Petrology and Provenance of Sands and Gravels from the Middle America Trench and Trench Slope, Southwestern Mexico and Guatemala

Abstract

The composition of sand from the Middle America Trench and slope varies significantly along strike and reflects local source areas with contrasting rock types. Terrigenous sands offshore Guerrero and Oaxaca, Mexico, are rich in quartz, feldspar, and metamorphic rock fragments and closely reflect the schist and gneiss that dominate the source area. The average composition of the fine sands is Q3iF50L19 and that of the coarse sands is Q4iF22L37. Plagioclase and potassium feldspar occur in roughly equal amounts; biotite and hornblende are the most common mafic minerals. Offshore Guatemala, terrigenous sands are volcaniclastic and rich in pyroclastic material, reflecting a source area dominated by Tertiary and Quaternary volcanic rocks and unconsolidated pyroclastic deposits. The average composition of the volcaniclastic sands is Q]F43L56. Almost all feldspar is plagioclase, almost all rock fragments are volcanic, and the ratio of glass shards to total rock fragments is 0.67. Pyroxene is the dominant mafic material. Gravel fragments from the base of scarps on the lower slope offshore Guatemala include prehnite-pumpellyite facies metabasite and laumontite-bearing argillite. The timing of the metamorphism is uncertain, but the common occurrence of prehnite-pumpellyite facies rocks in subduction complexes and their rarity on the seafloor suggest that the metamorphism of at least the metabasite is related to subduction and accretion along the Middle America Trench. These apparently recycled gravels form small olistostromes on the lower slope of this modern subduction system.