THE CARTHAGE COLTON MYLONITE (SHEAR) ZONE: A DEEPLY-SEATED STRIKE SLIP BOUNDARY IN THE GRENVILLE

The Cartage-Colton Mylonite (Shear) Zone: a deeply-seated strike slip boundary in the Grenville Province. JOHNSON Eric, Lee Geology Department, Hartwick College Oneonta, NY 13820 JOHNSONE@Hartwick.edu

The Carthage Colton Mylonite or Shear Zone (CCMZ)is a major structural boundary in the Grenville Province separating rocks belonging to the Central Metasedimentary Belt Terrane (CMB)to the west from rocks belonging to the Central Granulite Terrane (CGT)to the east. The zone is marked by strong l-s fabrics and is reported to have a northeasterly trend dipping to the northwest. This work reports on structural/petrologic data from three transects across the CCMZ. Deformation within the CCMZ ranges from anastomosing sets of cm-scale shear zones containing recrystallized mineral assemblages ranging from greenschist to granulite facies, to km+ wide zones of intense l-s tectonite with recrystallized mineral assemblages belonging to the upper amphibolite to granulite facies. Rock types studied included meta-igneous rocks belonging to the Diana Syenite Complex (syenite, granite), the Dana Hill Metagabbro Body, and a variety of leco-granitic gneisses and hornblend/pyroxene granite gneisses. The three transects studied all show a progression in deformation (shear fabric development) from west to east across the CCMZ. To the west, in the CMB (Diana Syenite Complex), deformation is dominated by a moderate to weak foliation with pronounced mineral streatching lineation giving a general N to NE strike and moderate dip (18-60^o) to the west. Lineations fall on the foliation planes and generally trend in a down-dip orientation. These rocks are cut by multiple generations of cm to m scale sear zones with a range in orientations. These cm to meter scale shears generally record greenschist-amphibolite facies conditions in the Diana Complex (CMB). To the east, fabric development becomes more intense and pervasive. Orientation of stretching lineations record well defined maxima across km scale distances, foliation planes tend to dip steeply (60-90^o) to the west and stretching lineations fall near to strike with shallow plunges of 0 to 25^o to the north and northeast. These wide zones of l-s tectonite fabric development tend to be confined to rocks belonging to the CGT and the rocks themselves record granulite facies conditions during shearing (T>700^oC). To the north, the CCMZ narrows and this transition is compressed but is still present. Narrow (cm-scale) anastomosing shear zones cut these rocks as well and record a range (GS -Granulite facies) in recrystallization conditions. The structural data support an early history (syn-post Ottawan) of intense ductile shearing along near vertical planes with on-strike to shallowly-plunging transport directions. This shearing occurs at granulite facies conditions and is generally confined to the CGT side of the CCMZ. These data strongly support a transpressive (strike slip) character for early deformation along the CCMZ. Later shearing, is confined to small m-cm wide zones of lower but variable metamorphic grade which may record late stage adjustments along the CCMZ bringing rocks of the CMB and CGT to a common structural level.