Premetamorphic down-to-basin faulting, folding, and tectonic dewatering, Rangeley area, western Maine

Abstract

The Rangeley area of western Maine is underlain by a thick sequence of dominantly eugeosynclinal metasedimentary rocks of Ordovician, Silurian, and Devonian age. The dominant structural pattern of these rocks is defined by tight, upright, northeast-trending passive flow folds and by three major normal faults along which younger rocks on the southeast are down-faulted against older rocks on the northwest. Each normal fault, together with a major syncline and a complementary anticline farther southeast, defines a geometrically related fault-fold unit. In best-exposed units, displacement along the faults increases in the direction of plunge of the synclines and of increasing structural relief in the syncline-anticline pairs. A genetic relation between normal faulting and folding is inferred. The dominant fault-fold pattern represents the oldest recognized deformation in the area. Slaty or phyllitic cleavage of this deformation is typically subparallel to the axial surfaces of folds, but locally crosses the faults and the axial surfaces of tight folds at low angles. Metamorphosed clastic dikes along the cleavage suggest that cleavage formation was in part a diagenetic dewatering process. This process probably graded, however, into low-grade metamorphism at depth. It was quickly followed by emplacement of large plutons, local superposed passive slip and flexural slip folding, and by two recognized events of greenschist and amphibolite facies metamorphism. Porphyroblasts of these events have grown across slip cleavages as well as older phyllitic cleavage, and metamorphic zones cross the dominant fault-fold pattern. Deformation, as well as sedimentation, is considered to have been controlled by the ancestral Merrimack synclinorium—a strongly linear two-sided trough that persisted at least from Late Ordovician through Early Devonian time. The fault-fold pattern is inferred to have evolved over a long period of time, as follows: (1) Rapid deposition of 15,000 to 20,000 ft of nearly-impermeable clastic sediments in Late Ordovician and Early Silurian time on the southeast-dipping slope of the sedimentary trough; mass weakened in depth by excess fluid pressure. (2) Continuing sedimentation, down-to-basin creep with associated slump faulting and folding, probably beginning in Middle Silurian time; faults flattened basinward in depth along lower boundary of zone of excess fluid pressure. (3) Horizontal compression developed parallel to slide direction as mass piled against material in the trough; incipient slaty cleavage developed normal to compression, improving vertical permeability. (4) Pore fluids expelled vertically, permitting the slumping mass to compact horizontally, and fold with at least 25 percent shortening. The process culminated in Early Devonian time, during and after deposition of the youngest exposed rocks in the area.