ZURICH, Dec 20 (Reuters) - Eric Varvel, chairman of Credit Suisse"s (CSGN.S) investment bank and head of its U.S. holding company, is leaving the Swiss group where he has worked for three decades, a staff memo seen by Reuters and confirmed by the bank said. James Walker has been appointed CEO ad interim of Credit Suisse Holdings (USA) Inc, the memo from Chief Executive Thomas Gottstein said. The departure extends the upheaval at Credit Suisse, which under new Chair Antonio Horta-Osario last month reined in its investment bankers and focused more on private banking as it tries to curb a freewheeling culture that has cost it billions in a string of scandals. read more Varvel was head of the asset management division which sold wealthy clients investments linked to UK supply chain financier Greensill. Credit Suisse had to suspend $10 billion worth of the investments as Greensill headed for collapse in March, heaping pressure on the bank and its top bosses. Varvel was removed as head of asset management in a leadership overhaul shortly after Greensill went bust. read more The memo gave no reason for Varvel"s exit. The Financial Times had reported last week that he was in talks to leave, owing partly to disagreement with Horta-Osario. read more "I speak for us all when I say we appreciated Eric"s optimism, his leadership and his winning attitude. He has developed remarkably durable and deep relationships with colleagues and clients alike," Gottstein wrote in the memo. "Based on a great many conversations with Eric, I know the difference he made for this institution, its people and our clients over the years."