The debate concerning eastward versus westward subduction along the western margin of Mesozoic North America involves three main arguments: The tomography argument (westward) claims subducted slabs are observed by tomographic processing of seismic waves; the geologic evidence argument (eastward) claims subduction polarity is recorded by tripartite accretionary complex–forearc basin–magmatic arc assemblages; and the crucial geologic test argument (eastward) claims endemic terranes are characterized by detrital zircons of appropriate age and abundance, particularly Precambrian zircons. Reconstruction of the arguments in standard logic form as categorical syllogisms indicates that all three arguments are valid. However, evaluation of the truth-falsehood of the propositions supporting the arguments suggests: (1) propositions for the tomography argument seem more truthful than false; (2) propositions for the geologic test argument seem more false than truthful; and (3) propositions for the crucial geologic test argument also seem more false than truthful. Thus, the tomography argument appears to be a “good” argument (i.e., valid and sound); the geologic evidence argument appears to be a “bad” argument (i.e., valid and unsound); and the crucial geologic test argument appears to be an “ugly” argument—in the spirit that some mathematical proofs are ugly (i.e., valid and unsound, in addition to being convoluted and couched in vague terms such as “appropriate”). Proposed tests for falsification of the arguments include application of an alternate tomographic processing method to obtain non-wall-like structures in the mantle, demonstration that tripartite successions are not consanguineous, and procurement of additional evidence showing that endemic terranes do not contain Precambrian detrital zircons of appropriate age and abundance.

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