed gravity anomalies; this was subsequently rescaled utilizing the slope of linear regression between actual Moho gravity anomalies and fitted gravity anomalies computed from CRUST1.0-derived Moho depths alongside our initial density contrast estimates. The results calculated via Parker-Oldenburg formula indicate that Zealandia's Moho depth primarily ranges from 8 to 28 km, while crustal thickness varies between 4 and 26 km. These values are significantly greater than those typical of oceanic crust but remain less than those characteristic of continental crust. Within our study area, identified ocean basins show a decreasing trend in crustal thickness relative to both oceanic crust age and increasing water depth. When considering only areas where crustal thickness is less than 16 km on scatter plots, Middleton Basin and Lord Howe Basin display substantially greater thicknesses compared to New Caledonia Basin and Fairway Basin. This observation suggests that if these four basins possess oceanic crusts beneath them, then it is likely that the oceanic crust underlying Middleton and Lord Howe Basins predates that found beneath New Caledonia and Fairway Basins, implying that post-Zealandia separation from Gondwana involved rifting predominantly linked to subduction rollback of the Pacific Plate.