COMPLEXITIES OF OROGEN-PARALLEL FAULTS: AN EXAMPLE FROM THE NORUMBEGA FAULT ZONE IN CENTRAL-EASTERN MAINE

Long-lived, orogen-parallel faults can be expected to have experienced complicated and significant episodic ductile and brittle faulting. A good example is the Norumbega fault zone within Northern Appalachian Mountains in central-eastern Maine. Recent mapping has confirmed that: (1) The fault zone was initiated by orogen-scale dextral ductile shearing that ductilely deformed Acadian granites and their Early-Middle Paleozoic host rocks in a 2-3-km-wide and several hundred mile-long ductile shear zone. Within the shear zone, the megacrystic granites were deformed into mylonite/ultramylonite, and the lower green-schist-facies meta-flysch turbidites and greywacke were converted to phyllonite and muscovite-quartz-schist (mylonite). The question is how the rheologically-contrasting granite and metasedimentary rocks accommodated the same shear strain? What caused discontinuous distribution pattern of several muscovite-quartz-schist slivers? (2) Significantly reactivated, transtensional brittle faulting produced nine pull-apart redbed basins along the fault. The redbeds are the youngest sedimentary rocks in Maine. Compared to pull-apart basins along other well-known orogen-parallel faults in the world, these basins are too small. Their aspect ratios are abnormally large (as large as 45:1). How could the basins be so narrow? (3) The Turner Mountain syenite “stock” is a fault sliver sandwiched between the ductile shear zone and the tilted redbeds. A recent LA-ICP-MS zircon dating yielded a U-Pb age of 410 Ma for the syenite, indicating that it was even older than the neighboring Lucerne batholith (385 Ma). However, the Turner Mountain syenite is immediately adjacent to the granitic ductile shear zone and in the middle of the Norumbega fault zone, but unlike the Lucerne batholith, it shows no evidence of ductile shearing. To answer the questions, we propose that a reactivated reverse faulting with considerable vertical displacement could address the above observations. However, a new question is: did this faulting extend into the central and southwestern Maine segment of the Norumbega fault zone?