Inherited lithospheric structures control arc-continent\ncollisional heterogeneity

Abstract

From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ∼4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis. INTRODUCTION The Indo-Australian plate is subducting beneath Eurasia in Indonesia, producing the ∼5200-km-long archipelago with hundreds of active volcanoes and a well-defined Wadati-Benioff zone (Fig. 1A). The northeastward motion of Australia with respect to Eurasia is ∼7 cm/ yr, but convergence is locally partitioned due to variable plate-boundary geometry and different types of convergent margins (e.g., Koulali et al., 2016). In eastern Indonesia, the incoming plate structure transitions from oceanic lithosphere to Australian continental lithosphere at the junction of the Sunda-Banda arc, resulting in arccontinent collision. This results in two chains of islands, the inner volcanic arc to the north, and the nonvolcanic islands (Timor, Sumba, Rote, and Savu) to the south. A section of the inner arc volcanoes is inactive from Alor to Romang north of eastern Timor (Fig. 1), which has been inferred to be due to the collision of the Australian continent (Harris, 2011; Hall, 2012). Despite these first-order controls on the collision, there is no clean jump between the oceanic and continental lithosphere within the Indo-Australian plate; instead, there is a complex transition due to the inherited structure of the precollisional Australian margin. During the breakup of east Gondwana in the Late Jurassic (Heine and Müller, 2005), a broad, long (several thousand kilometers) system of salients and embayments formed at the edge of the Indo-Australian plate (e.g., Charlton, 2000; Keep and Haig, 2010). These structures correspond to the rifted upperand lower-plate margins (e.g., Charlton, 2004), which are depicted in the “jagged-edge” geometry seen in the present-day bathymetry of the Wallaby, Exmouth, and Scott Plateaux offshore northwestern Australia (Fig. 1A). To study the spatiotemporal evolution of the transition from oceanic subduction to arccontinent collision along the western Banda arc, diverse geological, geodetic, seismic, and geomorphic data have been collected. This experiment included a deployment of 30 broadband seismometers (Miller et al., 2016), with the primary aim to image the structure of the crust and mantle beneath the region (Fig. 1B). At mantle depths, the tomographic images from these new data (Harris et al., 2020; Supendi et al., 2020) and from previous studies (e.g., Widiyantoro and van der Hilst, 1996; Spakman and Hall, 2010) show a steeply dipping, fast-velocity slab with some structural variations at depths >200 km. These morphological changes at depth are inferred to result from the transition from oceanic subduction to continental subduction and collision, imparting surface expression in terms of variations in volcanic activity and magmatic arc chemistry (e.g., Hilton et al., 1992; Elburg et al., 2004, 2005). While this first-order transition has dominated prior interpretation of structure in this region, our new crustal and uppermost mantle (<100 km) velocity models indicate a more subtle confluence of lithospheric structure and morphological inheritance as tectonic drivers. We argue that complexities found in the new tomographic images can be understood by linking the velocity anomalies to the incoming plate structures. The subaerial topography of the Banda arc appears to have evolved in response to these structural complexities in the downgoing slab, as previously identified on Sumba (e.g., Harris, 1991; Fleury et al., 2009) and on Savu and Rote (e.g., Roosmawati and Harris, 2009), Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G48246.1/5227444/g48246.pdf by Univ of Texas-Austin user on 08 February 2021 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America and as suggested here also for the highest elevations on Timor. These relationships indicate that heterogeneities in the subducting plate structure may control where and when exhumation and erosion of the accretionary wedge take place during initial phases of arc-continent collision. This highlights new avenues for understanding collisional processes in the context of long-term continental dynamics from a combination of geological and geophysical approaches, with implications for transport of continental material throughout the Wilson cycle. DATA AND METHODS Ambient Noise Tomography We used broadband seismic data (Miller et al., 2016) to produce ambient noise tomographic images of the crust and uppermost mantle. These were constructed by measuring short-period (6–37 s) Rayleigh wave dispersion from ∼4 yr of continuous records from 33 broadband stations (Fig. 1). First, we measured Rayleigh wave phase velocity dispersions at periods of 6 s to 37 s from the stacked cross-correlations of ∼4 yr of continuous vertical-component seismograms (March 2014–December 2018), following the procedures described by Bensen et al. (2007). Next, we performed a three-dimensional (3-D) surface wave inversion developed by Fang et al. (2015) to resolve the shear-wave velocity in the uppermost 70 km (Fig. S1 in the Supplemental Material1). The algorithm utilizes the fast-marching ray-tracing method to calculate synthetic traveltimes and ray paths and then directly inverts all data to a 3-D shear-wave velocity model by a linearized iteration (Zhang and Miller, 2021). Detailed processing and inversion parameters are described in the Supplemental Material. Uplift Modeling To explore the surface expression of deformation and its evolution in space and time, we analyzed the subaerial topography. We reconstructed uplift histories using the morphology of river profiles, which encode the history of rock uplift experienced by a drainage basin (e.g., Kirby and Whipple, 2001; Pritchard et al., 2009; Roberts et al., 2013). Given equal discharge and bedrock erodibility, a fluvial knick zone generated by uplift will erode more rapidly than neighboring reaches, and steepened reaches will propagate upstream. By parameterizing these erosional processes, observed river profiles can be inverted in a space-for-time substitution to estimate an uplift history (Kirby and Whipple, 2001). We applied a 1-D inversion (Pritchard et al., 2009) to the islands of the Banda arc (see the Supplemental Material). Data from uplifted marine terraces and synorogenic basins (Hantoro et al., 1994; Merritts et al., 1999; Muraoka et al., 2002; Nexer et al., 2015) provided independent constraints on the river profile inversions, making this region especially suited to this approach by allowing accurate parameterization of the uplift model. RESULTS AND DISCUSSION The ambient noise tomography shows a strong high-to-low velocity contrast that strikes in a southwest-to-northeast direction, roughly parallel to the Timor Trough from shallow crustal depths through to the mantle lithosphere (Fig. 2A; Fig. S1). However, there are significant, smaller-scale velocity patterns, such as the higher velocities in central Timor that are present at ∼30–60 km depth. These higher velocities are bounded by more moderate-shear-velocity 1Supplemental Material. Description of the methodology. Please visit https://doi .org/10.1130/ GEOL.S.13584929 to access the supplemental material, and contact [email protected] with any questions. A