THE STRATIGRAPHY AND STRUCTURE OF SMALL POINT, MAINE; D3 INVERTED, MAP-SCALE FOLDS IN THE TRANSTENSIONAL REGIME

The Cape Small Synform lies in an area of the Northern Appalachians that has undergone successive deformation from at least two major orogenic events, the Salinic Orogeny (440-423 Ma) and the Acadian Orogeny (421 – 400 Ma) (van Staal et al. 2009) and contains evidence for Norumbega shearing deformation (Early Devonian-Carboniferous) (Swanson 1999). With a grant from the USGS EDMAP program, a 1:10,000 scale map has been created which sheds light on the complex history of folding and shearing. The study identifies 5 new lithologic units within the Scarboro Formation (youngest to oldest- amphibolite, garnet rich member, muscovite rich member, quartz-rich member, and rusty-weathering member), 2 new units in the Spring Point Formation (a calc silicate and a marble), and articulates 6 seven other members which are similar to those mapped by Hussey and Berry (2006) in the Cape Elizabeth and Diamond Island Formations. A previously unseen graded bed, located in the Cape Elizabeth silver schist implies a downward facing stratigraphy, and designates the previously named Cape Small Synform as a synformal anticline. This folding generation, which dominates the area, is related to D₃, which took place during the Acadian Orogeny creating a complex regional fold that changes from open in the south to tight in the north. The upright folds deform two previous events: D₁, an isoclinal recumbent folding event during the Salinic orogeny observable in foliation of quartz-rich beds of Bald Head; and D₂, an isoclinal recumbent refolding deformation event evidenced by folded quartz veins and foliation at Bald Head related to a local Early Acadian event. Strike and dip measurements taken for both meso- and macro-scale D₃ folds suggest the existence of two synformal anticlines (Head Beach fold (190/21), and Hermit Island fold (191/39)), and two anticlinal synforms (Cape Small fold (193/22), the Seal Island fold (165/8)). The well-exposed trough of the Cape Small synformal anticline shows hinge parallel extension, which is linked to deformation outside of a purely compressive regime. Thus a wench dominated, transtensional environment (Venkat-Ramani and Tikoff 2002) is proposed to explain both hinge parallel extension and the connection of this Acadian orogenic folding to the subsequent Norumbega shearing.