Pretty much so. The subducting slabs do go pretty deep. Average crust thickness Is A mere three miles in the ocean to 20 to 30 miles beneath the continental crusts.
that seems strange if the crust is only abt 3 mi thick in the oceans how do we get away with drilling at least 5 miles below the sea floor in the near middle of the Gulf of Mexico and still have low enough temperatures for liquid hydrocarbons.