Trading space for time : an example of fold development from the Central Appalachians

Many investigators have postulated that deformation near a fold termination is equivalent to deformation during early amplification at the fold culmination. This concept is informally known as "trading space for time" when analyzing the deformation history of any structure. A purpose of this study is to test this concept, using a three-dimensionally exposed, periclinal thrust-modified buckle fold. The target anticline, located in eastern West Virginia, is a second-order parasite in the southeastern limb of a map-scale anticline, is 300 m long, deforms a 9 m thick quartz arenite of the Keefer Sandstone, and was examined primarily across three axial-perpendicular profiles from termination to culmination. These profiles in different parts of the fold share some similarities that might be consistent with the concept of "trading space for time:" NW-verging asymmetry, curved hinges with straight limbs, bedding-parallel slip as a common folding mechanism, and a late-stage set of vertical cross-axial veins. Many specific structural features at the termination are however, absent in the culmination: (1) ubiquitous bed-normal, strike- parallel veins accommodating outer-arc extension; (2) intensification of the layer-parallel shortening (LPS) strain by bed-normal transgranular stylolites and bed-parallel fluid inclusion planes (FIPS), and (3) a lack of outer-arc shortening structures to accommodate the decrease in curvature if the culmination had evolved sequentially from an equivalent of the fold shape to the termination. Similarly, the fold culmination for a thrust-related second anticlinal hinge lacks evidence for these features. Consequently, the fold culmination did not evolve from the termination and "space may not be traded for time."The deformation history of the quartz arenite ranged from diagenetic compaction through folding where temperatures were 165°-285°C and lithostatic pressures were 150- 225 MPa. Deformation progressed from pervasive grain-to-grain solution during compaction to grain-to-grain solution with localized development of transgranular stylolites and FIPS during LPS. Subsequent deformation during folding became even more heterogeneously distributed with micro- and mesoscale veins, and late-stage intense solution along preexisting faults and bedding surfaces. This increased scale of deformation dominated by mesoscale solution and fracture may have resulted from a loss of porosity for fluid conduits, and a lack of sufficient temperature and/or differential stress to generate dislocation-related intragranular structures.