Orogeny and metamorphism: A model for deformation and pressure‐temperature‐time paths with applications to the central and southern Appalachians

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Abstract
A model is presented for deformation, uplift, and exhumation in convergent orogens based on the following premises: (1) foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen; (2) the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton; on the seaward side of this transition, up to 20 km of overthrusts can be accumulated without creating topography; (3) critical shape theory can be applied to orogens as a whole, allowing predictions to be made concerning dynamic balance between mass addition and loss, uplift, exhumation, and growth of topography; and (4) pressure‐temperature‐time (P‐T‐t) data can be used to reconstruct particle paths within the deforming orogen.Three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition, as monitored by the foreland. Rapid exhumation of metamorphic terranes is most likely in the ‘steady state’ where erosion balances accretion; a ‘constructive state,’ where mass addition is greater than mass loss, results in crustal thickening and initially little exhumation; a ‘destructive state,’ where mass loss outweighs mass addition, results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension. P‐T‐t data on peak metamorphism, pluton emplacement, uplift, exhumation, and cooling can be used in conjunction with data from the foreland to distinguish among these various states. The topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen. Application of this model to the central and southern Appalachians suggests that during the constructive phases of the Taconian and Acadian orogenies, overthrust loads accumulated mainly on the seaward side of the ocean‐continent transition as marked by the Bouguer gravity gradient. Between the Acadian and Alleghanian orogenies, the Appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton. The constructive stage of the Alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton; this resulted in widespread uplift and westward transport of older metamorphic rocks. Post‐orogenic unroofing of the southern Appalachians is compatible with the predictions of flexural models; the post‐orogenic unroofing history of the central Appalachians and Ouachitas probably reflects Mesozoic extension in these areas.