Journal of Sedimentary Research, 2006, v. 76, 493–514 Research Article DOI: 10.2110/jsr.2006.044 SEDIMENTARY PROCESSES IN MODERN AND ANCIENT OCEANIC ARC SETTINGS: EVIDENCE FROM THE JURASSIC TALKEETNA FORMATION OF ALASKA AND THE MARIANA AND TONGA ARCS, WESTERN PACIFIC AMY E. DRAUT 1 AND PETER D. CLIFT 2 1 University of California, Santa Cruz/US Geological Survey, 400 Natural Bridges Drive, Santa Cruz California 95060, U.S.A. 2 School of Geosciences, Meston Building, University of Aberdeen, Aberdeen AB24 3UE, U.K. e-mail: adraut@usgs.gov ABSTRACT: Sediment deposited around oceanic volcanic arcs potentially provides the most complete record of the tectonic and geochemical evolution of active margins. The use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting: processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition. We review evidence from the modern Mariana and Tonga arcs and the ancient arc crustal section in the Lower Jurassic Talkeetna Formation of south-central Alaska, and introduce new data from the Mariana Arc, to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings. All three arcs are interpreted to have formed in tectonically erosive margin settings, resulting in long-term extension and subsidence. Debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes, forming relatively continuous mass-wasted volcaniclastic records in abundant accommodation space. There is little erosion or reworking of old volcanic materials near the arc volcanic front. Tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench; although some canyons deliver sediment to the trench slope, most volcaniclastic sedimentation is limited to the area immediately around volcanic centers. Arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning , 10 My. The chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to , 20 My but no longer, because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench, where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slope. INTRODUCTION Although arc volcaniclastic sediments can provide a valuable long-term record of arc magmatic and tectonic evolution, it is only in exceptional circumstances that a complete arc volcanic section is preserved and exposed. Efforts to understand petrogenesis, tectonic erosion, and mass recycling in oceanic subduction systems are thus hindered by the fact that only very small parts of modern arc systems can be readily sampled on volcanic islands or dredged from submarine volcanic centers. Much of the volcanic output is not exposed as lavas onshore but is dispersed by explosive eruptions as tephra and debris aprons over wide areas around magmatic centers. Processes of volcaniclastic sedimentation and preser- vation in oceanic arcs must be understood in order to assess the utility of these sediments in recording active-margin evolution. Documenting temporal evolution of arc volcanism is important because arc stratigraphy and geochemistry evolve in response to tectonic events such as arc rifting or collision with aseismic ridges and continental margins (e.g., Collot and Fisher 1991; Gill et al. 1994; Lee et al. 1995; Clift 1995; Clift and Lee 1998; Draut et al. 2004), as well as to changes in the rate of convergence and the sedimentary cover on the subducting plate (e.g., Tatsumi et al. 1983). Forearc basins have long been recognized as repositories of volcanic material from the adjacent arc (e.g., Dickinson 1974), making their sedimentary fill potentially ideal for reconstructing temporal arc evolution. Reconstructing arc evolution through the stratigraphic record has implications not only for modern subduction studies but also for understanding the growth of the continental crust. Because the volcanic and volcaniclastic component of island-arc crust constitutes a substantial proportion (30%) of the total crustal thickness (Suyehiro et al. 1996; Crawford et al. 2003), geochemical variability in the arc sedimentary history influences the net geochemistry of arc crust and thus also the bulk continental crustal composition following eventual arc– terrane accretion (e.g., Pearcy et al. 1990; Holbrook et al. 1999; Draut et al. 2002; Draut et al. 2004). Moreover, the long-term tectonic and geochemical evolution of arc crust can be reconstructed most readily through study of arc volcaniclastic sediment, aided by datable fossilif- erous stratigraphy. However, if the sedimentary record does not reliably represent the arc output at the time of sedimentation, then its use in deciphering the history of a particular margin will be severely limited. In this study we review bathymetric, drill, and dredge data from the Mariana and Tonga Arcs of the western Pacific, and introduce new bathymetry, backscatter imagery, and dredging data from the Mariana Arc, to develop a general conceptual model for sedimentation processes around modern arc volcanoes in non-accretionary subduction settings. We can thereby assess the potential for continuity (and, hence, reliability) of a sedimentary record of oceanic island arc activity in a tectonically Copyright E 2006, SEPM (Society for Sedimentary Geology) 1527-1404/06/076-493/$03.00