Landau will step down with immediate effect but will remain on the company's board as a non-executive director.
The move came as the company revealed it had appointed Rory Scott Russell as its new chief executive.
Russell had previously spent 10 years with Anglo-Dutch supermajor Shell, most recently serving as the company's finance manager for exploration in Europe and Russia.
While at Shell, he oversaw all financial aspects of the business, corporate governance and control, treasury management and sat on regional investment committees covering the North Sea, onshore Netherlands and Germany, Italy, Russia and Kazakhstan.
He
