  Using a 3D MHD simulation, we model the quasi-static evolution and the onset
of eruption of a coronal flux rope. The simulation begins with a twisted flux
rope emerging at the lower boundary and pushing into a pre-existing coronal
potential arcade field. At a chosen time the emergence is stopped with the
lower boundary taken to be rigid. Then the coronal flux rope settles into a
quasi-static rise phase during which an underlying, central sigmoid-shaped
current layer forms along the so called hyperbolic flux tube (HFT), a
generalization of the X-line configuration. Reconnections in the dissipating
current layer effectively add twisted flux to the flux rope and thus allow it
to rise quasi-statically, even though the magnetic energy is decreasing as the
system relaxes. We examine the thermal features produced by the current layer
formation and the associated "tether-cutting" reconnections as a result of
heating and field aligned thermal conduction. It is found that a central hot,
low-density channel containing reconnected, twisted flux threading under the
flux rope axis forms on top of the central current layer. When viewed in the
line of sight roughly aligned with the hot channel (which is roughly along the
neutral line), the central current layer appears as a high-density vertical
column with upward extensions as a "U" shaped dense shell enclosing a central
hot, low-density void. Such thermal features have been observed within coronal
prominence cavities. Our MHD simulation suggests that they are the signatures
of the development of the HFT topology and the associated tether-cutting
reconnections, and that the central void grows and rises with the
reconnections, until the flux rope reaches the critical height for the onset of
the torus instability and dynamic eruption ensues.
