Structural, metamorphic, and geochronological constraints on alternating compression and extension in the Early Paleozoic Gondwanan Pacific margin, northeastern Australia

The Ross-Delamerian orogenic belt formed along the early Paleozoic active Pacific margin of the newly merged Gondwana supercontinent. In its northern-most segment in the Townsville region of northeastern Australia, we have identified a short contractional phase of the Delamerian orogeny in the Argentine Metamorphics postdating formation of a mafic breccia with a U-Pb zircon age of 500 ± 4 Ma. Contraction was followed by widespread inferred extensional deformation with formation of flat-lying foliation, domal features, and amphibolite grade and greenschist retrograde metamorphism all synchronous with latest Cambrian to Early Ordovician extensional backarc volcanism, sedimentation and intrusions. One of these intrusions gives a U-Pb zircon age of 480 ± 4 Ma. Foliation related to the extensional deformation is cross-cut by a late granodiorite dyke with a U-Pb zircon age of 461 ± 4 Ma. Late east-west contractional deformation affected the higher grade part of the assemblage. In contrast to the Ross-Delamerian orogenic belt in the Transantarctic Mountains and southeastern Australia, the orogenic belt in northeastern Australia was affected by a short episode of contraction at ∼495 Ma followed by long-lived backarc extension from ∼490 Ma to 460 Ma with subsequent contractional deformation.