Miocene Extension in the Colorado River Extensional Corridor: A comparison with Slow-Spreading Ridges

Phillip B. Gans1, Pip Darvall2, and James Faulds3

1Geological Sciences, UCSB, Santa Barabara, CA 93106
2Geology Dept., Monash University, Victoria, Australia
3Dept. of Geology, University of Iowa, Iowa City, Iowa 52242

Recent geologic and geophysical studies have provided important constraints on the geometry and timing of extension and magmatism along the Colorado River Extensional Corridor (CREC). 30-60 kms of ~E-W extension across the CREC is indicated by multiple generations of rotated normal faults and drastic attenuation of the upper crust. These faults are closely spaced (0.5-2 km) and commonly initiated as planar, near vertical (tensile) fractures that then rotated to lower angles during progressive extension. At any given latitude within the CREC, much of the extension occurred in <1-3 Ma at an average spreading rate of 1-3 cm/yr. Extension accompanied and immediately followed voluminous, fundamentally mafic volcanism. Existing K-Ar data and new 40Ar/39 ages of syntectonic volcanic rocks indicate both volcanism and the inception of rapid extension migrated northward within the CREC at an average rate of 3-4 cm/yr from the vicinity of the Whipple mountains (20-17 Ma: peak=19.0 Ma) to the northern Eldorado Mountains (16-13 Ma: peak=15.0 Ma)

Most workers have concluded that extension across the CREC was fundamentally assymmetric or "rooted" on the basis of fault polarities, consistent shear sense indicators, and large displacements inferred for core complex "detachment faults". Several observations, however, are more compatible with symmetric, ("pure shear") extension. These include (1) the spatial coincidence of large magnitude extension and voluminous mafic magmatism within the corridor, (2) a prominent 20 km-wide, 10-20 mgal gravity high centered on the CREC and interpreted to be a shallow Miocene mafic intrusion (R.W. Simpson, 1991, pers comm.), (3) a symmetric upward bulge in the velocity structure of the middle crust beneath the southern CREC (McCarthy et al, 1991, JGR), and (4) the continuity of geological features and lack of significant strike slip displacement across a major accommodation zone in the central part of the corridor. We believe that the best modern analogues for the CREC are propagating slow-spreading mid-ocean ridges such as the Asal Rift in the Afar depression of Djibouti (e.g., Stein et al, 1991 JGR). Direct comparisons with this rift include (1) rate of propagation (Asal=3 cm/yr), (2) extension rate (Asal=1.6 cm/yr), (3) width of rift: (Asal=12-14 km, CREC=~10-20 km at onset, (4) fault spacing (Asal=1-2 km) , and (5) initial fault dips (Asal= >60¡). These similarities raise the intriguing possibility that the CREC may be a failed mid-Miocene propagating ridge wherein extreme stretching of the upper crust was triggered and in part accommodated by asthenospheric upwelling and a flux of basalt into the underlying lithosphere.

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