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.

The Ming's Bight Group of northwestern Newfoundland, an outlier of Humber Zone continental margin rocks, is entirely surrounded by ophiolitic rocks of the Dunnage Zone. Structures in the Ming's Bight Group and adjacent units record three main phases of deformation. The earliest structures relate to Silurian sinistral transpression previously documented in the region. Two later phases of extensional deformation produced a series of dextral obliquenormal shear zones and faults that now separate the Ming's Bight Group in the footwall from ophiolitic and granitoid rocks in the hangingwall. 40 Ar/ 39 Ar and U-Pb data constrain the times of oblique-normal shear and cooling. Metagabbro in the Point Rousse Ophiolite Complex, which lies in the hangingwall, preserves disturbed Ordovician hornblende 40 Ar/ 39 Ar ages, whereas adjacent shear zones record Devonian ages. Hornblendes in Pacquet Harbour Group amphibolites within extensional shear zones mainly record 40 Ar/ 39 Ar ages of 390-380 Ma. Synkinematic titanite and rutile porphyroblasts from an extensional shear zone on the northwestern margin of the Ming's Bight Group have been dated by the U-Pb method at 388 and 380 Ma, interpreted as growth and cooling ages, respectively. The titanite and hornblende ages suggest that the main phase of ductile oblique-normal shear was underway at 405-385 Ma. Ming's Bight Group schists and pegmatites produced concordant muscovite 40 Ar/ 39 Ar ages averaging 362 Ma, interpreted as the time of footwall cooling below 350ЊC. We suggest that the Ming's Bight Group is a mid-Devonian symmetrical core complex formed within a local transtensional regime developed during dextral oblique transcurrent movement along the Baie Verte Line. The timing and tectonic setting of extension do not support recent models for "extensional collapse" in the northern Appalachians.

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