BRITTLE STRUCTURES IN THE HOLYOKE BASALT OF THE HARTFORD BASIN: GROUND-TRUTHING LIDAR LINEARS

Hillshade models constructed from LiDAR-based DEM data reveal a complex pattern of brittle structures in the Jurassic Holyoke Basalt of the Hartford Basin in central Connecticut. The basalt is exposed over vast areas by glacial scouring, and has little till or soil cover. The brittle structures are manifested as linear depressions up to 25 m deep, and/or cliff faces that extend up to 2 km in length. Two structural areas were examined, the King Phillip Mt. (KPM) and Ragged Mt. (RM) fault blocks. In both areas, the linear trends make a conjugate pattern, with the average trend of the linears on the KPM block being 340° and 020°, with an average spacing of 129 m and 114 m respectively; while the trend of the linears on the RM block are 015° and 050°, with an average spacing of 152 m and 124 m respectively.

In order to ground-truth the origin of the LiDar linears, joints were measured across both fault blocks. The joints are normal to layering in the basalt which dips 4° to 15° to the east, and are not cooling joints. They are planer and continuous for 10s of meters and occur primarily in massive basalt, and rarely extend into the columnar jointed rocks below. Joint spacing within the linears on the KPM block cluster around 2 m, while on the RM block cluster around 0.5 m. The joint spacing may be related to differences in the thickness of the massive basalt between the two blocks. Outside the linears, joint spacing ranges from 5 to >20 m. Joints on the KPM block have orientations of 353±15°, while the joints on the RM block are oriented 030±15°. The joints do not parallel the trend of the linears, instead they intersect the linears at a slight angle (15° to 22°) and create broadly stepped walls to the valleys.

Because of the relationship between linear and joint orientation, the linears are interpreted to be shear zones, and the joints are interpreted to be pinnate joints formed in association with shearing. However, the floors of the linears are covered by soil, and therefore a definitive fault origin cannot be proven. If the linears represent conjugate shear zones, it suggests that the structures in the KPM block formed under N-S compression, while structures in the RM block formed under a NE-SW compression. However, since there is no definitive evidence for fault block rotation, the source of this stress difference will be the subject of future work.