AN IMPROVED METHOD FOR DETERMINING AND CHARACTERIZING ALIGNMENTS OF POINT-LIKE FEATURES AND ITS IMPLICATIONS FOR THE PINACATE VOLCANIC FIELD, SONORA, MEXICO

We present an improved method for determining statistically significan t alignments of pointlike features. One of the principal such methods now in use, the two-point azimuth method, depends on a homogeneous dis tribution of points over the region of interest. Modification of this approach by use of the relatively new statistical technique of kernel density estimation permits treatment of heterogeneous point distributi ons without introducing substantial dependence on choice of the grid e mployed in the test for significance of apparent preferred orientation s. The improved method can selectively reveal alignments on different spatial scales and can suggest the locations of alignments as well as their orientation. We use this method to analyze the spatial distribut ion of 416 vents, largely of Pleistocene age, in the Pinacate volcanic field, Sonora, Mexico, just east of the northern end of the Gulf of C alifornia. Apart from a few sets of aligned cinder cones, the distribu tion of Pinacate vents appears nearly random on aerial and space photo graphy. However, when treated statistically, old Pinacate vents exhibi t structural control trending approximately N10 degrees E throughout t he field and in all its subareas. In contrast, vents with ages estimat ed by comparison with dated cones to be younger than about 0.4 Ma show not only the N10 degrees E control but also N20 degrees W and N55 deg rees W alignments significant at the 95% confidence level. The N10 deg rees E alignment probably reflects the current Basin and Range horizon tal stress regime in this particular area, which is atop the mantle ma gma source of the Pinacate lavas. The N55 degrees W direction is relat ed to a major regional fracture of that orientation passing through th e middle of the field and possibly related to normal faults associated with opening of the adjacent Gulf of California. The distribution of vents relative to the fracture trace is consistent with magma having b een guided upward along a SW dipping fault plane. The origin of the N2 0 degrees W alignment is unknown but of pre-Pleistocene heritage. We f ound no evidence to support control of the Pinacate vent alignments pa rallel to rifting or transform directions in the adjacent Gulf. Intrus ion along N20 degrees W and N55 degrees W fractures at or since about 0.4 m.y. ago could reflect either a shift in the crustal stress field or an increase in magma pressure in Pinacate conduits that allowed mag ma to ascend along structures that were not parallel to the maximum ho rizontal compressive stress.