Sept 16 (Reuters) - The S&P 500 and the Dow Jones indexes opened flat on Thursday as gains in economically sensitive sectors on the back of stronger-than-expected retail sales data were offset by losses in heavyweight technology stocks. The Dow Jones Industrial Average fell 4.12 points, or 0.01%, at the open to 34,810.27. The S&P 500 opened lower by 3.61 points, or 0.08%, at 4,477.09. The Nasdaq Composite dropped 41.44 points, or 0.27%, to 15,120.09 at the opening bell. (Reporting by Ambar Warrick; Editing by Maju Samuel)