SEDIMENTOLOGICAL EXPRESSION OF TECTONOMAGMATIC PROCESSES RELATED TO EXTENSION AND CONTRACTION IN RESPONSE TO PLATE CAPTURE AND CONSEQUENT MANTLE MOVEMENT

Extensional deformation, recorded by structural basins, commonly characterized by buttress unconformities and associated magmatism, distinguishes widespread domains within oceanic and continental settings. Extension commonly follows plate collision, coupling and plate capture that are inferred to cause changes in plate organization and motion, which are manifest as plate drag or rotation and recorded by stretching and transtension, focused upon coupled plate margins. Accumulations of clastic strata, concurrent with volcanism, generally include dark-gray to black, pyritic, laminated shale, uncommonly rich in total organic carbon content, that accumulate in restricted oceanic basins and seaways on continental shelves. Syn-extension volcanism, which locally is manifest by very widespread ash layers, may emit large quantities of gases (e.g. CO_2, SO₂, H₂S) leading to global warming, and/or cooling. During movement of captured megaplates, downward protruding continental roots and /or coupled oceanic slabs disrupt and displace mantle that flows away from the moving plate assemblage. Where the flowing mantle encounters a continental plate margin “subduction” is induced and entrained crust may be emplaced to a depth of 100 or more km as recorded by formation of eclogite, localized commonly along globally distributed plate sutures. Under coherent continental crust, passage of mantle is inferred to coincide with regional uplift accompanied by stream incision and erosional exhumation, recorded by a regional unconformity above which development of widespread, sheet-like, clastic, units, may take place as developed especially during the Paleozoic. After passage of the mantle wave that interrupts extension, stretching resumes, commonly concurrent with large volcanic eruptions, which move through ruptures in thickened crust. Some isotopic excursions and variations in elemental abundances are contemporaneous with the resumption of extension and volcanism. Where acidic seawater, which at times may have contributed to extinction of susceptible organisms and the dissolution of carbonate, is pushed against and under a plate, mineralization and formation of ore deposits are inferred to develop especially along major, deep, faults. Examples of the orogenic processes and related sedimentologic features are recorded in eastern North America during the Taconic and Acadian orogenies.