Sandstone and chert boudins that have syntectonic extensional veins are dominant structural elements in melanges of the Kodiak accretionary complex, Alaska. Detailed analyses of cogenetic methane-rich and water-rich fluid inclusions in syntectonic quartz veins indicate that fluid pressures dropped by 20% to 45% during crystal growth. This drop occurred faster than uplift of the rock and probably represents the local effect of fluid escaping to shallow levels in the subduction complex during melange deformation, possibly along an interconnected fracture network. An important implication of channelized fracture flow to higher levels in a subduction complex is that fluids may transport heat and solutes, thereby generating thermal and chemical anomalies along shallow faults.

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