(Reuters) -Shares of global automakers took a dive on Thursday after President Donald Trump put a wall of tariffs around the U.S. automotive sector, exacerbating worries over the impact on global trade and potential hits to industry profits. Trump on Wednesday followed through on weeks of threats for new tariffs on imported cars, saying a 25% import tax on vehicles not built in the U.S. would begin on April 3. Although the duties have been well flagged, shares in automakers across the globe tumbled on Thursday, with U.S. auto giant General Motors sliding 7% in premarket trading and Ford Motor shedding about 4%.