Mergers Among Securities Markets

With both Nasdaq and the NYSE now publicly traded, they both have stock they can use in acquisitions of other markets.  Both markets are willing to spend to acquire and are sniffing around the purchase of the London Stock Exchange (LSE).  These acquisitions will sorely test the SEC's view of monopolization in the securities markets.  What is the relevant market to assess market share and thereby to assess competitive effect of such mergers?  I am not confident that the SEC is up to the task and hope that Congress will start hearings on the matter before the acquisitions get to far along.