New insights into the geological evolution of West Papua from recent field and laboratory studies

Our understanding of the geological evolution of West Papua (or the Bird\u27s Head Peninsula and Bird\u27s Neck) predominantly stems from a systematic mapping campaign conducted by Indonesian and Australian geologists during the 1970\u27s and 80\u27s, together with the findings of mineral and hydrocarbon exploration by Dutch geologists in the early 1900\u27s. Most of the research that has been conducted since these initial, but comprehensive studies have been associated with continued exploration for hydrocarbons (e.g., around the Salawati and Bintuni Basins) and precious metals (e.g., associated with Grasberg-Ertsburg). Here we present an overview of research conducted over the past five years that was largely funded by several oil exploration companies. Our research did not focus on oil exploration, but instead attempted to update our understanding of the age of magmatic, metamorphic and deformation events, and to improve our knowledge of the regional stratigraphy of West Papua. These new data have been used to develop new tectonic models and paleogeographic maps that provide a framework for future studies in the region to build upon

The Tamrau Block of NW New Guinea records late Miocene-Pliocene collision at the northern tip of the Australian Plate

The Tamrau Block is a geological terrane located in the Bird\u27s Head Peninsula of NW New Guinea. Relatively little is known about this terrane and how it relates to other parts of western New Guinea. The geological history and provenance of this terrane is difficult to resolve because it is found at the boundary between the Australian, Philippine Sea, Caroline, and Pacific plates. This segment of the plate boundary records episodes of deformation, magmatism, and exhumation associated with the interaction of these three tectonic plates. For instance, the southern edge of the Tamrau Block is bounded by a major strike-slip fault zone (the Sorong Fault Zone), previous studies imply that the terrane may have been translated some distance along this fault zone during the Cenozoic. We seek to understand the geological history and provenance of the Tamrau Block. To do this, we report new field observations made during a three-month field campaign in a poorly exposed region, together with new petrographic and geochronology data. U-Pb detrital zircon age spectra were obtained from five sedimentary and metamorphic sequences and the results were combined with existing biostratigraphic age data to reassess the stratigraphy and tectonic history of the region. The oldest rocks in the Tamrau Block (the Tamrau Formation) represent Jurassic-Cretaceous passive margin sediments. These were deformed and metamorphosed in at least three distinct events - the first phase of which involved amphibolite facies conditions (and produced the first reported instance of metamorphic kyanite from the Bird\u27s Head), possibly in the Oligo-Miocene. These Jurassic-Cretaceous rocks are unconformably overlain by the Ajai Limestone. Both units are cross-cut and overlain by intrusives and eruptives of the middle Miocene Moon Volcanics. The heat associated with these intrusives baked the Tamrau Formation and the overlying Ajai Limestone, resulting in andalusite growth in the Tamrau Formation and hornfelsing of the limestones. Deposition of the overlying Koor Formation occurred from the middle-late Miocene, and was partially contemporaneous with volcanism. An episode of crustal shortening occurred after the deposition of the Koor Formation with the development of asymmetric folds and steeply-inclined reverse faults. This is marked by a late Miocene-Pliocene break in deposition between the Koor Formation and undeformed Opmorai Formation (∼10.5-4.5 Ma) and records the collision of part of an oceanic island arc (Tosem Block) to the Tamrau Block, an event also recognised in other parts of New Guinea. A comparison of the U-Pb detrital zircon age spectra from sedimentary rocks within the Tamrau Block and those reported in other studies indicate that there is no relationship between the rocks of the Tamrau and Kemum blocks (which are in faulted contact). Instead, the U-Pb detrital age data share similarities with rocks from the Lengguru Fold and Thrust Belt and Weyland Overthrust in the south-east. We therefore propose that the Tamrau Block was transported westwards ∼300 km to its current position along the Sorong Fault Zone after the late Miocene-Pliocene collision between the Tosem Block and the northern margin of the Australian Plate

The history of Cenozoic magmatism and collision in NW New Guinea - New insights into the tectonic evolution of the northernmost margin of the Australian Plate

Evidence of Cenozoic magmatism is found along the length of New Guinea. However, the petrogenetic and tectonic setting for this magmatism is poorly understood. This study presents new field, petrographic, U-Pb zircon, and geochemical data from NW New Guinea. These data have been used to identify six units of Cenozoic igneous rocks which record episodes of magmatism during the Oligocene, Miocene, and Pliocene. These episodes occurred in response to the ongoing interaction between the Australian and Philippine Sea plates. During the Eocene, the Australian Plate began to obliquely subduct beneath the Philippine Sea Plate forming the Philippine-Caroline Arc. Magmatism in this arc is recorded in the Dore, Mandi, and Arfak volcanics of NW New Guinea where calc-alkaline and tholeiitic rocks formed within subduction-related fore-arc and extension-related back-arc settings from 32 to 27 Ma. Collision along this plate boundary in the Oligocene-Miocene jammed the subduction zone and caused a reversal in subduction polarity from north-dipping to south-dipping. Following this, subduction of the Philippine Sea Plate beneath the Australian Plate produced magmatism throughout western New Guinea. In NW New Guinea this is recorded by the middle Miocene (18-12 Ma) Moon Volcanics, which include an early period of high-K to shoshonitic igneous activity. These earlier magmatic rocks are associated with the subduction zone polarity reversal and an initially steeply dipping slab. The magmatic products later changed to more calc-alkaline compositions and were emplaced as volcanic rocks in the fore-arc section of a primitive continental arc. Finally, following terminal arc-continent collision in the late Miocene-Pliocene, mantle derived magmas (including the Berangan Andesite) migrated up large strike-slip faults becoming crustally contaminated prior to their eruption during the Plio-Pleistocene. This study of the Cenozoic magmatic history of NW New Guinea provides new data and insights into the tectonic evolution of the northern margin of the Australian Plate

Isotopic mapping reveals the location of crustal fragments along a long-lived convergent plate boundary

New Guinea has acted as the boundary between the Australian and Pacific plates for hundreds of millions of years. Strike-slip movement and arc–continent collisions along this boundary during the Cenozoic have shuffled rocks of different age and composition in a series of terranes along the plate boundary making mapping them a considerable challenge. Here we report results of SrNd isotopic data obtained from rock samples from western New Guinea that are representative of the different terranes. These isotopic data reveal the crustal affinity of the terranes and we have used these data to map their spatial distribution. The isotopic data show three distinct crustal domains underlying western New Guinea; Palaeozic–Mesozoic Australian continental crust (87Sr/86Sr = 0.719594 to 0.710921; εNd = −13.85 to 1.373); thinned transitional crust intruded by Miocene–Pleistocene magmatic rocks (87Sr/86Sr = 0.706524 to 0.704019; εNd = 6.67 to 2.13); and accreted island arc crust (87Sr/86Sr = 0.704053 to 0.703759; εNd = 6.63 to 4.97). These data, together with crustal contamination models, indicate that the northern-most extent of Australian continental crust exists beneath the northern-most section of western New Guinea. We also combined our isotopic data with existing data across New Guinea and used these to develop an isotopic map that shows the position of the ancient Australian–Pacific Plate boundary, producing results that are also consistent with broad-scale seismic tomography imagery. Our findings provide a framework for mapping other plate boundaries, particularly ancient systems where only fragmentary data exist

Reconciling regional continuity with local variability in structure, uplift and exhumation of the Timor orogen

Along-strike variations in orogenic development can be difficult to constrain. Resulting assumptions projecting similarity or variability along strike can lead to erroneous conclusions at the orogen scale. Young orogens provide opportunities to document limits of along-strike projection and test factors that may control lateral variability. Here we present new constraints on the history of uplift, exhumation and shortening in the Timor orogen from West Timor, Indonesia. Structural mapping documents a foreland thrust stack of Jurassic-Miocene Australian margin strata and a hinterland antiformal stack of Permo-Triassic Australian continental units duplexed below Banda Arc lithosphere. Biostratigraphy within the piggyback Central Basin reveals earliest deepwater synorogenic deposition at 5.57–5.53 Ma, uplift from lower to middle bathyal depths at 3.35–2.58 Ma, and uplift from middle to upper bathyal depths at 2.58–1.30 Ma. Hinterland Permo-Triassic strata yield apatite (U-Th)/He ages of 0.33–2.76 Ma, apatite fission track ages of 2.19–3.53 Ma and partially reset zircon (U-Th)/He ages. These thermochronology ages are youngest or most strongly reset in the center of the antiformal stack and yield modeled exhumation rates of 0.45–3.31 km/Myr. A balanced cross section reveals a minimum of 300 km of shortening including 210 km of Australian continental subduction below the Banda forearc. When compared to published results from Timor-Leste, these data show that the timing of initial collision, synorogenic basin uplift, and total shortening amount were broadly similar across the island. Therefore, despite along-strike changes in orogen morphology and significant small-scale spatial variability in deformation, first-order structural similarity dominates at large scales. We suggest that along-strike variations in orogen morphology are due to changes in the distribution of deformation within the orogen driven by differences in backstop strength, internal wedge strength and basal décollement friction as well as the presence of the wedge-top Central Basin in West Timor

Reconciling regional continuity with local variability in structure, uplift and exhumation of the Timor orogen

Along-strike variations in orogenic development can be difficult to constrain. Resulting assumptions projecting similarity or variability along strike can lead to erroneous conclusions at the orogen scale. Young orogens provide opportunities to document limits of along-strike projection and test factors that may control lateral variability. Here we present new constraints on the history of uplift, exhumation and shortening in the Timor orogen from West Timor, Indonesia. Structural mapping documents a foreland thrust stack of Jurassic-Miocene Australian margin strata and a hinterland antiformal stack of Permo-Triassic Australian continental units duplexed below Banda Arc lithosphere. Biostratigraphy within the piggyback Central Basin reveals earliest deepwater synorogenic deposition at 5.57–5.53 Ma, uplift from lower to middle bathyal depths at 3.35–2.58 Ma, and uplift from middle to upper bathyal depths at 2.58–1.30 Ma. Hinterland Permo-Triassic strata yield apatite (U-Th)/He ages of 0.33–2.76 Ma, apatite fission track ages of 2.19–3.53 Ma and partially reset zircon (U-Th)/He ages. These thermochronology ages are youngest or most strongly reset in the center of the antiformal stack and yield modeled exhumation rates of 0.45–3.31 km/Myr. A balanced cross section reveals a minimum of 300 km of shortening including 210 km of Australian continental subduction below the Banda forearc. When compared to published results from Timor-Leste, these data show that the timing of initial collision, synorogenic basin uplift, and total shortening amount were broadly similar across the island. Therefore, despite along-strike changes in orogen morphology and significant small-scale spatial variability in deformation, first-order structural similarity dominates at large scales. We suggest that along-strike variations in orogen morphology are due to changes in the distribution of deformation within the orogen driven by differences in backstop strength, internal wedge strength and basal décollement friction as well as the presence of the wedge-top Central Basin in West Timor