STRUCTURAL CONTROL FOR EMPLACEMENT OF THE ANDESITIC DIKES IN ROMENJAN AREA, SOUTH OF BIRJAND, EAST OF IRAN

Early Neogene andesitic mass and dikes in the Romenjan area of southern Birjand, east of Iran show evidence that their emplacement was structurally controlled. Andesitic dikes in this area formed by dilating in trans-tensional zones on pre-existing vertical to steeply N-dipping strike-slip faults. Magma propagation along these faults must have required less energy than the creation of a self-propagated fracture at dike dips and the magma pressure must have been greater than the compressive stress perpendicular to the fault surface. NWW- to W-trending en echelon dikes formed locally and are not obviously attached to the four main dikes in the area. The en echelon segments are probably pieces of deeper dikes, which are segmented perhaps as a result of a documented rotation of the regional stresses. Alternatively, changes in orientation of principal stresses in the vicinity of each en echelon dike could have resulted from local loads associated with paleotopographic highs or nearby structures.

We suggest that the change in stresses near the earth’s surface played the main rolls for created open space for magma emplacement in this area.

The longest dikes intruded normal to σ3, along pre-existing faults with critical dips and strikes: only NNW steeply dipping planes were injected. To achieve this geometry, magma pressures must have been greater than σ3, and also have exceeded friction across the fault surfaces. Driving pressures were apparently not sufficient to intrude neither along faults of other orientations nor along self-propagated paths. En echelon dikes trend at an angle to the fault-conforming dikes, and could have formed during the documented 15ͦ stress-field rotation from east to west.