Dynamics of collapse of an impact central uplift: Evidence from folds and faults in the collar of the Vredefort Dome, South Africa

ABSTRACT Structural analysis of overturned metasedimentary strata of the lower Witwatersrand Supergroup in the inner collar of the Vredefort Dome reveals the presence of tangential folds and faults associated with the 2.02 Ga impact. The folds are distinct from previously identified subradially oriented, vertical to plunging-inclined, gentle folds that are interpreted as the products of convergent flow (constriction) during the initial stages of central uplift formation. The tangential folds comprise disharmonic, open, asymmetric, horizontal to plunging-inclined anticline-syncline pairs with centripetally dipping axial planes and right-way-up intermediate limbs. They display centripetal-down vergence (anticline radially outward of the syncline) that is consistent with steep inward-directed shear of the overturned strata. We attribute this kinematic pattern to subvertical collapse of the Vredefort central uplift during the latter stages of crater modification. The folds are cut by pseudotachylite-bearing steep to vertical tangential faults that display center-down slip of <10 m up to ~150 m. Both the tangential folds and the faults suggest that the large-scale overturning of strata related to outward collapse of the Vredefort central uplift was accompanied by a component of inward-directed collapse via layer-parallel shearing and folding, followed by faulting. Subradially oriented faults with conjugate strike separations of 1–2 km in the NNE collar of the dome suggest penecontemporaneous tangential extension of the inner collar rocks. This evidence indicates that second-order structures in the metasedimentary collar of the Vredefort Dome preserve a complex, multistage record of evolving strain associated with both initial convergent and upward flow (constriction) related to central uplift rise and later divergent and downward flow (flattening) linked to its collapse, and that centripetally directed collapse features may be important components of the structural inventory of very large central uplifts.

Modified Convergent Flow Tracing Method for Evaluating Advective Velocity and Effective Porosity in Fractured Rock Aquifers

This study presented the analysis of the modified convergent flow tracing method, which is a modified virtual solute transport approach to retrieve tracer masses from a pulse image (virtual) well to an extraction well. In the convergent flow tracer test, approximate analytical solutions were extended for the pulse image well using a single-well tracing method. This method transformed the drift-and-pumpback conditions of the single-well tracing method. The method requires a prior information of the effective porosity. Using sodium chloride as a tracer mass, the tracer data sampled through field-scale tests were used to obtain breakthrough curves. This modified method was different from the pre-existing single method because it considers both the ambient groundwater movement (the two classes of drifts) and the constant volumetric flow rate during the pumping phase. The method was applied to the tracer test at underground research tunnel for verifying the theory inductively derived from the single tracing method. Through field tests, the values of velocity and porosity were compared to the results of the drift-and-pumpback equations of the single-well test, and the several different equations related to breakthrough curves of the two-well tests conducted on a field scale.

Multiphase boudinage: a case study of amphibolites in marble in the Naxos migmatite core

In multiply deformed terrains multiphase boudinage is common, but identification and analysis of these is difficult. Here we present an analysis of multiphase boudinage and fold structures in deformed amphibolite layers in marble from the migmatitic centre of the Naxos metamorphic core complex. Overprinting between multiple boudinage generations is shown in exceptional 3-D outcrop. We identify five generations of boudinage, reflecting the transition from high-strain high-temperature ductile deformation to medium- to low-strain brittle boudins formed during cooling and exhumation. All boudin generations indicate E–W horizontal shortening and variable direction of bedding parallel extension, evolving from subvertical extension in the earliest boudins to subhorizontal N–S extension during exhumation. Two phases of E–W shortening can be inferred, the first associated with lower crustal synmigmatic convergent flow and the second associated with exhumation and N–S extension, possibly related to movement of the North Anatolian Fault.