A NEOPROTEROZOIC (?) BASIN BENEATH ITHACA, NEW YORK

An exploratory geothermal borehole (ESH No. 1; 31109-30003-00) was drilled in Ithaca, NY on the campus of Cornell Univ. in 2022, and was completed at a depth of 2984 m in crystalline low-grade meta-sedimentary/volcanic rocks, below the Paleozoic Appalachian Basin strata. Borehole imagery and the BHI dip log are consistent with seismic profiles, with subhorizontal reflectors (Paleozoics) overlying rocks with possible bedding/foliation, dipping NNW to N. About 120 m of rock was penetrated below the unconformity. Accounting for dip, the true thickness is less than 60 m. In addition to ubiquitous younger rock fragments, petrographic and SEM analyses of cuttings revealed that most of the rocks below the unconformity are fine-grained (silt-sized) lower greenschist facies pelitic/semipelitic phyllites (white mica, chl, minor bi), intermingled with felsite, greenstone and minor crystalline dolostone. Minor calcite, quartz and granitic micro-veins (100-200 microns) were observed. The lithologies and metamorphic fabric that occurs in ESH No. 1 are not typical of the Grenvillian basement of the NY promontory. The USGS magnetic map shows a low anomaly that spans the Ithaca, NY region, and is surrounded by magnetic highs that can be traced to Grenvillian high-grade rocks in the Adirondack Mountains, while the extensive, Scranton Gravity high occurs ca. 75 kilometers to the SE. We propose that the Ithaca magnetic low is caused by these siliciclastics within a remnant Iapetan Neoproterozoic basin. A basement well (31109-03973-00) located south of Ithaca, NY was reported to contain hydrothermally altered granitoid, and zircons from a kimberlite dike in the Ithaca area show decoupling of U-Pb and Lu-Hf isotopes, and U-Pb resetting with lower intercept ages of ca. 600 Ma. These data support an extensive pre-Iapetan basin well inland of the Iapetan margin. The low grade metamorphism and steep dips of the rocks suggest considerable burial and heating of this basin related to crustal extension during the early/initial continental breakup in a possible basin and range style event. Further, a thick section of less metamorphosed and deformed rock would have to have been eroded to create the abruptness of the Paleozoic unconformity. This suggests a possible highly active and complex Neoproterozoic extensional tectonic history in Central New York of deep burial, uplift and extensive erosion.