Abstract

The northern Colorado River extensional corridor is a 70- to 100-km-wide region of moderately to highly extended crust along the eastern margin of the Basin and Range province in southern Nevada and northwestern Arizona. It has occupied a critical structural position in the western Cordillera since Mesozoic time. In the Cretaceous through early Tertiary, it stood just east and north of major fold and thrust belts and also marked the northern end of a broad, gently (~15°) north-plunging uplift (Kingman arch) that extended southeastward through much of central Arizona. Mesozoic and Paleozoic strata were stripped from the arch by northeast-flowing streams. Peraluminous 65 to 73 Ma granites were emplaced at depths of at least 10 km and exposed in the core of the arch by earliest Miocene time.

Calc-alkaline magmatism swept northward through the northern Colorado River extensional corridor during early to middle Miocene time, beginning at ~22 Ma in the south and ~12 Ma in the north. Major east-west extension followed the initiation of magmatism by 1 to 4 m.y., progressing northward at a rate of ~3 cm/yr. The style of volcanism changed during the course of east-west extension. Eruptions of calc-alkaline to mildly alkaline mafic to intermediate magmas predated extension. Calc-alkaline to mildly alkaline mafic, intermediate, and felsic magmas were prevalent during major extension. Tholeiitic and alkalic basalts were then erupted after significant block tilting. The most voluminous volcanism occurred in early Miocene time and was accompanied by mild north-south extension. Belts of east-west extension bordered the region to both the north and south in early Miocene time. Large-magnitude east-west extension engulfed nearly the entire region in middle Miocene time, beginning in most areas ~16 Ma and ending by ~9 Ma. Tilt rates commonly exceeded 80°/m.y. during the early stages of east-west extension. Although less voluminous than that in the early Miocene, volcanism generally spanned the entire episode of extension south of Lake Mead. Thus, thick volcanic sections, as opposed to sedimentary rock, accumulated in many growth-fault basins. The northward advancing magmatic front stalled, however, in the Lake Mead area along the southern margin of the southern Nevada amagmatic gap. Thus, Tertiary sections in the Lake Mead area are dominated by sedimentary units, including alluvial fan, continental playa, and lacustrine deposits. During middle Miocene extension, strain was partitioned into a west-dipping normal-fault system in the north and an east-dipping system in the south. The two fault systems and attendant opposing tilt-block domains overlap and terminate within the generally east-northeast-trending Black Mountains accommodation zone. Major east-west extension was contemporaneous on either side of the accommodation zone. The west-dipping normal fault system in the north is kinematically linked to major strike-slip faults along the northern margin of the corridor, where a complex three-dimensional strain field, involving both east-west extension and north-south shortening, characterized the middle to late Miocene.

The transition between the Colorado Plateau and the Basin and Range is unusually sharp along the eastern margin of the northern Colorado River extensional corridor and is marked by a single west-dipping fault zone, the Grand Wash fault zone. Subhorizontal, relatively unfaulted strata on the Colorado Plateau give way to moderately to steeply east-tilted fault blocks across the Grand Wash fault zone. Topographic and structural relief across this boundary developed during middle Miocene extension and was established by 9 Ma. The location and abruptness of the Colorado Plateau-Basin and Range transition in this region may have been controlled by an ancient north-trending crustal flaw, inasmuch as it follows a diffuse boundary between Early Proterozoic crustal provinces.