Tripping through the basin and range country of the Southwestern United States

Wednesday, May 24, 1995

Students in most Earth Sciences programs at the University of Waterloo are required to attend a major field trip around the time they commence their final undergraduate year. This year, one group of students chose a trip to the southwest corner of the United States with the goal of exploring the geological development of that fascinating region. Most of the route we traversed with them lies within the tectonic region known as the Basin and Range Province. We thought that these notes which provide a brief precis of what is known about the geological history of the region might be helpful to anyone traveling there.

"Basin and Range" is defined in the Glossary of Geology as a "physiographic province characterized by a series of tilted fault blocks forming longitudinal, asymmetric (sic) ridges or mountains and broad intervening basins; specif. the Basin and Range physiographic province in the SW U.S., where the ridges have steep eastern faces and gentler western slopes" (Fig.1). It occupies most of the region between the Colorado Plateau to the east and the Sierra Nevada to the west (close to 1000 km in width) and from southern Idaho in the north into Mexico to the south.

Besides a characteristic fault pattern, the Basin and Range Province is distinguished by elevated levels of contemporary seismicity, high crustal heat flow, abnormally thin continental crust yet a generally high regional elevation. Whereas most of the Cordilleran fold belt to the north and the south is characterized by a predominantly compressional style of crustal deformation, the Basin and Range deformation is manifestly extensional, at least over the past 30 million years.

In terms of topography, the crest to crest spacing of the high 'ranges' is 25 to 35 km with the intervening low 'basins' 10 to 20 km in width. The contrast in elevation is especially evident in Death Valley where it amounts to almost 3.5 km between the playa lake surface at Badwater at - 87 m (-281) and Telescope Peak in the Panamint Range immediately to the west where it is +3,368 m (11,0495. The tilting of the blocks in the ranges is produced by the particular style of normal faulting known as listric normal faulting in which the angle of dip of the fault becomes shallower (less steep) with depth so that the fault block rotates as it slides down the fault, (see Fig.1 again).

To understand the reasons for this exceptional history in the Basin and Range Province we have to go back over 30 million years and chart the movements of North America as it moved on its plate away from the Mid- Atlantic Ridge spreading ridge to the east. Along its western margin North America initially encountered an oceanic plate moving easterly from the East Pacific Rise spreading centre (Fig.2). This plate, known as the Farallon Plate was being subducted beneath the margin of western North America in the same way as its remnants, the Juan de Fuca Plate offshore from northern California to Vancouver Island and the Cocos Plate off the west coast of Mexico, are doing today. Subduction of this oceanic plate produced a succession of volcanic arcs through Mesozoic to Mid-Tertiary time which periodically collided with the western North America plate margin producing intermittent compressional orogenic events, e.g. Nevadan orogeny (140- 150 Ma), Sevier orogeny (80-130 Ma) and the Laramide orogeny (50-80 Ma). But during all this time North America was advancing toward the ancestral East Pacific Rise while subducting progressively younger and therefore hotter oceanic crust of the Farallon Plate.

The outline of the crest of the East Pacific Rise at that time is known from the shape of dated magnetic stripes in the present Pacific Plate; it was oriented roughly N-S but with an abrupt westerly offset north of an E-W transform fault zone known as the Mendocino Fracture Zone. Thus the first section of the spreading centre to be reached by North America was the 90 degree angle where this offset occurred. Thirty million years ago the first section of the East Pacific Rise began to be over-ridden by North America and to be subducted. Once geologists realized that a section of the spreading ridge must have been subducted, this raised the highly intriguing question of what are the consequences of such an event.

Some early scientists thought that the centre would continue spreading and generating lithospheric plates on either flank as it had been doing before subduction (Fig.3). This idea ignores the fact that for oceanic lithosphere to form there must be an efficient cooling mechanism which exists in oceanic water but which does not exist within a subduction zone. Furthermore this model would require that oceanic crust must be emerging from beneath central and southern California but there is no trench there now - the interplate boundary in that region is the San Andreas Fault system, a right-handed transcurrent fault.

Once it was realized that subduction of a spreading ridge would prevent any further lithospheric plate production it was realized that the plate being subducted beneath western North America (the old Farallon Plate) must have a gap or hole in it and that this gap must be enlarging as North America drifts further across it and as the length of the spreading ridge being subducted consequently grows. The concept of a "slab gap" or "slabless window" in the subducting plate therefore arose (Fig.3). Furthermore, when the current extent of this gap or window was plotted it was found to coincide roughly with the outline of the Basin and Range Province

So how are these two intriguing facts connected? The most truthful answer to this question is that tectonicists are still arguing over the relative importance of a number of causative connections but a simple- minded explanation must include the nature of the stresses existing between North America and the plate to the west of it. Wherever and whenever these two plates are in a convergent movement pattern these stresses are predominantly compressional, especially when the subducting plate is relatively young, hot and buoyant and therefore resists being pressed downwards into the asthenosphere. But when the plates in contact are both moving along westerly vectors (not necessarily co-linear) then the compressive stresses must be greatly reduced if not almost eliminated in that region. Extension will therefore be expected in the region with the least compressive stress. Geological estimates of the amount of extension that has occurred range up to ca. 350 km; curiously, this amount is about equal to the width of the westward bulge of the Californian coast from Cape Mendocino south to the Gulf of California.

Another factor must also be considered. A "slab gap" or "slabless window" in the subducting plate cannot mean that there is a vast cavern beneath North America in that region. Instead the ductile asthenosphere beneath must rise up into it. The base of the North American plate over the gap will therefore be in contact with hot asthenosphere whereas elsewhere there will be an intervening slab of relatively cooler former oceanic lithosphere

The extensional tectonics of the Basin and Range country can therefore be thought of in terms of a relaxation of the compressive stresses created during convergent subduction, heating of the lithosphere by hot asthenosphere below combined with thermal expansion and buoyant rise of the lithosphere above. Add to these the expressions of Cenozoic and Recent volcanic activity throughout the region produced by this sub-plate heating and the ductile rise of metamorphic complexes created closer to the heat source and you have a few of the mechanisms that may have been involved.

So will the Basin and Range Province continue to expand as a greater length of the East Pacific Rise is over-ridden? The answer must be "Yes" but with one important caveat Spreading ridges are Earth's mechanism of disposing of excess heat; they are Earth's radiators. As heat dissipation is progressively made less efficient along the current spreading centre, the likelihood is increased that the spreading centre will jump to a new location away from the North America continental margin, possibly further to the west in the Pacific Ocean where its operation would be unencumbered by stray trespassing continents. If this were to happen, North America would be presented with a new convergent plate along its western margin, a new trench would be established under California and the state of stress on the continent would click back to being compressional. And Californians would have to contend with volcanoes in addition to earthquakes, fires, floods and smog. Never a dull moment!

Edward C Appleyard