Paleosubduction

Paleosubduction refers to subduction processes that occurred in Earth's past and are inferred from the rocks and structures preserved in ancient orogens. In the geologic record these zones are not observed directly as active plate boundaries, but as slices of oceanic lithosphere and overlying sediments that were subducted, metamorphosed, and later exhumed within continental belts.

Evidence includes high-pressure, low-temperature metamorphism (blueschist and eclogite facies) that forms under subduction conditions, ophiolites representing former oceanic crust and mantle, and accretionary complexes and mélanges created at subduction interfaces. Arc-related volcanic rocks with distinctive geochemical signatures, and metasomatic alteration by slab-derived fluids, also support subduction histories. Geochronology and isotopic data (for example U-Pb ages on zircons; Sm-Nd and Lu-Hf systematics) are used to constrain the timing of subduction, metamorphism, and exhumation. Geochemical tracing of slab components helps distinguish subduction signatures from other tectonic histories.

Interpretation relies on plate tectonic reconstructions that combine paleomagnetic data, stratigraphy, and structural geology to locate ancient subduction zones within past supercontinents and orogens. Thermobarometry on high-pressure rocks helps estimate depths and temperatures; mantle-derived xenoliths and serpentinites provide additional clues. Because direct observation is unavailable, paleosubduction models are tested against multiple lines of evidence and refined as new data emerge.

Studying paleosubduction advances understanding of how continental crust forms and evolves, how oceans close, and how subducted material is recycled into the mantle. Notable examples include ancient subduction-related rocks and ophiolites such as those in the Semail nappe in Oman and HP-LT units within the Franciscan Complex of California, which record Paleozoic subduction processes, as well as older belts in Caledonian and Variscan orogens.