SLANT STACKS OF REFRACTION DATA FROM MAINE - EFFECTS OF LATERAL VARIATIONS IN VELOCITY STRUCTURE

We discuss some of the advantages and pitfalls associated with tau(p) analysis of wide-angle profiles recorded over terranes characterized b y significant lateral heterogeneity. We construct composite slant stac ks for a pair of reversed shots recorded along the axis of the Merrima ck Synclinorium in southwest Maine and invert picks for bounds on aver aged one-dimensional slowness-depth structure. Extremal bounds on tota l crustal thickness derived from shot point 12 are 40 to 43 km. Averag e crustal velocities for individual slowness models corresponding to t he extremal depths range from 6.5 to 6.6 km/sec. Predicted normal-inci dence two-way travel times for the Moho lie between 12.2 and 13.2 sec. The utility of one-dimensional inversion of tau(p) data is largely in estimating the maximum resolving power of the data set; this is infor mation 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 t he pitfalls associated with interpretation of tau(p) data in terms of one-dimensional structure. In some cases, the principal arrival may be come multi-valued, resulting in misidentification of phases and compou nding biases in estimates of average crustal parameters. Slant stacks can be highly useful, however, in refining two-dimensional models deri ved 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 configur ation of the Moho beneath southwest Maine. Our final model shows a ste p in the Moho that is consistent with variations in normal two-way tra vel times inferred from analysis of postcritical fan shots. This abrup t change in crustal thickness may mark the northeast extent of signifi cant underthrusting of Avalon crust beneath Medial New England basemen t during the Acadian Orogeny. The preservation of offsets in Moho that were formed during Paleozoic compression is consistent with the limit ed amount of rifting experienced by this portion of the orogen during the opening of the Atlantic.