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Mahony, 74, is expected to retire when he turns 75, as the church requires.
"I welcome Archbishop Gomez to the Archdiocese of Los Angeles with enthusiasm and personal excitement," Mahony said. "The Auxiliary Bishops and I are looking forward to working closely with him over the coming months until he becomes the Archbishop early in 2011."
Gomez, 58, was named coauditor-archbishop of Los Angeles Archdiocese, the nation's most populous. The pope made the decision Monday, and the Los Angeles Archdiocese announced it early Tuesday
In 2005, TIME named Gomez one of the 25 most influential Latinos in the United States.
Mahony was expected to introduce Gomez today at a 10 a.m. news conference at the Cathedral of Our Lady of the Angels.
Gomez is a Monterrey, Mexico-born priest of the conservative Prelature of Opus Dei and has headed the church in San Antonio since 2004.
At a future concistory, the pope will likely name Gomez a cardinal, given that Los Angeles is such a large and important archdiocese whose leader has traditionally worn a red hat.
Gomez would become the first archbishop of Mexican descent to head a North American archdiocese. A Mass and reception for Gomez is set for May 26, according to the archdiocese, which claims about 5 million Roman Catholics, about 70 percent of whom are Hispanic. The archdiocese covers Los Angeles, Ventura and Santa Barbara counties.
"This just recognizes the reality on the ground that the center of gravity of U.S. Catholicism is moving to the South and West and is becoming increasingly Hispanic," David Gibson, a Catholic author who covers religion for PoliticsDaily.com, told the Associated Press.
Gomez said he was grateful for the appointment and the trust that the Vatican had in him.
"I'm very grateful to the Holy Father for giving me this opportunity to serve the Church with a mentor and leader like Cardinal Roger Mahony," Gomez stated. "I'm grateful to the Apostolic Nuncio, Archbishop Pietro Sambi, for supporting the Holy Father's confidence in me. I will try with all my strength to earn that trust."
Appointment Comes Amid Abuse Scandal Fallout
The appointment means Gomez will be the man to deal with fallout from the abuse scandal that came to light during Mahony's tenure. In 2007, Mahony agreed to a record-setting $660 million settlement with more than 500 alleged victims of clergy abuse.
A federal grand jury is also investigating how the Archdiocese of Los Angeles handled claims of abuse, and has subpoenaed several witnesses, including a former Los Angeles priest convicted of child molestation and a monsignor who served as vicar for clergy under Mahony.
Mahony's attorney has said the cardinal, the longest-serving U.S. cardinal since the Second Vatican Council, is not a target of the investigation.