Introduction to Orogenic Bridge Theory

Orogenic Bridge Theory proposes that orogens striking highly oblique to orthogonal to active rifts hinder rifting and breakup. The highly oblique character and low angle geometry of the thrust systems and shear zones in these orogens make them unable to efficiently accommodate crustal thinning and transform faulting which are necessary for breakup. Thus, upon intersecting such orogens, rifts step, and/or locally reorient, and/or bypass the oblique orogen. While breakup and seafloor spreading occur in adjacent areas, the orogenically thickened crust at oblique orogens continues to stretch and thin until breakup occurs there also or until rifting stops. Unlike historical theoretical “land bridges”, orogenic bridges are dynamic features and they deform together with adjacent oceanic and anorogenic continental crust.Orogenic bridges where full breakup has not yet occurred are continuous domains of orogenically thickened continental crust, which were (hyper) extended during rifting. They may be separated from adjacent oceanic crustal domains by major transform faults, which form along inherited rift-orthogonal orogenic thrusts. Examples of continuous orogenic bridges are the late Paleoproterozoic Laxfordian–Ammassalik–Nagssugtoqidian–Torngat Orogen, which gave rise to the Greenland–Iceland–Faroe Ridge and Davis Strait, and possibly to the late Neoproterozoic Timanian Orogen in the Fram Strait.Should sufficient extension occur, orogenic bridges eventually rupture. Ruptured orogenic bridges generally form hyperextended salients of continental crust offshore and coincide with major steps and/or reorientation of the main rift axis. Examples of ruptured orogenic bridges include the Permian Cape Fold Belt in South Africa and the Falkland Plateau and Maurice Ewing Bank, the late Neoproterozoic East African–Antarctica Orogen in southeastern Africa and Antarctica, and the latest Neoproterozoic–early Paleozoic Delamerian–Ross Orogen in eastern Australia and Antarctica.Orogenic bridges have significant implications for several branches of marine Earth science, including but not limited to the biogeodynamics, plate tectonics, structural geology, and natural resource distribution and geohazards. For example, orogenic bridges provide prolonged topographical links between continents during supercontinent breakup, thus allowing continued exchanges of terrestrial fauna and flora between rifted continents, e.g., prolonged faunal exchanges between Greenland and Europe and western Africa and Brazil. Conversely, they form topographical barriers, which prevent biological exchanges of marine fauna and flora between oceanic domains across orogenic bridges, e.g., discrete early Paleozoic trilobite assemblages in Svalbard and Scandinavia.Orogenic bridges explain the occurrence of anomalously thick crust offshore as remnants of oblique (hyper) extended orogenic crust and localize the formation of major transform faults. In addition, Ridge–Ridge-Ridge triple junctions localize at the intersection of two orogenic bridges. Thus, orogenic bridges have a considerable impact on plate tectonics and paleogeographic reconstructions.Orogenic bridges extend the continent–ocean boundary farther offshore at various margins worldwide. Thus, they have significant implications for offshore mineral deposits, hydrocarbon exploration, and the Law of the Sea. Furthermore, the mapping of orogenic structures connected with orogenic bridges will further aid geohazard risk assessment, and exploration for white and orange hydrogen and geothermal resources along fault zones.