Search

Slab rollback during subduction plays a key role in controlling continental growth at convergent plate boundaries. The dynamics of currently subducting slabs can be precisely constrained using geophysical techniques. In contrast, ancient episodes of slab rollback can be difficult to constrain, yet are critical to unlocking the tectonic evolution of long‐lived orogens such as the Phanerozoic Australian Tasmanides of eastern Gondwana. Recognition of ancient slab rollback relies on the identification of the progressive migration of magmatic arcs. Here, we investigate the timing and isotopic variation of ∼90 km of the trans‐orogen migration of the Carboniferous Bathurst Batholith as a potential indicator of slab rollback. U–Pb–Hf isotopes, combined with a regional zircon Hf isotope data set, suggest that the eastward migration of the batholith over ∼18 Myr, from 340.1 to 322.4 Ma, records a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a relatively stable trench setting. These results provide a magmatic record of Carboniferous slab rollback and establish a missing link between the long‐lived Lachlan and New England orogens of the eastern Gondwanan Tasmanides. Plain Language Summary: Slab rollback occurs at convergent plate boundaries, which is key to understanding the evolution of subduction processes. However, for ancient subduction zones where the geometry of long since subducted slabs no longer exists, the evolution of slab dynamics remains unclear. Occasionally though, the progressive migration of magmatic trends is observable and provides constraints on slab rollback, which aid in reconstructing the tectonic evolution of long‐lived orogens, such as the Phanerozoic orogens of eastern Australia. Here, we focus on how the unique magmatism of the Carboniferous Bathurst Batholith progressively migrated in the direction of the trench and analyze and interpret the geochronological and geochemical evolution of the batholith to better constrain subduction dynamics. We find that the Bathurst Batholith records the Carboniferous slab rollback of eastern Gondwana and reveals the relationship between the two key orogens of eastern Australia. Thus, the migration of magmatism in convergent settings can be used to track the slab rollback processes and evolution of the long‐lived orogens. Key Points: The Bathurst Batholith is part of a Carboniferous continental arc formed as a result of westward subduction in eastern GondwanaEastward migration of the batholith over ∼18 Myr recorded a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a stable trench settingThe batholith as a trans‐orogen "missing link" reveals continental arc magmatism extended across the Lachlan and New England orogens [ABSTRACT FROM AUTHOR]

Abstract Slab rollback during subduction plays a key role in controlling continental growth at convergent plate boundaries. The dynamics of currently subducting slabs can be precisely constrained using geophysical techniques. In contrast, ancient episodes of slab rollback can be difficult to constrain, yet are critical to unlocking the tectonic evolution of long‐lived orogens such as the Phanerozoic Australian Tasmanides of eastern Gondwana. Recognition of ancient slab rollback relies on the identification of the progressive migration of magmatic arcs. Here, we investigate the timing and isotopic variation of ∼90 km of the trans‐orogen migration of the Carboniferous Bathurst Batholith as a potential indicator of slab rollback. U–Pb–Hf isotopes, combined with a regional zircon Hf isotope data set, suggest that the eastward migration of the batholith over ∼18 Myr, from 340.1 to 322.4 Ma, records a maximum slab steepening rate of ∼0.6–1.2° Myr−1 in a relatively stable trench setting. These results provide a magmatic record of Carboniferous slab rollback and establish a missing link between the long‐lived Lachlan and New England orogens of the eastern Gondwanan Tasmanides.

The hafnium isotopes of zircon have recently been identified as a new means of detecting magmatic cyclicity. A hierarchy of possible magmatic cycles has been verified, ranging from the ∼600 Myr supercontinent cycle to 10–20 Myr plutonic cycles. While the supercontinent cycle is well-established, the shorter mantle convective and plutonic cycles require further work to understand how magmatic dynamics may give rise to such cyclicity. In this study, we analyze in detail the long-lived orogenic system of the Australian Tasmanides, and a hierarchy of cycles is visible, including a ∼ 290 Myr accretionary/orogenic cycle, a ∼ 65 Myr upper mantle cycle, and 20–10 Myr magmatic and plutonic cycles. These cycles correlate with the evolution of the orogens and episodic continental growth of eastern Gondwana. The surprising similarity in period length between the opening/closing of a typical Atlantic-like Wilson cycle to the case here of the lifecycle of a West Pacific-type arc system may imply that the two distinct tectonic settings have a similar mantle convective effect, which requires further empirical testing and theoretical study. Similar to magmatic cycles identified in the Hadean and in modern arc settings, the hierarchy of cycles in the Tasmanides implies a uniformitarian similarity in arc magmatism throughout Earth history. • A hierarchy of magmatic cyclicity has been identified in the Australian Tasmanides. • Geological processes of the cycles have been systematically addressed. • Mechanisms of cyclicity and episodic continental growth have been proposed. • Geological significance of modulation of the cycles has been proposed. • The hierarchy of cycles displays a uniformitarian affinity throughout Earth history. [ABSTRACT FROM AUTHOR]