Accretion, Soft and Hard Collision: Similarities, Differences and an Application from the Newfoundland Appalachian Orogen

Abstract

We argue there is no distinction between accretion and collision as a process, except when accretion is used in the sense of incorporating small bodies of sedimentary and/or volcanic rocks into an accretionary wedge by off-scraping or underplating. There is also a distinction when these terms are used in classifying mountain belts into accretionary and collisional orogens, although such classifications are commonly based on a qualitative assessment of the scale and nature of the accreted terranes and continents involved in formation of mountain belts. Soft collisions occur when contractional deformation and associated metamorphism are principally concentrated in rocks of the leading edge of the partially pulled-down buoyant plate and the upper plate forearc terrane. Several young arc-continent collisions show evidence for partial or wholesale subduction of the forearc such that the arc is structurally juxtaposed directly against lower plate rocks. This process may explain the poor preservation of forearcs in the geological record. Soft collisions generally change into hard collisions over time, except if the collision is rapidly followed by formation of a new subduction zone due to step-back or polarity reversal. Thickening and metamorphism of the arc's suprastructure and retro-arc part of upper plate due to contractional deformation and burial are the characteristics of a hard collision or an advancing Andean-type margin. Strong rheological coupling of the converging plates and lower and upper crust in the down-going continental margin promotes a hard collision. Application of the soft–hard terminology supports a structural juxtaposition of the Taconic soft collision recorded in the Humber margin of western Newfoundland with a hard collision recorded in the adjacent Dashwoods block. It is postulated that Dashwoods was translated dextrally along the Cabot-Baie Verte fault system from a position to the north of Newfoundland where the Notre Dame arc collided ca. 10 m.y. earlier with a wide promontory in a hyperextended segment of the Laurentian margin.

Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

“…In New Brunswick, oblique accretion was accompanied by blueschist facies metamorphism around 445 Ma in the Québec reentrant (van Staal et al, 2008). Arrival of the Victoria terrane at the Newfoundland promontory was similarly timed but resulted in a "soft" collision ( van Staal and Zagorevski, 2020) without exhumation of high-pressure rocks. • The Gander-Lakesman assemblage arrived in the Wenlock-Ludlow in another "soft" collision (e.g.…”

“…The Gander Group is tectonically overlain in its type area by a fragmented ophiolite package forming the "Gander River ultrabasic belt" or GRUB, emplaced in a Penobscottian "soft" collision ( van Staal and Zagorevski, 2020) during the Early Ordovician.…”

“…Detrital zircon data from "Avalonia" (Murphy et al, 2004b(Murphy et al, , 2004cWalsh et al, 2007;Pollock et al, 2009;Satkowski et al, 2010;Barr et al, 2012Barr et al, , 2020aDorais et al, 2012b;Thompson et al, 2012Thompson et al, , 2014Willner et al, 2013;Henderson et al, 2016;Waldron et al, 2019a). protracted and relatively "soft" ( van Staal and Zagorevski, 2020) compared with that of the Miramichi terrane.…”

“…The Victoria terrane records a much "softer" collision ( van Staal and Zagorevski, 2020) with the Laurentian margin, and continued to receive sediment into its foredeep well into the Llandovery (Waldron et al, 2012 and references therein). Sinistral transport of the Victoria terrane may have carried it outboard of the Newfoundland promontory (arrow V in Fig.…”

A plate tectonic view from the top of the world

“…It is possible that this “whole‐crust” deformation reflects a change in collisional style from “soft”, with contractional deformation and metamorphism concentrated in rocks at the leading edge of the under‐thrusting plate (i.e. India), to “hard”, where thickening and metamorphism occurs across both the lower and upper plates (see: van Staal & Zagorevski, 2020). While soft collision can progressively evolve into hard collision, such a transition can also be reflected by punctuated, large‐scale changes in the orogenic system.…”