DEVELOPMENT OF EXTENSIONAL FAULT-PROPAGATION FOLDS IN THE MODOC PLATEAU, NORTHEASTERN CALIFORMIA

It is observed that vertical displacement is typically greatest near the centers of normal faults and decreases to zero at the tips. This implies that normal faults propagate out from their centers as they grow. Consequently, along-strike variations in the surface structure of faults can be viewed as analog to their temporal evolution.

To better understand the process of fault growth, we visited the near-tip regions of 8 surface breaking normal faults in the Modoc Plateau, northeastern California. Normal faults in this region express east-west extension related to either Basin and Range or Cascadian back-arc tectonics. The faults come to the surface through a blanket of Miocene-Pliocene basalt flows. Thus, degradation of the scarps is slow and surface deformation in the tip regions is well preserved.

Using a Differential Global Positioning System, we produced detailed topographic maps of the near-tip regions of eight scarps along with spatially constrained observations of surface character and structural deformation. Scarps in the Modoc Plateau are defined by vertical cliffs, controlled by columnar jointing, with large talus piles (~1/2 scarp height) at their bases. Near the tips of the scarps, there is typically a transition to a gentle monocline (recorded by tilted basalt columns) with a strike-parallel axis. Fold amplitude decreases along strike until only a gentle regional tilt to the hanging wall remains. Tilted basalt surfaces in the near-tip regions are typically parallel to the overall topographic slope, implying that topography in these regions is controlled, at least partially, by fault-related folding. We interpret these structures to show that the growth of normal faults in the Modoc Plateau is preceded by gentle monoclinal folding beyond the tip of the propagating fault.

We use a boundary-element code to test the mechanical controls on folding in the near-tip regions of normal faults. The code employs a displacement discontinuity in a linear elastic half-space. By varying elastic properties, the shape and orientation of the discontinuity, and the distribution of slip along the discontinuity, we produce models for the development of fault-related surface deformation. Our field observations provide modeling constraints and a test of model validity.