Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

Abstract

The submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

Transient stripping of subducting slabs controls periodic forearc uplift

“…Furthermore, the good agreement between natural and modelled topographic profiles (Figs. 1b and 2d) provides an independent confirmation that ongoing underplating activity is a plausible mechanism to account for the present-day coastal high, which is in line with geological and geophysical evidences for deep accretion along active margins 1,14,24 , as well as with earlier wedge-scaled analogue and numerical studies [32][33][34][35][36] . The coastal topography generally localizes directly above a 30-40-km-depth plate interface, trenchward from the intersection of the continental Moho (Supplementary Fig.…”

“…This is especially true for tectonic underplating (i.e. deep accretion at the base of the forearc crust) that occurred in the past and is probably still active along many present-day subduction zones, as evidenced along the Circum-Pacific belt from geological records 13,[19][20][21][22] and geophysical imaging 1,23,24 (Fig. 1a).…”

Subduction erosion and arc volcanism

“…As the capacity of the subduction channel to consume eroded material is a function of several parameters, including the topography of the incoming plate, the convergence rate and the hydrogeology of the forearc and plate boundary 90-92 , erosion rates can be difficult to constrain in the highly dynamic forearc environment. For example, the subduction channel might widen at depth, and thus allow more eroded material to enter the plate boundary shear zone, or shrink, causing underplating down to at least ∼30-40 km depth (as imaged by reflection and refraction seismic profiles and inferred from telltale forearc uplifts) [93][94][95] . As a result, a single margin might display along-strike changes from an erosive to an accretionary and underplating regime 4,96,97 .…”

“…As a result, a single margin might display along-strike changes from an erosive to an accretionary and underplating regime 4,96,97 . Similarly, a margin with a large, long-lived accretionary prism can turn erosive, removing the accreted material by frontal erosion [G] 95 .…”

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

“…When exhumed from subsurface depths of 10–30 km in these settings, these sediment underplates form belts of high- P / T metamorphic assemblages. Identifying the existence of such sediment underplates at modern accreting convergent margins has been possible through seismic imaging in recent years, as Scholl (2019) demonstrates, from the accretionary record of coastal North and South Americas (see orange boxes with number 7 in Fig. 1).…”