Continuous restoration of deformed shapes through the construction of a reverse displacement gradient and its application to granite emplacement in central North China

This paper presents a new technique to restore 2D/3D deformed shapes by minimization of the misfit between measured and theoretical reverse displacement gradients. Owing to its relatively direct and objective nature, the procedure ensures continuity of displacement across the boundaries between abutting elements. The procedure also minimizes the additional distortion of the elements that results from the compatibility of retro-deformation between abutting elements. We apply the technique to the Mesozoic Fangshan granodioritic pluton (FGP) in western Beijing, central North China. Based upon the measured strain of deformed enclaves, about 78% area of the present central zone can be restored by retro-deformation of the marginal and transitional zones. As a result of the emplacement of the central zone, deformation also occurred in both the outer zones and the wall rocks, predominantly by ductile flow. The associated deformation in the outer zones, with or without the involvement of the wall rocks, created space for the emplacement of the central zone. In addition, vertical expansion was effective in creating space for subsequent emplacement, producing approximately one third of the volume created by later expansion. Thus, ballooning is a viable mechanism for emplacement of the FGP.