If Gabriel García Márquez had been born in the United States, everyone would be referring to him as "García" NOT "Márquez." His first last name is García, his mother's maiden name was Márquez. This is the common practice of Latin America and Spain--the mother's maiden name is retained as the second last name. Hence Vargas Llosa, García Lorca, Pardo Bazán, Cabrera Infante, Eustasio Rivera, Cervantes Saavedra, etc.
As the redoubtable Teaching Assistant has noted,
Another extremely perverse thing is that when I'm in a bookstore, I can never find authors like Gabriel García Márquez or Federico García Lorca or Emilia Pardo Bazán where they are should be located according to proper alphabetization by last name. I've always got to resort to looking under Márquez, Lorca, and Bazán, even in the foreign language section of my university's official bookstore. I never know where I'll end up finding Lope de Vega or Sor Juana Inés de la Cruz, either... This doesn't change whether I look for the works of these authors in Spanish or in translation.Let the bookstores learn from the litbloggers, as we continue to develop the excellence of this necessary medium.