Petrographic similarities between minor intrusive rocks and their volcanic host rocks in the Mesozoic–early Tertiary eastward-dipping monoclinal sequence of the Coast Range of central Chile are evidence that the intrusions were the feeders of the volcanic host units (or immediately overlying units). Each successively younger volcanic host unit is located east of the preceding one; consanguineous dikes intrude only younger host units or immediately underlying rocks, never older units to the west. This demonstrates a relative eastward shift of the volcanic centers through time. A similar shift is known to occur for plutonic intrusions and nonmagmatic features. An explanation consistent with these facts is an eastward-dipping subduction zone existing during Mesozoic and early Tertiary times.

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