Sutures are zones of weakness within orogenic belts that have the potential to become reactivated during orogenic evolution. The Robertson Lake shear zone marks a major tectonic boundary in the southeastern Grenville orogen of Canada that has been intermittently active for at least 130 m.y. The shear zone played a major role in the compressional stage of the orogenic cycle as well as during postorogenic collapse. The zone separates the Elzevir terrane to the west and the Frontenac terrane to the east. Sphene ages (U-Pb) indicate that these two terranes have distinct tectonothermal histories and that the shear zone represents a ''cryptic snture.'' In its current state, the shear zone is a low angle (30 degrees ESE dip) plastic to brittle extensional shear zone that separates the Mazinaw (footwall) and Sharbot Lake (hanging wall) domains. Integration of structural, metamorphic. and chronologic data leads to a model that describes the complete evolution of this fundamental tectonic boundary that evolved fr om al? early compressional zone (ca. 1030 Ma) to a late extensional zone (until at least 900 Ma).

Rocks regionally metamorphosed and deformed at middle- to lower-crustal levels in the contraction-dominated Mesoproterozoic Grenville orogen are exposed in southern Ontario, Canada. Investigation of the Robertson Lake shear zone (RLSZ) indicates that extension is a significant component in the late tectonic evolution of this deeply eroded orogen. The RLSZ is a discrete zone 0.5–1.0 km thick that can be traced for nearly 100 km, cross-cutting regional structures and lithologic units. The zone is composed of a thick mylonite zone and a narrow overlying brecciated zone. Mylonite foliation shallowly dips east-southeast and contains a down-dip lineation. Mylonites are characterized by microstructures indicative of crystal-plastic deformation and contain a variety of shear-sense indicators, including SC and CC′ composite fabrics, sigma porphyroclasts, mica fish and other shape fabrics oblique to shear planes, consistently indicating a normal (down-to-the-east) sense of shear. A zone of cataclastic structures overlies the mylonite zone, and brittle deformation overprints the mylonitic fabrics. Slickensides in the zone generally strike parallel to foliation in the mylonites but have steeper dips. Sense of slip indicators and brittle fault orientations also indicate down-to-the-east displacement during east-west extension. Juxtaposition of brittle and crystal-plastic structures is resolved with a model of displacement during exhumation whereby localization of cataclasis occurred along a previously active mylonite zone. Metamorphic facies are upper greenschist in the hanging wall and upper amphibolite in the footwall and metamorphic grade increases from west to east in both the hanging wall and footwall. The regional variations in metamorphic grade and the low-angle shear zone geometry are a result of isostatic flexural rotations that accompanied extension. Combined with the crystal-plastic to cataclastic nature of the RLSZ, new 40Ar/ 39Ar isotope data from biotite constrain the timing of shear zone displacement until at least 901 ± 1 Ma, late in the evolution of the Grenville orogen.

The Robertson Lake shear zone is a major plastic to brittle extensional shear zone in the Grenville orogen that bounds the Mazinaw and Sharbot Lake domains and provides information on the style of late extension and the unroofing history of the orogen. Argon isotope data were collected from hornblende and micas to determine 40Ar/39Ar ages, constrain the temperature‐time histories of these two domains, and infer the unroofing history of the region. Hornblende cooling ages across the Mazinaw domain (footwall) show little variation, indicating uniform unroofing of the footwall since 950 Ma. Phlogopite, muscovite, and biotite cooling ages of the footwall are 924 to 890 Ma. The cooling history of the Mazinaw domain is characterized by slow cooling after peak metamorphism (circa 1000 Ma), accelerated cooling (4°–5°C/m.y.) from 950 Ma to 890 Ma, and an average cooling rate of ∼1°C/m.y. to circa 590 Ma, when these rocks were at or near the surface. The cooling history of Sharbot Lake (hanging wall) domain is drastically different than that of the Mazinaw domain. Hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 Ma, respectively, indicating a cooling rate of 5°C/m.y. after slow cooling from metamorphic temperatures. Biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve, indicating that the cooling history varied across the domain. Cooling rates in the hanging wall adjacent to the shear zone are low (2°C/m.y.). A biotite cooling age (1029 Ma) and preservation of an amphibole growth age (1205 Ma) in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 Ma. These data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone. The varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement.

The Robertson Lake shear zone (RLSZ) is a low-angle (∼30° dip), plastic to brittle extensional shear zone located in the southeastern Grenville Orogen, Ontario, Canada. Thermobarometric data have been collected to evaluate the effects of extension on regional metamorphic field gradients. Upper greenschist facies metamorphic conditions are preserved in the footwall and upper amphibolite facies metamorphic conditions in the hanging wall. However, steep gradients in metamorphic pressure are present adjacent to the Robertson Lake shear zone. In the footwall pressures increase from 550 MPa to 800 MPa within 8 km of the RLSZ and in the hanging wall pressures decrease from 900 MPa to 500 MPa within 12 km of the RLSZ. The pressure gradients are interpreted to be a product of isostatic flexural rotations in response to extension along the zone. The degree of rotation indicated by the baric gradients in the hanging wall and footwall are compatible and require an initial shear zone orientation of 60–90°, which suggests reactivation of an initially high-angle terrane boundary. Previous studies provide constraints of a depth extent of 40 km for the RLSZ and crustal thickness of 60 km during extension. Thus, extensional rotations were accommodated by decoupling and regional-scale flow in the lower third of the crust.

Calcite textures in several microstructural domains from the Bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions. The coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration. Porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms. The textures are consistent with high-temperature deformation experiments where r, f and c slip systems were active. Complete dynamic recrystallization produced equant, medium-sized grains (50 mu m) with a texture that is nearly random. Secondary calcite growth observed under cathodoluminescence and variation in delta(18)O value with microstructure indicate large fluid fluxes during mylonitization. Fluid-assisted grain-boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms. Finer grained S-C mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great-circle girdle attributed to attainment of an 'easy slip' orientation for most grains. Ultramylonites have homogeneous textures with fine, elongate grains (20-30 mu m) mantled by disseminated secondary phases. Textures in ultramylonites are characterized by single point maxima and great circle girdles. The most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction.Texture/microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size, strain and secondary phases in addition to temperature, strain rate and differential stress. The varied deformation mechanisms and application of experimentally-derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains. These differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain-boundary sliding to dislocation creep.