FRACTAL NATURE AND SCALING OF NORMAL FAULTS IN THE ESPANOLA BASIN, RIO-GRANDE RIFT, NEW-MEXICO - IMPLICATIONS FOR FAULT GROWTH AND BRITTLE STRAIN

Quaternary faults in the western Espanola Basin of the Rio Grande rift show a power-law size (displacement) distribution suggesting that fau lting in this region is scale invariant, and that faults are self simi lar. The power law, or fractal, distribution is characterized by a fra ctal dimension of 0.66 to 0.79 and represents a young, immature, activ e fault population in a continental extensional regime. Based on this distribution. it is estimated that unobserved faults with very small d isplacements account for up to 6% of the total strain. Since 1.2 Ma, t otal extension in this part of the basin has been at least 5%. A direc t correlation exists between maximum displacement and length of faults in this area suggesting that they obey a scaling relationship in whic h the ratio of log d(max)/log L, is 5 x 10(-3). This ratio is nearly c onstant for faults whose lengths span three-orders of magnitude, indic ating that there is no difference in the scaling relationship of displ acement and length between faults of all sizes. Considering previous m odels, these fault characteristics suggest that, in the western Espano la Basin: (1) host rock shear strengths are low; (2) remote shear stre sses were probably high; and (3) most faults do not extend throughout the brittle crust. Finally, displacement profiles on five of the large st faults are asymmetric and show a rapid decrease in displacement fro m the point of maximum displacement toward the fault tip. The fractal nature, scaling relationship and distribution of displacement on fault s are used to suggest that faults grew by nearly proportional increase s in displacement and length, perhaps by mechanisms dominated by propa gating shear fractures rather than by linkage of pre-existing joints o r faults.