Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

Abstract

At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (∼19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small‐to‐medium‐sized prisms have formed (∼16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (∼8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid‐volume sediment subduction at convergent ocean margins (∼43,500 km) is calculated to be 1.5 km³/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long‐term rate of sediment subduction at 1.0 km³/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km³/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long‐term rates, which range from 25 to 50 km³/m.y. per kilometer of margin. A conservative long‐term rate applicable to many sectors of converge...

The tectonic regime along the Andes: Present‐day and Mesozoic regimes

“…If the shortening rate of the compressional system or stretching rate of an extensional system is taken into consideration, the relationship between trench roll-back rate and absolute motion can be expressed as shown in Figure 2. The system could be better depicted if the crustal removal of subduction erosion is taken into consideration (von Huene and Scholl 1991;Ranero and von Huene 2000).…”

“…On the other hand, the contrasting models of active convergent margins show that some subduction zones are dominated by a huge accretionary prism of largely oceanic sediments and rocks, while others show tectonic erosion where the forearc is composed of continental rocks that are progressively fractured and subducted, resulting in subsidence and extension (Hussong 1980;von Huene and Scholl 1991). Along the Andes and the eastern Pacific subduction zones there are good examples of the different subduction types, and the two main conceptual models are illustrated in Figure 1.…”

Long-term sedimentary recycling of rare sulphur isotope anomalies

“…53, is to estimate the theoretical maximum for this parameter on the modern Earth by combining rates of sediment subduction [54][55][56][57] with the typical reactive Fe content of deep-sea sediments 58 and the assumption that most or all this reactive Fe is sequestered as a constituent of pyrite. This approach yields an estimate on the order of 4 -9 x 10 11 molS y -1 , a very large sulphur flux in the context of a surface system driven almost entirely by volcanic sulphur input (see below).…”

From seamount accretion to tectonic erosion: Formation of Osa Mélange and the effects of Cocos Ridge subduction in southern Costa Rica

“…[2] Exploration of active convergent margins indicates that subduction is accompanied by either erosion or accretion of material to the upper plate [von Huene and Scholl, 1991;Clift and Vannucchi, 2004]. The two processes can coexist along the same margin or alternate in time, so that mass removal can develop on a previously growing margin and completely remove an accretionary prism.…”