Neotectonic joints

Neotectonic joint systems are the most recent joint systems to form within a region subject to uplift and erosion. An inventory of neotectonic joint attributes was compiled from observations in case study terrains containing flat-lying sedimentary sequences in the platform covers of cratons: the Appalachain Plateau, southeast England-northeast France, the Arabian platform, and the Ebro basin in Spain. Neotectonic joint systems are simple, generally consisting of sets of vertical extension fractures or less commonly steep conjugate fractures striking parallel to, or symmetrically about, the extension fractures. Shallow neotectonic joints propagate within the upper 0.5 km of the crust where effective σ₃ is both tensile and horizontal and σ₁ - σ₃ is small. These shallow joint systems generally form within the upper 0.5 km: of the crust because unloading as a result of denudation and lateral relief consequent on uplift are prerequisites for their propagation. These structures are of potential value for tracking the contemporary stress field in regions where in situ measurements are not available. As a test of this possibility, late-formed joints were studied in the Appalachian Valley and Ridge of Pennsylvania. Outcrop studies showed that late-formed joints have the characteristics of neotectonic joints and, furthermore, propagated parallel to or approximately parallel (N75°-90°E) to directions of contemporary horizontal maximum stress (S_H) known from in situ stress measurements or fault-plane solutions of earthquakes. The latter study lends strong support to our notion that late-formed or neotectonic joints in some terrains are likely to reflect the orientation of the neotectonic or contemporary tectonic stress field.