A renewed chase for yield has sparked a "sudden and unexpected" surge in the market valuations of the big four banks this year, pushing share prices to record highs and beyond analysts' targets, and raising questions over how much further they can run. The moves have been particularly pronounced in the case of Commonwealth Bank of Australia stock, which at $93.27 a share is $10 ahead of where the broker analyst community believes the stock will trade in a year's time, on Bloomberg data.