( ( Tectonically speaking ) ( , ( ( the area ) ( ( was ( near ( ( a ( continental ( plate ( subduction zone ) ) ) ) ( as ( part ( of ( a ( back-arc basin ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (S (ADVP (RB Tectonically)) (VP (VBG speaking))) (, ,) (NP (DT the) (NN area)) (VP (VBD was) (PP (IN near) (NP (NP (DT a) (JJ continental) (NN plate) (NN subduction) (NN zone)) (PP (IN as) (NP (NP (NN part)) (PP (IN of) (NP (DT a) (JJ back-arc) (NN basin)))))))) (. .)))
Tectonically speaking, the area was near a continental plate subduction zone as part of a back-arc basin.
(ROOT (S (NP (DT A) (JJ volcanic) (NN arc)) (VP (VBZ is) (VP (VBN formed) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (VBG subducting) (NN plate)) (VP (VBZ flows) (PP (IN under) (NP (DT another) (JJ tectonic) (NN plate)))))))) (. .)))