The Laramide Orogeny, which began during the late Mesozoic about 80 MA, continued to disturb Arizona into the early Cenozoic. Continued construction of the Mogollon Highlands in the southwestern half of the state resulted in the development of northeastern lowlands that were an heterogeneous assemblage of local uplifts and sedimentary basins. During this time, north-northeast trending drainages stripped away the Phanerozoic cover over the highlands and deposited sediments in the northern lowlands. Monoclinal folding in northern Arizona associated with the Laramide defined local uplifts, such as the Defiance and Kaibab uplifts, that flanked major basins including the Coconino basin and Kaiparowits-Black Mesa downwarp.
By 50 million years ago, the Laramide Orogeny ended and for the next 15 to 20 million years during a tectonically inactive period the Mogollon Highlands continued to erode. As the core of the highlands were exposed Proterozoic metamorphic, plutonic, and volcanic clasts were transported northward and today are found perched on the Colorado Plateau. The Mogollon Rim Formation, or "rim gravels", indicate a southwesterly source by clast composition and sedimentary structures, such as imbrication, and confirm a topographic inversion occurred after deposition of the rim gravels.
Basin sedimentation in the northeast and highland erosion in the southwest was disrupted by renewed tectonism called the Mid-Tertiary Orogeny which began about 35 MA. This orogenic episode is defined by the emplacement of granites and the eruption of large volumes of ash and lava in the southern and western portions of the state. Intermediate to felsic magmatic activity shifted through time from southwestern New Mexico about 32 MA, to southeastern Arizona, and ended in southwestern Arizona by about 21 MA as the orogeny proceeded. Along with this magmatic trend, crustal extension accommodated by detachment faulting began to dismember the Mogollon Highlands. Detachment faults are low angle normal faults which allow large amounts of extension as the footwall of the fault, usually referred to as the lower plate, is pulled from under the hanging wall, usually called the upper plate.
Extension during the Mid-Tertiary orogeny was accompanied by the formation of metamorphic core complexes. Dynamic metamorphic processes focused along the plane of the detachment faults formed a metamorphic veneer of rock called mylonites along the detachment surface. Mylonites form as existing minerals are stretched and elongated parallel to the direction of slip on the fault. New mineral crystal growth, with cyrstal orientations that are also parallel to the direction of slip on the fault, also occurs as chemically rich fluids are directed along the detachment. Thus, the mylonitic fabric of metamorphic core complexes indicate the direction of slip on the detachment fault from which the fabric formed. Exposures of mylonitic rocks can be found capping the granites within the lower plate of the South Mountain detachment and in the Buckskin, Harcuvar, Harquahala, Vulture, Bellmont, Tortolita, Santa Catalina, Rincon, and Pinaleno Mountains.
Along with mylonitization of the lower plate, the upper plate of a detachment also records a unique set of features. As the detachment fault moves, the upper plate fractures and faults as a series of listric normal faults which sole into the detachment. With further movement on the detachment, additional rotation occurs and the result is a domino effect as the fault blocks rotate. The strata included in these fault blocks dip toward the developing core complex as the detachment moves and away from the direction of slip.
Sedimentary basins form within the upper plate of detachment faults in response to displacement. As listric normal faults develop in the upper plate they rotate forming a series of half grabens. As the half grabens develop they are contemporaneously filled by eroding sediments from the exposed hanging wall block and the footwall block and represent syntectonic sediments. Such basins are deepest adjacent to the hanging wall scarp where displacement is great. Coarse clastic sediments are mostly trapped adjacent to the hanging wall scarp and finer sediments occur closer to the footwall as the half graben fills. It is also common to find lakes and river systems occupying half graben valleys. Such aquatic features initially are trapped close to the hanging wall but may migrate toward the footwall as sediments fill the graben.
The Mid-Tertiary Orogeny marks the beginning of the development of what can be considered the modern physiography of Arizona. As the orogeny continued, the Mogollon Highlands were extended and structurally lowered and the Colorado Plateau began to gain elevation. The ancestral Mogollon Rim was a structural feature which separated these two tectonic realms: a southerly and western province driven by extensional subsidence and a northerly and eastern province driven by uplfit. Existing river systems were dramatically disrupted. As the Colorado Plateau was uplifted, it was effectively isolated from receiving further sedimentation derived from the Mogollon Highlands. Instead, sediment from the subsiding highlands encountered the ancestral Mogollon Rim forming the Beavertail Butte Formation, or Beavertail Butte gravels, which have been dated at 18 to 20 MA.
The Mid-Tertiary Orogeny gave way to the Basin and Range Disturbance around 15 MA. Volcanism, which was dominantly mafic in composition but does include more felsic occurrences, and extensional high angle normal faulting scars the southeastern, southern and western half of the state. In contrast to detachment faulting, high angle normal faulting accommodates much less extension. Additionally, these faults dissect detachments throughout the southern and western portion of the state. Displacement on some of the larger high angle normal faults, such as the Verde fault, can range between 6,000 and 10,000 feet and tend to flank northwest-southeast trending mountains and valley systems which define the southern and western portion of the state so well. High angle faults also cut the Colorado Plateau, especially along it's southern and southwestern margin, but rarely have more than 500 feet of displacement.
The hanging wall associated with high angle normal faulting formed deep valley floors that have accumulated great thickness' of volcaniclastic sediments. The footwall blocks remained high flanking the valleys as mountains and providing a sediment source for the deepening valleys. As these valleys developed, through-going drainage was frequently not possible and, as a result, many valleys formed by basin and range high angle normal faulting were the sites of lake development. Many of these lakes contain lacustrine sediments and fossils typical of freshwater terrestrial systems. Evaporite deposits indicating periodic desiccation are also known from valley fill in such places as Verde Valley, Chino Valley, and the Tonto Basin. As the Basin and Range Disturbance ceased, subsidence stopped and the valleys began to fill. This allowed for the establishment of through-going river systems and the development of the modern drainage system of Arizona.
For the last 2 million years or so, most of Arizona's basin have been the sites of erosion rather than deposition. As a result, the river systems which were once depositing material in valleys began to cut through their own sediments. Most valleys in the western, southwestern and southern part of the state contain incised river channels. In some places, such as in the Verde Valley, the river has exposed a dramatic record of its own history. This erosive phase may be due, in part, to the opening of the Sea of Cortez about 5 MA.
The Modern Physiography of Arizona
The state of Arizona can be
divided into three distinct physiographic provinces based on topography,
geology, and associated structures. The Colorado Plateau Province,
which is in part, located in northeastern Arizona, is characterized by
topographically flat and elevationally high terrain which is rather undisturbed
geologically. That is, the strata of the plateau are generally flat
lying with only minor local tilting, folding and faulting. The Basin
and Range Province, in contrast, is topographically irregular and elevationally
low and contains strata that are highly faulted, folded, and tilted.
In between these two provinces is the northwest-southeast trending Transition
Zone, which is topographically, elevationally, structurally, and geographically
between the two extremes recorded by the Plateau and the Basin and Range.
Cenozoic Paleogeography
courtesy of Ron Blakey
- Eocene
The end of Laramide orogenesis in Arizona marked the begining of a tectonically quiescent period. The Mogollon Highlands in the central part of the state shed clastic sediment northward onto what has become the Colorado Plateau. During this time, the northern portion of the state represented interior lowlands that flanked highlands to the south. A broad veneer of sediment, called the rim gravels, is found today as far north as the nearly 9000' high North Rim of the Grand Canyon but can be traced by clast lithology back to southern sources in the Transition Zone with intervening elevations around 4000'. This topographic reversal was intiated during extensional tectonics of the Mid-Tertiary Orogeny.
Cenozoic Outcrop Photos
courtesy of Ron Blakey