Synaccretionary sedimentary and volcanic rocks in the Ordovician Tetagouche backarc basin, New Brunswick, Canada: Evidence for a transition from foredeep to forearc basin sedimentation

Abstract

Development of an Upper Ordovician to lower Silurian (Llandovery) accretionary wedge (Brunswick subduction complex) along the composite Laurentian margin accompanied subduction of the Tetagouche backarc basin and coincided with synaccretionary sedimentation in the Bathurst and Fournier supergroups in northern New Brunswick. These dominantly turbiditic synaccretionary units conformably to unconformably overlie Upper Ordovician pelagic shale and chert in the upper stratigraphic levels of several of the nappes that compose the imbricate thrust stack of the subduction complex. Local occurrences of tectonic mélange at the contacts between the turbidites and their pelagic substrate are consistent with deposition of at least some of the former during thrust-related deformation in the accretionary wedge. Detrital zircon data indicate maximum depositional ages ranging from 454 ± 3 to 459 ± 8 Ma, coeval with initiation of subduction of the Tetagouche backarc basin. In the Elmtree inlier, arc tholeiitic basalt disconformably or unconformably overlies Darriwilian MORB-type pillowed flows that constitute backarc oceanic crust of the Tetagouche basin. Sedimentary rocks immediately below and intercalated with the arc tholeiites contain detrital zircons ranging from 444 ± 6 Ma to 455 ± 10 Ma, suggesting that the tholeiites are related to backarc subduction.

Detrital zircon age spectra from all sampled units exhibit a distinct Laurentian signature, indicating an abrupt change in provenance coeval with closure of the backarc basin and Ganderia – Laurentia collision. The lithology and implied ages of the Bathurst and Fournier synaccretionary sedimentary rocks support a correlation with siliciclastic and carbonate-rich turbidites of the Matapedia cover sequence, which were deposited farther west in a forearc setting (Matapedia forearc) with respect to subduction of Tetagouche backarc oceanic lithosphere. The implication of Late Ordovician influx onto the accretionary wedge and foredeep of sediments having a Laurentian affiliation (like those in the Matapedia forearc), demonstrates trenchward migration of forearc sedimentation between ca. 450 and 430 Ma. This southeastward (present coordinates) expansion occurred in concert with episodic accretion of buoyant crustal blocks that populated the Tetagouche – Exploits basin, and concomitant southeastward step-back of the subduction zone.