  We study the violent phase of the merger of massive binary white dwarf
systems. Our aim is to characterize the conditions for explosive burning to
occur, and identify a possible explosion mechanism of Type Ia supernovae. The
primary components of our model systems are carbon-oxygen (C/O) white dwarfs,
while the secondaries are made either of C/O or of pure helium. We account for
tidal effects in the initial conditions in a self-consistent way, and consider
initially well-separated systems with slow inspiral rates. We study the merger
evolution using an adaptive mesh refinement, reactive, Eulerian code in three
dimensions, assuming symmetry across the orbital plane. We use a co-rotating
reference frame to minimize the effects of numerical diffusion, and solve for
self-gravity using a multi-grid approach. We find a novel detonation mechanism
in C/O mergers with massive primaries. Here the detonation occurs in the
primary's core and relies on the combined action of tidal heating, accretion
heating, and self-heating due to nuclear burning. The exploding structure is
compositionally stratified, with a reverse shock formed at the surface of the
dense ejecta. The existence of such a shock has not been reported elsewhere.
The explosion energy ($1.6\times 10^{51}$ erg) and $^{56}$Ni mass (0.86
M$_\odot$) are consistent with a SN Ia at the bright end of the luminosity
distribution, with an approximated decline rate of $\Delta m_{15}(B)\approx
0.99$. Our study does not support double-detonation scenarios in the case of a
system with a 0.6 M$_\odot$ helium secondary and a 0.9 M$_\odot$ primary.
Although the accreted helium detonates, it fails to ignite carbon at the base
of the boundary layer or in the primary's core.
