Their relationship is, for the most part, based upon Ennis being the top and Ellery being the bottom, with mild dominance and submission elements to it. I say "mild" because a true Dom/sub relationship doesn't general permit for switching, and Ennis has gradually become more and more interested in bottoming and oral sex. However, I don't personally see this as a departure from Annie Proulx's story since Ennis topped rather aggressively in her story on their first sexual encounter, and I merely extrapolated from that, and from Ennis's other aggressive tendencies.
However, they do not carry this mild dom/sub aspet into other parts of their relationship.