Birth and evolution of the Virgin River fluvial system: ∼1 km of post–5 Ma uplift of the western Colorado Plateau

Abstract

The uplift history of the Colorado Plateau has been debated for over a century with still no unified hypotheses for the cause, timing, and rate of uplift. 40 Ar/ 39 Ar and K/Ar dating of recurrent basaltic volcanism over the past ~6 Ma within the Virgin River drainage system, southwest Utah, northwest Arizona, and southern Nevada, provides a way to reconstruct paleoprofiles and quantify differential river incision across the boundary faults of the Colorado Plateau-Basin and Range boundary. We compare differential incision data with patterns of channel steepness, bedrock erodibility, basaltic migration, and mantle velocity structure to understand the birth and evolution of the Virgin River system. New detrital sanidine ages constrain the arrival of the Virgin River across the Virgin Mountains to less than 5.9 Ma. Virgin River incision rates and amounts show an eastward stair-step increase in bedrock incision across multiple N-S-trending normal faults. Using block incision values away from fault-related flexures, average bedrock incision rates are near zero since 4.6 Ma in the Lower Colorado River corridor, 23 m/Ma from 6.8 to 3.6 Ma in the Lake Mead block, 85 m/Ma from 3 to 0.4 Ma in the combined St. George and Hurricane blocks, and 338 m/Ma from 1 to 0.1 Ma in the Zion block. Steady incision within each block is documented by incision constraints that span these age ranges. We test two end-member hypotheses to explain the observed differential incision magnitudes and rates along the Virgin River system over the past ~5 Ma: (1) as a measure of mantle-driven differential uplift of the Colorado Plateau relative to sea level; or (2) due to river integration across previously uplifted topography and differential rock types with down-dropping of Transition Zone blocks but no post-5 Ma uplift. We favor headwater uplift of the Colorado Plateau because basalt-preserved paleoprofiles indicate that eastern fault blocks have been the "active" blocks that moved upwards relative to western blocks with little base-level change of the lower Colorado River corridor in the past 4.6 Ma. Block-to-block differential incision adds cumulatively such that the Zion block (Colorado Plateau edge) has been deeply incised 880-1200 m (~338 m/Ma) over the 2.6-3.6 Ma period of Hurricane fault neotectonic movement, which has a slip magnitude of 1100 m. Mantle-driven uplift is implicated by a strong correlation throughout the Virgin River drainage between high normalized channel steepness (k sn) and low underlying mantle velocity, whereas there is a weaker correlation between high k sn and resistant lithologies. Basaltic volcanism has migrated northeastward at a rate of ~18 km/Ma parallel to the Virgin River between ca. 13 and 0.5 Ma, also suggesting a mantle-driven mechanism for the combined epeirogenic uplift of the western Colorado Plateau, recurrent slip on its bounding faults, and headward propagation and differential incision of the Virgin River. Thus, we interpret the Virgin River to be a <5 Ma disequilibrium river system responding to ong...

Spatiotemporal history of fault–fluid interaction in the Hurricane fault, western USA

“…Using the published slip rate estimates of 0.44 to 0.57 mm yr −1 (Lund et al, 2007), this equates to vein formation depths from ∼ 40 to 300 m. However, this assumes negligible exhumation of the footwall over the last 540 ka, and they are minimum estimates. Using the local incision rate of the Virgin River through the footwall of 338 m Myr −1 (Walk et al, 2019) as a maximum estimate of footwall exhumation, this suggests a maximum depth of vein formation from 70 to 480 m. Consistent with the findings at thermal springs along the Hurricane fault zone (Crossey et al, 2006;Crossey et al, 2009;Nelson et al, 2009), this indicates that deeply circulated thermal fluids are moving up the fault zone, advecting deeper geotherms towards the surface, and mixing with shallow meteoric fluids. The relatively shallow depth of these processes is notable, and has been observed at other major normal faults.…”

Interpreting and reporting 40Ar/39Ar geochronologic data

“…Detrital sanidine geochronology has the potential for utilization for many Phanerozoic sedimentary deposits (Copeland and Harrison, 1990;Chetel et al, 2011). Recent applications have mostly focused on Paleocene/Late Cretaceous chronostratigraphic studies and river terrace dating in the southwest United States (Hereford et al, 2016;Karlstrom et al, 2017;Leslie et al, 2018aLeslie et al, , 2018bAslan et al, 2019;Walk et al, 2019). The power of the method lies in the robustness of sanidine to produce ultraprecise and accurate dates by single-crystal total fusion.…”

A constraint on post–6 Ma timing of western Grand Canyon (Arizona, USA) incision removed: Local derivation indicated by ca. 5.4 Ma fluvial deposits below Shivwits Plateau basalts north of Grand Canyon

“…Deposits from paleo-Lake Hualapai suggest the region immediately west of the Grand Wash fault was internally drained ca. 10-5.6 Ma (Lucchitta, 1966;Spencer et al, 2013;Faulds et al, 2016) and a major south-flowing drainage was established by 4.72 Ma (Walk et al, 2019). However, disruption of any transport across the fault likely began closer to the onset of fault motion ca.…”

“…The Shivwits Plateau is a broad erosional surface cut across the Permian Kaibab Limestone north of the modern canyon that is bounded on the west by the Grand Wash fault. Preservation of geomorphic surfaces by Cenozoic volcanism displays a general southwest-to-northeast trend of escarpment retreat (Lucchitta, 1975(Lucchitta, , 1984(Lucchitta, , 1989Lucchitta and Jeanne, 2001), with remnants of Triassic Moenkopi Formation and later Mesozoic units preserved below capping resistant basalt flows, which follow the same southwest-to-northeast trend (Crow et al, 2011;Walk et al, 2019). South of the Hualapai and Coconino Plateaus is the transition zone of central Arizona, which separates the southwestern edge of the Colorado Plateau from the Basin and Range province.…”