An improved method for determining and characterizing alignments of pointlike features and its implications for the Pinacate volcanic field, Sonora, Mexico

Abstract

We present an improved method for determining statistically significant alignments of pointlike features. One of the principal such methods now in use, the two‐point azimuth method, depends on a homogeneous distribution 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 distributions without introducing substantial dependence on choice of the grid employed in the test for significance of apparent preferred orientations. 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 distribution of 416 vents, largely of Pleistocene age, in the Pinacate volcanic field, Sonora, Mexico, just east of the northern end of the Gulf of California. Apart from a few sets of aligned cinder cones, the distribution of Pinacate vents appears nearly random on aerial and space photography. However, when treated statistically, old Pinacate vents exhibit structural control trending approximately N10°E throughout the field and in all its subareas. In contrast, vents with ages estimated by comparison with dated cones to be younger than about 0.4 Ma show not only the N10°E control but also N20°W and N55°W alignments significant at the 95% confidence level. The N10° alignment probably reflects the current Basin and Range horizontal stress regime in this particular area, which is atop the mantle magma source of the Pinacate lavas. The N55°W direction is related to a major regional fracture of that orientation passing through the 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 been guided upward along a SW dipping fault plane. The origin of the N20° W alignment is unknown but of pre‐Pleistocene heritage. We found no evidence to support control of the Pinacate vent alignments parallel to rifting or transform directions in the adjacent Gulf. Intrusion along N20°W and N55°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 magma to ascend along structures that were not parallel to the maximum horizontal compressive stress.