Geochronological and fluid-inclusion constraints on the Ediacaran-early Cambrian tectonothermal evolution of the Ithaca Basin, eastern Laurentia

Abstract

In 2022, an experimental geothermal deep borehole (ESH-1) was drilled 2978 m through the Appalachian Basin to test the viability of unconventional geothermal energy as a heat source for Cornell University in Ithaca, New York, U.S.A. The borehole encountered unexpected greenschist facies, Neoproterozoic interlayered phyllite and metavolcanics (Cayuta Formation, Ithaca Basin) from 2859 to 2978m (below the surface) beneath the Paleozoic sedimentary rocks. Rocks of the Ithaca Basin record a previously documented tectonothermal event. New geochronology was completed to constrain the timing of the event, and was integrated with petrology and fluid inclusion studies. The new results suggest that deposition, tectonism and exhumation occurred between ca. 560–500 Ma. Because the Cayuta Formation experienced greenschist facies metamorphism, exhumation likely involved the removal as much as 15 km of overburden prior to deposition of the basal Cambrian Potsdam Group. Eleven zircon analyses from Cayuta Formation yield an age of 559 ± 6 Ma, dating volcanism or the age of contemporaneous volcanic detritus. Interbedded phyllite and metavolcanics show middle greenschist facies assemblages indicative of ∼2–4 kbars (0.2–0.4 GPa) pressure. Fluid inclusions in quartz record a change in conditions from 252 °C to ≥3.0 kbar (0.3 GPa) to 150 °C and ≥2.0 kbar (0.2 GPa). Considering that quartz in phyllite exhibits crystal plasticity textures, under normal metamorphic conditions the estimated temperature reached was >350 °C and pressure is extrapolated to >4.0 kbar (0.4 GPa). These rocks are strongly deformed indicating substantial tectonism predating deposition of unconformably overlying ca. <503 Ma Potsdam Group sandstones. This deposition, bimodal volcanism, deep burial, metamorphism, deformation, uplift and erosion occurred in <56 Ma which is uncharacteristically rapid for a rift basin. Although speculative at this time, the preferred tectonic model appeals to transtention transitioning to transpression in a shifting stress field along the New York-Alabama lineament structure during the Ediacaran–Early Cambrian resulting in positive tectonic inversion.