TAKING TO THE SKIES TO STUDY THE GEOLOGY ON THE GROUND
To the west of the basin lies the Circle Cliffs Uplift and its Waterpocket Fold, a portion of which has been set aside as the
Who was Henry?
Two groups of high peaks
The Henry Mountains are a 56 mile-long and 19 mile-wide, isolated string of five rugged, high peaks. From north to south, the range is clustered into two main groups, each dome being 6-10 miles in diameter. The larger northern group consists of
The Laramide Orogeny, a continuation of Cretaceous mountain-building, provided compression on the Colorado Plateau that resulted in numerous high-relief uplifts separated by small intervening basins. The uplifts and monoclines that we flew over on the earlier portion of our flight (my Posts I and II) demonstrated these landforms. One such basin is the Henry Mountains Basin that received localized intrusions of magma into shallow crustal levels, and that "pushed up" the Henry Mountains. More on that later.
Having been stripped of its Tertiary deposits, the synclinal basin’s surficial bedrock is composed largely of the multi-membered Mancos Shale (which has experienced numerous revisions). These marine mudstone, siltstone, shale and sandstone deposits were deposited during the initial transgression of the Western Interior Seaway during the Early Cretaceous. The sedimentary section in the Henry Mountains is dominated by sandstones and shales ranging in age from Permian to Cretaceous.
In western regions of the basin are found the mesa-capping, fluvial sandstones of the Tarantula Mesa Sandstone that border the Waterpocket Fold. As we shall see, exposures of strata underlying the Cretaceous deposits down to the Permian are dramatically revealed by the formation of the
Early sketch from the field notebook of G.K. Gilbert in 1875 of his conceptual model of a laccolith
(From Hunt, 1988).
(Report on the Geology of the
USGS of the Rocky Mountain Region, 1877)
The geology and geometry of the Henrys
Many of the intrusive centers are surrounded on their periphery by clusters of smaller laccoliths, bysmaliths, dikes and sills. Their partially exposed, eroded pieces and remnants were visible from the air, as we shall see. In varying stages of exposure and surrounding the base of the centers are the basin's exposed sedimentary rocks, ranging in age from Late Permian to Late Cretaceous, which have been uplifted and deflected by the igneous intrusions that have arched their overburden skyward. Erosion has unroofed the cover from the summits of the intrusive centers and differentially exposed the verticalized rocks around their bases.
Emplacement ages for the Henry Mountains intrusions are from about 31 to 23 Ma, which have been radically revised from earlier calculations. A clear pattern in terms of spatial migration of emplacement ages amongst the various intrusive centers does not appear to exist. The entire complex appears to have been assembled in less than one million years.
What is the most likely emplacment scenario?
Uplifted and reflected overburden
The following cross-sectional diagram of
Are the intrusive centers of the Henry Mountains laccoliths or stocks?
Gilbert hypothesized that sill intrusion preceded the inflation of an underlying laccolith. Hunt believed the central intrusions are cylindrical stocks that are sheathed with a zone of shattered sedimentary rocks and that laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. Recent findings have confirmed the presence of a floored laccolithic intrusion but have not ruled out a stock at depth.
Hunt’s concept of relationships between the stocks and uplift of beds of
(Modified from Hunt 1953. From Processes of Laccolithic Emplacement,
An enigmatic intra-plate locale
Looking below the surface
Modern geochronology and geochemistry to the rescue
Revised ages of the intrusions have made it clear that mid-Tertiary magmatism on the Colorado Plateau was part of voluminous regional magmatism in the North American Cordillera. The data suggests the existence of an essentially continuous, thousand-mile plus, intra-continental magmatic zone that extended from western Nevada through southern Utah to southwestern Colorado, and south to west Texas during the Oligocene to Miocene transition. As we shall see, the length of the zone and its perpendicular orientation to the trend of subduction along the western coast, help to explain Mid-Tertiary igneous activity. In addition, the isotopic geochemical signature of the Henry's rocks tells us that the magma was derived from partial melting of subducted oceanic crust in the mantle, the characteristic mark of a magmatic arc.
The Farallon Big Picture
The rapid subduction of the oceanic Farallon Plate beneath the continental North American Plate proceeded at a flatter trajectory in the region that drove the Laramide Orogeny. It was the presence of the Farallon Plate that provided the voluminous and widespread source for arc-related magmatism. But how? Plate convergence presumably slowed by 50 Ma, which drove the dense Farallon deeper into the mantle causing it to founder and break up. That allowed underlying buoyant, hot mantle to rise and heat the base of the crust resulting in its partial melting.
That's not all. As the less-dense melt ascended through the mantle, it pooled at the base of the silicic crust, owing to its greater mafic-density. That facilitated a silicic, crustal melt. The final result was the shifting of the original mafic composition of the mantle-derived melt to a melt with a more intermediate composition (our diorite!). That further retarded its journey of ascension, eventually stalling its rise into the shallow crust in a neutral-buoyancy state. Voila! That produced the magma that fed the Henry Mountains (and the other laccolithic-derived landforms of the Colorado Plateau), identifiable by its mafic, arc-like affinity. Amazing stuff.
In summary, magmatism at the laccolithic centers is likely a consequence of the subduction of Farallon oceanic lithosphere. That exerted control over the composition, distribution and timing of magmatism after the Laramide Orogeny. The transport of relatively small volumes of magma within the laccoliths to shallow crustal environments indicates suppression by the unique physical properties of the high-strength lithosphere of the Colorado Plateau relative to contemporaneous magmatism in the Great Basin to the west and the San Juan Mountains to the east.
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