DEFORMATION POST-DOMING? NEW MAPPING OF THE OLIVERIAN JEFFERSON DOME, MT. DARTMOUTH 7.5’ QUADRANGLE, NEW HAMPSHIRE

The Mt. Dartmouth 7.5’ Quadrangle, located west of the Presidential Range, contains the southeastern portion of the Jefferson Dome, the largest and northernmost of the Oliverian Domes. As part of a USGS NHGS EdMap StateMap project, the purpose of this study was to remap this part of the Jefferson Dome and perform a structural and mineralogical analysis of its rock units to develop a more complete understanding of the tectonic history of this part of the Bronson Hill Anticlinorium (BHA). The Dome was last mapped by the Billings in the late 1930s and their map was published in 1946. Their map consisted of three Oliverian units that were renamed but not remapped on the New Hampshire bedrock geologic map of Lyons et al. (1997). Our work has significantly changed the map pattern of the internal Dome units as well as added three new units including a coarse-grained and a porphyritic, hornblende-bearing, alkali feldspar granite and a third unit composed of porphyritic biotite granite. The other Dome units are phaneritic and include biotite, quartz alkali feldspar syenite, hornblende syenogranite, and biotite quartz syenite. The rocks show highly altered feldspars as well as myrmekite, perthite, sericite alteration, and biotite-chlorite alteration.

The typical well-foliated dome margin giving way to a weakly or non-foliated dome interior, commonly seen throughout the BHA, was not found. Instead a variety of foliation orientations and intensities were observed throughout the Dome. Several zones of steeply dipping, strongly foliated regions have been mapped and represent possible shear zones. Shear sense appears to be a mix of sinistral and dextral based on sigmoidal subgrains between foliation planes, and S-C fabrics, respectively. The well- to weakly-foliated fabric defines the dome structure and shows the limbs oriented at 61°,41° SE and 279°,10° NW, with an interlimb angle of 130°, and a dome hingeline of 62°,13°. This classifies the dome as a gently plunging, steeply inclined, gentle fold. The doming foliation appears to be older than the foliations related to the possible shear zones.

If the doming is accepted to be late Acadian, as observed in adjacent portions of the BHA, then the shear zones may be younger, perhaps NeoAcadian.