We present results from a newly developed fully coupled thermomechanic
al model of the continental crust in which crustal shortening at a con
vergent plate boundary is driven by a basal velocity discontinuity whi
ch represents delamination and subduction in the underlying mantle. Th
is new dynamical model incorporates complex rheologies (elasticity, th
ermally activated creep and brittle frictional behaviour), allows for
extremely large deformation, and is coupled along its top boundary to
a complex erosion/ sedimentation model. The model is based on the 'dyn
amic Lagrangian remeshing' (DLR) method, which uses Lagrangian spatial
discretization of the crust and therefore; allows for an accurate tra
cking of rock particles as they travel through the deforming orogen; t
his information is, in turn, used to produce synthetic PTt paths. We h
ave used this model to predict the distribution of apparent ages for a
wide range of isotopic systems at the surface of an actively deformin
g orogen. We have also predicted other geophysical and geological obse
rvables, such as the metamorphic grade of exposed rocks (regarded here
as a first-order approximation for total denudation), topography, sur
face heat flux, and the thickness of sediment deposited in the adjacen
t foreland basins. Results clearly demonstrate that the highest exhuma
tion rates (and thus the youngest isotopic ages) are found in regions
of maximum topography near the centre of the orogen, but that the most
deeply exhumed rocks are found on the side of the orogen, in the vici
nity of the retro-shear zone, a crustal-scale 'fault' which accommodat
es most of the crustal shortening within the orogen. It is also in thi
s region that the isotopic systems characterized by the greatest closu
re temperatures display the youngest ages and the highest grade metamo
rphic rocks are found. These conclusions are derived from the assumpti
on of uniform erosion across the orogen; in cases where rainfall (and
thus erosion) is orographically controlled, the tectonic style of the
orogen is different from the uniform erosion case, as is the distribut
ion of isotopic ages and metamorphic grades across the orogen.