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The timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume. Plain Language Summary: Rifting is the process by which continents are broken apart and is generally associated with the production of magma. Magmas that are produced during early rifting are chemically different than those produced during later rifting. To understand which rocks deep in the earth may be melting to generate the different magmas at different times, we developed new computer code to model rifting scenarios in different places. Using this code, we estimate to what extent different rocks would melt during rifting and how much magma they would produce. We tested four different rock types: basalt, dry peridotite, wet peridotite, and pyroxenite. Our findings show that there is a relative order regardless of the rifting scenario in which each rock type starts to melt and when they produce the most magma. If all the rocks tested are present in the earth before rifting, then the earliest magmas produced during rifting will be from the melting of basalts and wet peridotite, followed by the melting of pyroxenite, and finally dry peridotite. These findings help to inform us of the sources of early rift magmas and what rocks may have been present deep in the earth before rifting took place. Key Points: Melt produced from a compositionally heterogeneous mantle during rifting is assessed with a new 1‐D geodynamic modeling code, MELT1DLithosphere thickness, mantle composition, and temperature are the primary controls on the timing of melt production from each lithologyModels predict a transition from melting of wet and/or mafic components to fertile ultramafic components and finally to ambient mantle [ABSTRACT FROM AUTHOR]

International Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian continental margin, in the deep water Mentelle Basin (MB) and on the neighboring Naturaliste Plateau (NP). The drillsites are located on continental crust that continued rifting after seafloor spreading began further north on the Perth Abyssal Plain (PAP) between magnetochrons M11r and M11n (133–132 Ma), ending when spreading began west of the NP between chrons M5n and M3n (126–124 Ma). Drilling recovered the first in situ samples of basalt flows overlying the breakup unconformity on the NP, establishing a magnetostratigraphically constrained eruption age of >131–133 Ma, and confirming a minimal late Valanginian age for the breakup unconformity (coeval with the onset of PAP seafloor spreading). Petrogenetic modeling indicates the basalts were generated by 25% melting at 1.5 GPa and a potential temperature of 1380°C–1410°C, consistent with proximity of the Kerguelen plume during breakup. Benthic foraminiferal fossils indicate that the NP remained at upper bathyal or shallower depths during the last 6 Myr of rifting and for 3–5 Myr after breakup between India and Australia. The limited subsidence is attributed to heat from the nearby Kerguelen plume and PAP spreading ridge. The margin subsided to middle bathyal depths by Albian time and to lower bathyal (NP) or greater (MB) depths by late Paleogene time. Periods of rapid sedimentation accompanied a westward jump of the PAP spreading ridge (108 Ma), rifting on the southern margin (100–84 Ma), and opening of the southern seaway between Australia and Antarctica (60–47 Ma). Plain Language Summary: The southwestern Australian margin formed during the breakup of the supercontinent Gondwana during the Late Jurassic and Early Cretaceous Periods. International Ocean Discovery Program Expedition 369 drilled four sites on the southwestern Australian rifted continental margin in order to better understand the subsidence and magmatic behavior of the margin during the final stages of rifting. Drilling shows that a widespread unconformity imaged in seismic data on the margin correlates with the onset of seafloor spreading on the Perth Abyssal Plain at 132–133 Ma. This was followed by eruption of basalts on the Naturaliste Plateau, which were generated from melting of the underlying mantle that was enhanced by the nearby Kerguelen mantle plume. Proximity of the plume kept the margin at shallow depths during rifting and for about 3–5 Myr after final separation of India and Australia at 126 Ma. The margin subsided to > 1,000 m depths between about 121 Ma and the present, with intervening periods of relatively rapid sedimentation associated with adjustments to the spreading ridge configuration in the Indian Ocean at 108 Ma, and rifting events between southwestern Australia and Antarctica at 100–84 Ma and 60–47 Ma. Key Points: Continental breakup occurred at 132–133 Ma on the Perth Abyssal Plain and at 126 Ma west of the Naturaliste PlateauVolcanism on the Naturaliste Plateau with both plume and mid‐ocean ridge compositional affinities accompanied Perth Abyssal Plain breakupLate synrift and early postrift subsidence was limited by proximity of the Kerguelen plume and Perth Abyssal Plain spreading ridge [ABSTRACT FROM AUTHOR]

The Mentelle Basin, located off the southwestern continental Australian margin, is an under-explored deep-water basin that developed during the breakup of eastern Gondwana in the Middle Jurassic through to the Early Cretaceous. There is a high degree of uncertainty in stratigraphic interpretations of the basin owing to the lack of well and seismic data in the region. As a consequence, there has been a heavy reliance on data from the neighbouring Perth Basin to infer the geological history of the Mentelle Basin. During Expedition 369 of the International Ocean Discovery Program, a single hole was drilled with a rotary core bit to 517.10 m below seafloor at Site U1515 on the continental slope in the eastern Mentelle Basin. The aim was to sample and date the inferred Permian to Jurassic pre-breakup strata. The presence of palynomorphs including Murospora florida and Retitriletes watherooensis in the fluvio-lacustrine succession below the Valanginian breakup unconformity indicates a Jurassic to earliest Cretaceous age for the upper part of the pre-rift sedimentary succession. Notable characteristics of this succession include intervals with high percentages of total organic carbon (up to 44 wt%) and high hydrogen index values (Type I source rocks) associated with an increase in Botryococcus and other freshwater algae that accumulated in shallow lakes and waterlogged floodplains. A possible example of dinoturbation was also noted in this core. The discovery of Jurassic to earliest Cretaceous pre-breakup strata at Site U1515 suggests that Jurassic rifting was not constrained to the western depocentre of the Mentelle Basin, but also extended to a number of depocentres in the eastern Mentelle Basin. This has significant implications for the understanding of the regional tectonic history and the Mentelle Basin's petroleum potential. Pre-breakup strata sampled in the eastern Mentelle Basin for the first time by IODP Expedition 369 Upper Jurassic–Lower Cretaceous non-marine strata below the Valanginian Unconformity High TOC's, high abundances of Botryococcus algae in a few horizons and a possible example of dinoturbation Jurassic rifting not constrained to the western Mentelle Basin, but to depocentres in the east [ABSTRACT FROM AUTHOR]

Models for extension-related magmatism based on decompression melting of asthenospheric mantle poorly simulate fluxes and bulk compositions of magmas produced during early stages of continental extension. For the Great Basin of western North America, it is proposed that magmatism proceeded in two stages, the first involving melting of lithospheric mantle sources between 40 and approximately 5 million years ago (Ma), followed (since approximately 5 Ma) by melting of upwelling asthenospheric mantle in areas where extension has exceeded about 100 percent. This transition in magma sources is diachronous, depending on initial variations in lithosphere thickness and on rates of lithospheric thinning.