Fluids expelled during deformation at convergent margins lead to a variety of diagenetic reactions, including carbonate cementation, which alter the rheology of the deforming sediment. The volume of carbonate cement in shallow sediments at modern convergent margins is small, but sandstones in an ancient accretionary complex in Alaska contain significant amounts of carbonate cement that formed before underplating occurred. Other than vein calcite, carbonates are early calcite replacing framework grains or filling pore spaces, followed by ankerite that occurs mainly as pore-filling cement. Amounts range from 2--16 weight percent in typical samples, averaging about 9 weight percent. Mean porosity filled by carbonate in Kodiak sandstones corresponds to porosities expected at depths of 4--5 km in normal compacting sands underthrust at modern convergent margins. The temperatures of formation are difficult to determine precisely, but were probably between 100 ¿C and 200 ¿C. The bulk of cementation occurred before the formation of thrust faults, folds and slaty clevage related to underplating. Lithification by cementation changed the rheology of sandstones by (1) filling pore space and increasing peak strength, and (2) increasing cohesion so that particulate flow was not longer a viable deformation mechanism. Lithification before significant deformation contributed to underplating of the Kodiak Formation as relatively coherent duplexes. ¿American Geophysical Union 1990