Slant stacks of refraction data from maine: Effects of lateral variations in velocity structure

Abstract

Abstract We discuss some of the advantages and pitfalls associated with τ( p ) analysis of wide-angle profiles recorded over terranes characterized by significant lateral heterogeneity. We construct composite slant stacks for a pair of reversed shots recorded along the axis of the Merrimack Synclinorium in southwest Maine and invert picks for bounds on averaged one-dimensional slowness-depth structure. Extremal bounds on total crustal thickness derived from shot point 12 are 40 to 43 km. Average crustal velocities for individual slowness models corresponding to the extremal depths range from 6.5 to 6.6 km/sec. Predicted normal-incidence two-way travel times for the Moho lie between 12.2 and 13.2 sec. The utility of one-dimensional inversion of τ( p ) data is largely in estimating the maximum resolving power of the data set; this is information that forward modeling approaches do not provide. Comparisons of slant stacks of reversed shot gathers with intercept times predicted for a two-dimensional model derived by ray tracing highlight some of the pitfalls associated with interpretation of τ( p ) data in terms of one-dimensional structure. In some cases, the principal arrival may become multi-valued, resulting in misidentification of phases and compounding biases in estimates of average crustal parameters. Slant stacks can be highly useful, however, in refining two-dimensional models derived from travel times because of the sensitivity of ray parameters to dip and lateral velocity gradients. We use slant stacks in conjunction with the original shot gathers to refine our estimate of the configuration of the Moho beneath southwest Maine. Our final model shows a step in the Moho that is consistent with variations in normal two-way travel times inferred from analysis of postcritical fan shots. This abrupt change in crustal thickness may mark the northeast extent of significant underthrusting of Avalon crust beneath Medial New England basement during the Acadian Orogeny. The preservation of offsets in Moho that were formed during Paleozoic compression is consistent with the limited amount of rifting experienced by this portion of the orogen during the opening of the Atlantic.