Episodic reactivation of a Late Precambrian mylonite zone on the Gondwanan Margin of the Appalachians, southern Newfoundland

Abstract

The Grand Bruit Fault Zone of southern Newfoundland is a fundamental structure within Late Precambrian basement on the Gondwanan margin of the Appalachian orogen. Within the fault zone, a sequence of structures documents changes in the sense of ductile displacement from (1) reverse dip slip, to (2) dextral strike slip, to (3) sinistral oblique slip, and, finally, to (4) dextral lateral offsets. Fault movements along this structure were punctuated by emplacement of a variety of plutons and minor intrusions which, when precisely dated, allow these movements to be bracketed at between 571 Ma and 564 Ma, 497 Ma and 427 Ma, 424 Ma and 420 Ma, and 421 Ma and 387 Ma, respectively. The tectonic evolution of the Gondwanan inlier of southern Newfoundland is mirrored, in large part, by the record of mylonite development within the Grand Bruit Fault Zone. These tectonic events are attributable to well‐constrained, regional orogenic events of both the Pan‐African and Appalachian cycles. Newly formed shear zones in the fault zone reactivate parts of much older faults of similar regional orientation and are, in some cases, kinematically indistinguishable from the ancestral structures. Integration of precise geochronological data with the sequence of overprinted fault structures demonstrates that, although the role of progressive deformation in shear zone development was important, the observed disposition of structures and rock units is primarily a function of polyorogenic accretion. As a multiple‐reactivated structural lineament in a Gondwanan basement inlier, the fault zone exerted fundamental control over the tectonic development of the leading edge of the convergent southeast margin of the orogen.

Late Neoproterozoic Avalonian and related peri-Gondwanan rocks of the Newfoundland Appalachians

“…These areas record differing thermal histories in the Ordovician, but by the Silurian, the inboard margins of both Neoproterozoic blocks again record effects of similar orogenic events: in this case, transpressionrelated thrusting and wrench faulting associated with Salinic collision (B. H. O'Brien et al, , 1993Holdsworth, 1994; G. R. Dunning, S. J. O'Brien, R. E. Holdsworth, and R. D. Tucker, 1995, unpublished data). The intensity and diversity of Silurian orogenic overprint on Neoproterozoic rocks is greater in the Hermitage Flexure than in the Avalon Zone.…”

“…J. O'Brien and Tomlin, 1984;Blackwood, 1985;B. H. O'Brien and S. J. O'Brien, 1989;B. H. O'Brien et al, 1993; Fig.…”

“…Metamorphic monazite, dated at 543 ± 3 Ma, has been separated from Roti Intrusive Suite dikes that crosscut mylonite in the Grand Bruit Fault Zone (B. H. O'Brien et al, 1993). The monazite data reflect cooling from temperatures in the vicinity of 700 °C in the late Neoproterozoic basement in the earliest Paleozoic.…”

“…The crystalline metamorphic rocks of the Hermitage Flexure basement have been most extensively studied in the Cinq Cerf Bay area (e.g., B. H. O'Brien et al, , 1993Fig. 11).…”

“…The stratigraphic and thermal history in that area suggests that the Neoproterozoic rocks evolved in a similar fashion and in close proximity to those of the Avalon Zone during much of the Neoproterozoic, perhaps within a contiguous block, but may have remained detached from the type Avalon Zone, in some fashion, in the early Paleozoic. Both were again juxtaposed during complex Silurian transpressional orogenesis related to the final stages of Iapetan closure (Holdsworth, 1991;, B. H. O'Brien et al, 1993; see following discussion). Significantly, the Paleozoic history of the Neoproterozoic rocks of the Hermitage Flexure implies pre-Silurian interaction with Iapetan rocks, either along or offshore the southeastern Iapetus margin.…”