The mechanical anisotropy of Four‐mile gneiss has been investigated in a series of uniaxial and triaxial, compression and extension experiments performed at confining pressures Pc up to 400 MPa, constant strain rates ε from 1.6×10−6 to 1.5×10−4 s−1, and temperatures T from 25° to 800°C on cylindrical and notched samples oriented with respect to foliation (S) and lineation (L). Differential stresses measured both at the onset of yielding and at failure vary with specimen orientation, with maximum compressive strengths exhibited by samples cored perpendicular to S and minimum strengths exhibited by samples cored at 45° to both S and L. While failure strengths are influenced most strongly by the orientation of S, they appear to depend upon the orientation of L as well. An orthorhombic failure criterion, generalized from a nonlinear Mohr‐Coulomb relation, has been considered with quadratic and linear stress terms resembling those of invariants J2 and I1, respectively, and material parameters estimated by nonlinear regression methods. Satisfactory fits were achieved for results at T = 25°C as well as T = 700°C. Fracture strengths are relatively insensitive to changes in T and ε and the anisotropy exhibited at T = 700°C is remarkably similar to that measured at T = 25°C. Relatively small reductions in strength observed at elevated temperatures are probably due to the influence of thermally induced microcracks. Mechanisms of deformation and sources of anisotropy have been identified by examining microstructures developed in deformed specimens and observing their relationships to those fabric elements initially present in the starting material. Throughgoing shear fractures developed in samples shortened in all orientations with respect to S and L by the coalescence of microcracks in feldspar and quartz grains, as reported for isotropic granites. However, inelastic strains within mica grains were accommodated by slip, frictional sliding, and kinking, and deformation of favorably oriented micas appears to have led to local stress concentrations in neighboring phases that result in nucleation of tensile microcracks. Both S and L are defined by the preferred orientations of micas, and a simple model involving crack nucleation around oriented mica grains is proposed to explain the anisotropy observed.

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To constrain the age of deformational/metamorphic events in the south-central Brooks Range, we analyzed 16 samples of white mica, amphibole, and biotite using the 40 Ar/ 39 Ar incremental heating technique. Metamorphic rocks in the study area (between 151°W and 148°W) occur in three principal east-west-trending faultbounded belts. These are, from south to north, the pumpellyite-actinolite-facies rocks of the Phyllite belt, the high-pressure/low-temperature (HP/LT) metamorphic rocks of the Schist belt, and the predominantly greenschist-facies metamorphic rocks of the Skajit allochthon. All three belts have been affected by two penetrative deformational/metamorphic events. The oldest of these (D 1a ) resulted in tight to isoclinal folding and was accompanied by pumpellyite-actinolite-facies metamorphism in the Phyllite belt, blueschist-facies metamorphism in the Schist belt, and blueschist-to greenschist-facies metamorphism in the Skajit allochthon. Two white micas samples, one from the Schist belt, and one from the sodic-amphibole-bearing schists north of the Minnie Creek thrust (MCT) yielded convex-upward Ar-release spectra with maximum apparent ages of 142 and 129 Ma, respectively; we interpret maximum apparent ages as a minimum age for D 1a deformation and HP/LT metamorphism in the south-central Brooks Range.A second synmetamorphic deformational event (D 1b ) affected all but the northernmost rocks of the Skajit allochthon, resulting in pervasive dynamic recrystallization accompanied by growth of metamorphic minerals; in the Schist belt and in sodicamphibole-bearing schists north of the MCT, D 1b occurred under lower amphibolite-to greenschist-facies metamorphic conditions, and in the Phyllite belt, D 1b occurred under lower greenschist(?)-facies conditions. Plateau, preferred, and isochron dates on white mica, amphibole, and biotite are Early Cretaceous in age, and range from 135 to 110 Ma. We argue that the predominance of Early Cretaceous dates result from the degassing of HP/LT metamorphic minerals during dynamic recrystallization associated with D 1b folding and the pervasive growth of syn-D 1b neoblasts. These ages are significantly older than Late Cretaceous to Tertiary(?) extensional structures in the southern Brooks Range, and support the interpretation that D 1b structures formed during Brookian contractional deformation.

Eclogite containing the prograde assemblage garnet + sodic-augite + glaucophane + actinolitic-hornblende + epidote + rutile + apatite (+ sphene) occurs within the Schist belt of the south-central Brooks Range along the Middle Fork Koyukuk River near Wiseman, Alaska. Prograde minerals in eclogite are overprinted and partially replaced by a secondary assemblage of zoned amphibole (actinolite to hornblende, core to rim), epidote, chlorite, garnet, albite, and sphene in diffuse veins and in pervasively altered patches. Veins and patches of secondary mineralization are crosscut, in turn, by monominerallic actinolite veins.Host rocks consist mainly of pelitic and semipelitic quartz-mica schists with subordinate intercalated layers of mafic schist, albite schist, massive metabasite, metagabbro, marble, calc-schist, and granitic orthogneiss. These rocks have been severely deformed and recrystallized under lower amphibolite-to greenschist-facies conditions, but an older high pressure/low temperature (HP/LT ) metamorphic event is indicated by the presence of relict crossite in metabasite, and the common occurrence of pseudomorphs after glaucophane in quartz-mica schists. Textural relations indicate that the high-pressure assemblage quartz + phengite + paragonite + chlorite + glaucophane + chloritoid was widespread in quartz-mica schists prior to retrograde metamorphism. Mineral geothermometry, phase equilibria, and microstructural and textural relations indicate that eclogite and the enclosing rocks of the Schist belt both experienced peak metamorphic conditions within the blueschist facies at temperatures between 350 and 550°C and pressures above the lower stability limit of glaucophane (greater than approximately 7 kbar); retrograde metamorphism occurred within the lower-amphibolite to greenschist facies at temperatures between approximately 380 to 480°C and at pressures below the lower stability limit of glaucophane.The occurrence of the assemblage garnet + Na-pyroxene + rutile in metabasites appears to be compositionally controlled. Whereas eclogite was derived from a Fe-Tirich basaltic protolith, mafic schists, metabasites, and metagabbros from the surrounding schists were derived from rocks of normal basaltic composition and contain lower amphibolite-or greenschist-facies mineral assemblages.