Geosynclines v. Accretionary Prisms Structure

Geosynclines v. Accretionary Prisms

Structure and formation of geosynclines

Geosynclines such as the Neoproterozoic Wonoka canyons now located on the southern portions of the Australian continental land mass form as sediments deposit in submarine basins, eventually leading to the compressions and fracture of these deposits and the emergence of canyons or linear troughs -- the geosyncline proper (Giddings et al., 2010). The formation process of these submarine geological features is largely dependent on the destruction of continental crust, especially along its margins, as it is broken down into the sediment that then collects in the submarine basins to form geosynclines, which can either be subjected to further oceanization, moving away from the continental crust, or drawn up out of its submarine birthplace as in the Neoproterozoic Wonoka canyons system (Giddings et al., 2010; Marakushev & Marakushev, 2008). This has led to some measure of debate regarding the origins of certain specific geographic features.

The Neoproterozoic Wonoka canyons, for instance, were hypothesized to be of non-marine origin due to a lack of defining marine features such as wave patterns and an abundance of crustal material (Giddings et al., 2010). The material is now, however, hypothesized to have collected as sediment from runoff and erosions of the continental crust, and the lack of wave patterns and other shallow-water formations is believed to be due to the deep-water nature of the Neoproterozoic Wonoka canyon's origins (Giddings et al., 2010). The depth of the canyons and the degree of fracture is indicative of the scale and mechanisms of a geosyncline's formation.

These mechanisms can also be determined at least in part by the specific substances found in certain geosynclines. South Australia again provides an excellent real-world example; the types of material found in the Wokona Formations and in neighboring canyons are definitely of continental curst origin, yet there exists no explanation for this material's appearance in these formations other than a transference from some other crust via erosion and oceanic currents (Von der Borch 1985). The specific carbonate sediments that comprise layers of the rock in these formations, that is, did not originate on the continental crust where they are now found, but rather from some other area that was eroded away during the Proterozoic era. The sediment that resulted from this erosion was deposited in a submarine basin and eventually coalesced through compression, faulted, and fractured into the formations seen today (Von der Borsch 1985).

As diagram one in the Appendix clearly shows, plate movement from divergence occurring in oceanic plates puts lateral pressure on the basins where geosynclines form, resulting in a pattern of sediment build up and faulting which eventually leads to the creation of the geosyncline. This external thrust causes the migration of sediment material, creating lateral compression in addition to the vertical compression that occurs from the sheer weight of the deposits. The combination of lateral and vertical forces as well as compression lead to the solidifying and mineralization of the sediment deposits, as well as to their faulting and fractures. This is what results in the canyon and mountain formations that are the defining features of these undersea geosynclines, and of land-based features such as the Neoproterozoic Wonoka canyons that likely formed underwater.

2. Structure and formation of accretionary prisms

Accretionary prisms also form from the sedimentary deposits of continental runoff and erosion, but these structures are fundamentally different in both the mechanism of their formation and their eventual structure. Forming over or adjacent to areas of subduction, accretionary prisms or wedges are ultimately shaped by both the rate of sediment deposition and the rate of subduction of the same sediment, both of which can and do change regularly and independently of each other (Simpson, 2010). Accretionary prisms are large geometric structures that remain composed of sediment and can drastically shift their weight distribution and stress patterns based on these varying levels of subduction and sediment deposition (Simpson, 2010; Kimura et al., 2010). This in turn allows for a greater flow of gaseous elements through accretionary prisms, carving out even more unique formations and leading to even more delicate and unstable structures (Kimura et al., 2010).