  We present a high-resolution study of the cold molecular gas as traced by
CO(1-0) in the unlensed z$\sim$3.4 submillimeter galaxy SMM J13120+4242, using
multi-configuration observations with the Karl G. Jansky Very Large Array
(JVLA). The gas reservoir, imaged on 0.39" ($\sim$3 kpc) scales, is resolved
into two components separated by $\sim$11 kpc with a total extent of 16 $\sim$3
kpc. Despite the large spatial extent of the reservoir, the observations show a
CO(1-0) FWHM linewidth of only 267 $\pm$ 64 km s$^{-1}$. We derive a revised
line luminosity of L'$_\mathrm{CO(1-0)}$ = (10 $\pm$ 3) $\times$ 10$^{10}$ K km
s$^{-1}$ pc$^2$ and a molecular gas mass of M$_\mathrm{gas}$ = (13 $\pm$ 3)
$\times$ 10$^{10}$ ($\alpha_\mathrm{CO}$/1) M$_{\odot}$. Despite the presence
of a velocity gradient (consistent with previous resolved CO(6-5) imaging), the
CO(1-0) imaging shows evidence for significant turbulent motions which are
preventing the gas from fully settling into a disk. The system likely
represents a merger in an advanced stage. Although the dynamical mass is highly
uncertain, we use it to place an upper limit on the CO-to-H$_2$ mass conversion
factor $\alpha_\mathrm{CO}$ of 1.4. We revisit the SED fitting, finding that
this galaxy lies on the very massive end of the main sequence at z = 3.4. Based
on the low gas fraction, short gas depletion time and evidence for a central
AGN, we propose that SMM J13120 is in a rapid transitional phase between a
merger-driven starburst and an unobscured quasar. The case of SMM J13120
highlights the how mergers may drive important physical changes in galaxies
without pushing them off the main sequence.
