Which phrase best describes the melange found at convergent plate margins?

Which phrase best describes the melange found at convergent plate margins?

• A. Basalt lava flows
• B. Limestone sequences
• C. Pelitic shale
• D. Melanges of feldspathic sandstone, siltstone and shale

Answer: (D)

Convergent plate margins produce mélanges, which are chaotic mixes formed when rocks from different origins are pressed and sheared together in subduction-zone environments. In an accretionary prism, fragments of sandstone, siltstone, shale and other rocks get crushed and torn, ending up embedded in a mixed, clay-rich or fines-rich matrix. This creates a heterogeneous, jumbled unit rather than a single clean rock type. The phrase describing a mélange of feldspathic sandstone, siltstone and shale fits this idea precisely, as it emphasizes the assemblage of varied clastic rocks within a deforming, mixed matrix—typical of tectonically generated mélanges at subduction zones. In contrast, basalt lava flows reflect volcanic igneous activity, limestone sequences are carbonate sedimentary deposits, and pelitic shale denotes a more uniform, fine-grained sedimentary/low-grade metamorphic rock. These options do not capture the characteristic heterogeneity of mélanges formed at convergent margins.

Convergent plate margins produce mélanges, which are chaotic mixes formed when rocks from different origins are pressed and sheared together in subduction-zone environments. In an accretionary prism, fragments of sandstone, siltstone, shale and other rocks get crushed and torn, ending up embedded in a mixed, clay-rich or fines-rich matrix. This creates a heterogeneous, jumbled unit rather than a single clean rock type.

The phrase describing a mélange of feldspathic sandstone, siltstone and shale fits this idea precisely, as it emphasizes the assemblage of varied clastic rocks within a deforming, mixed matrix—typical of tectonically generated mélanges at subduction zones.

In contrast, basalt lava flows reflect volcanic igneous activity, limestone sequences are carbonate sedimentary deposits, and pelitic shale denotes a more uniform, fine-grained sedimentary/low-grade metamorphic rock. These options do not capture the characteristic heterogeneity of mélanges formed at convergent margins.