MICROGRAVITY AND MICROMAGNETIC EVIDENCE FOR SHALLOW SUBSURFACE TERMINATION OF THE NORTHEAST DIKE AT SHIP ROCK, NM

The northeast dike at Ship Rock, NM is composed of en echelon segments whose offsets do not appear to be formed by post-emplacement deformation. Delaney and Pollard [1981] modeled the subsurface structure of the northeast dike based on the surface expression and concluded that the en echelon segments formed due to rotation of the maximum horizontal compressive stress direction as the dike propagated vertically. They proposed that a main dike is located at depth, connecting the en echelon segments.

We modeled the structure under the northeast dike based on gravity and magnetic field measurements collected over a pair of en echelon dike segments and used that modeling to constrain the emplacement mechanism. Once we reduced the gravity data, the measured anomaly appears to be random, which suggests that the actual gravity anomaly is within the error of the Lacoste and Romberg gravimeter used for this study. For this reason, we only used the magnetic data to determine a best-fit model. To constrain the subsurface structure, we developed several models of the subsurface using the University of British Columbia's Mag3D programs and based the initial models on Delaney and Pollard's model. However, the initial models produced anomalies that are both larger and wider than the anomalies in the measured field data. To produce a smaller and narrower anomaly, dike material must be removed from the model. In the best-fit model for the magnetic field data, the dike terminates at approximately six meters below the surface. This surprising result suggests that the northeast dike was originally emplaced above the present-day land surface and propagated both upwards and downwards. The remnants of the northeast dike appear to be the base, not the top, of the original dike, which has since been eroded away, and the en echelon segments formed due to rotation of the maximum horizontal compressive stress direction at the bottom edge of the dike.

Our results also have implications for the other dikes at Ship Rock. Ship Rock is surrounded by several smaller dikes with less surface expression than the northeast dike, but these dikes were likely more substantial in the past and have since been eroded. Furthermore, the south and west dikes likely terminate in the shallow subsurface as well, but we need to collect additional data to determine the termination depth of the west and south dikes.