1 Introduction
The upper-plate location of continental arc magmatism is broadly correlated with the geometry of the subducting slab, with melt generation and individual volcanic centers generally positioned above the subducting slab 100 ± 40 km depth contours (Isacks and Barazangi et al., 1977; England et al., 2004; Syracuse and Abers, 2006). Numerous factors can influence subducting slab geometry, including the age of the incoming slab (Jarrard, 1986), buoyant topographic highs (e.g., oceanic plateaus: Van Hunen et al., 2002, 2004; aseismic ridges: George et al., 2022), the thickness of the overriding plate (e.g., Sharples et al., 2014), trench shape and curvature (e.g., Chiao et al., 2002), convergence rate (e.g., Billen and Hirth, 2007), and convergence obliquity (e.g., Laurencin et al., 2018). Thus, overall arc localization should be limited, given the dynamic nature of long-lived subduction systems, wherein slabs can break off and active ridges can subduct, the age of the incoming plate can change over time (Grow & Atwater, 1970), incoming plate speed and direction can both vary with time (Stock & Molnar, 1988; Richards & Lithgow-Bertelloni, 1996), and bathymetric highs can be fully subducted (Gutscher et al., 1999) or accreted (Yang et al., 2015). Thus, it is accepted that long-lived arcs should migrate large trench-perpendicular distances as both slab and upper-plate geometries morph over time (e.g. Gianni and Luján, 2021).
Hence, how arcs can be localized on 107-year time scales, hundreds of kilometers inboard of a non-stationary trench (Kirsch et al., 2016; Rabiee et al., 2020; Humphreys and Grunder, 2022; Ma et al., 2022), remains an enigma. Furthermore, during periods of slip activity, crustal blocks along strike-slip faults are translated to new positions, intrinsically creating a natural struggle against a clear geological record of arc localization (Alvarado et al., 2016; Trop et al., 2022). Moreover, overriding plates themselves are always in relative motion to the incoming slab (DeMets et al., 2015), and trench-to-arc distance also can vary with time due to accretion (Hughes and Pilatasig, 2002) and subduction erosion (Stern, 2011). Potential arc localization is compounded further by the fact that fluids and hence melt can be generated from a large area of a subducting slab and channeled up-dip (Wilson et al., 2014).
In this work we document arc-localization inboard of the proto and modern Pacific trench along the mobile Alaska Range Suture Zone (ARZS; Ridgway et al., 2002; Trop et al., 2019) since ca. 100 Ma (Figure 1). Variability of incoming subducting slab characteristics and convergent margin configurations—including both normal oceanic plate and oceanic plateau subduction, plate vector changes, oroclinal bending and reconfiguration of trench shape, terrane accretion, long distance translation (>2000 km; e.g., Stamatakos et al., 2001; Tikoff et al., 2022) and a Paleocene slab break off/slab window event—have not drastically affected ARSZ arc location with time. These observations naturally lead to the deduction that a) inherited upper-plate lithospheric thickness and temperature contrasts across the ARSZ and northern Alaska (Miller et al., 2018; Gama et al., 2022) and along the western-Northern Cordillera (Clowes et al., 1995; Estève et al., 2020) have played a role in subducting slab geometry under the Wrangell composite terrane of southern Alaska, b) arc localization, since ca. 100 Ma has been in part controlled by inferred upper-plate related hydrodynamic (viscous) mantle wedge “suction” forces, c) upper-plate crustal heterogeneity preferentially focuses magma ascent, perhaps through melt ponding at Moho offsets and d) upper-plate lithospheric-scale strike-slip faults can act as passive and active conduits for arc magmatism (e.g. Regan et al., 2021).