Complex sentence: Most of the Ceraunius Fossae are located in the northern Tharsis quadrangle, although a portion extends northward into the southwestern part of the Arcadia quadrangle where the fossae diverge around the flanks of Alba Mons to form the Alba and Tantalus Fossae systems.
Split into simpler sentences: Most of the Ceraunius Fossae are located in the northern Tharsis quadrangle. A portion extends northward into the southwestern part of the Arcadia quadrangle where the fossae diverge around the flanks of Alba Mons to form the Alba and Tantalus Fossae systems.