Continental convergent margins are characterized by mélanges of which rock types?

• A. Feldspathic sandstone, siltstone, and shale
• B. Quartz sandstone
• C. Evaporites
• D. Dolostone

Answer: (A)

Continental convergent margins form subduction-zone accretionary prisms, where rocks from the descending plate and adjacent crust are ground up and intermingled in a chaotic blend. In these mélanges, fragments of crustal sedimentary rocks get incorporated into a fine-grained matrix, creating a characteristic mix of varied rock pieces within a clay-rich groundmass. Feldspathic sandstone, siltstone, and shale are common components because they are typical crustal sediments that are readily eroded, transported, and recycled during subduction and accretion. This combination reflects the crustal, sedimentary origin of much of the material being torn from the continental margin and incorporated into the mélange. By contrast, rocks like quartz sandstone (which is more quartz-rich and less feldspar-bearing) and carbonate-dominated rocks such as evaporites or dolostone are less representative of the clastic, crustal fragments that define continental-margin mélanges.