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(Abridged) We present the first high-resolution (5.6''x5.1'') images of theemission of silicon monoxide (SiO) in the nearby spiral IC342, obtained withthe IRAM Plateau de Bure Interferometer. We have mapped the emission of theSiO(2-1) and H13CO+(1-0) lines in a 0.9kpcx1.3kpc region around the nucleus ofIC342. The bulk of the line emission comes from a 290pc spiral arm and a r~80pcnuclear ring. The distribution of SiO clouds is mostly anticorrelated with thecontinuum emission at 87GHz, dominated by thermal free-free bremsstrahlung. TheSiO/H13CO+ intensity ratio increases by an order of magnitude from the nuclearring to the spiral arm. Furthermore the gas kinematics show significantdifferences between SiO and H13CO+ over the spiral arm, where the linewidths ofSiO are 2 times larger than that of H13CO+. The average abundance of SiO in theinner r~320pc, X(SiO)>2x10^{-10}, evidences that shock chemistry is at work inIC342. We have compared the emission of SiO with another tracer of molecularshocks: the emission of methanol (CH3OH). The significant difference of theX(SiO) measured between the spiral arm (~a few 10^{-9}) and the nuclear ring(~10^{-10}) is not echoed by a comparable variation in the SiO/CH3OH intensityratio. This implies that the typical shock velocities should be similar in bothregions. In contrast, the fraction of shocked molecular gas should be 5-7 timeslarger in the spiral arm (up to ~10% of the available molecular gas mass overthe arm region) compared to the nuclear ring. We conclude that the large-scaleshocks revealed by the SiO map of IC342 are mostly unrelated with starformation and arise instead in a pre-starburst phase. Shocks are driven bycloud-cloud collisions along the potential well of the IC342 bar. The generalimplications for the current understanding of galaxy evolution are discussed.Comment: 15 pages, including 9 figures (6 color) and 3 tables; accepted in  Astronomy & Astrophysic

Large-scale molecular shocks in galaxies: the SiO interferometer map of IC 342