Gary J. Axen

On the role of isostasy in the evolution of normal fault systems

The footwalls of west-dipping normal faults that separate the west-central Colorado Plateau from the Basin and Range province record at least 5-7 km, and perhaps as much as 15-20 km, of west-side-up Neogene uplift, with an axis just 10-20 km west of undeformed plateau strata. The uplift is expressed as folding and steep faulting in pre-Tertiary cratonic and disconformably overlying Neogene strata, forming a basement-cored anticline and coincident topographic high on the western margin of the plateau. The authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin. Profound, isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains, calling into question several assumptions fundamental to existing models of the evolution of normal fault systems.

Evolution of fault-surface roughness with slip

Principal slip surfaces in fault zones accommodate most of the displacement during earthquakes. The topography of these surfaces is integral to earthquake and fault mechanics, but is practically unknown at the scale of earthquake slip. We use new laser-based methods to map exposed fault surfaces over scales of 10 µm to 120 m. These data provide the fi rst quantitative evidence that fault-surface roughness evolves with increasing slip. Thousands of profi les ranging from 10 µm to >100 m in length show that small-slip faults (slip <1 m) are rougher than large-slip faults (slip 10‐100 m or more) parallel to the slip direction. Surfaces of small-slip faults have asperities over the entire range of observed scales, while large-slip fault surfaces are polished, with RMS values of <3 mm on profi les as long as 1‐2 m. The large-slip surfaces show smooth, elongate, quasi-elliptical bumps that are meters long and as high as ~1 m. We infer that these bumps evolve during fault maturation. This difference in geometry implies that the nucleation, growth, and termination of earthquakes on evolved faults are fundamentally different than on new ones.

Exhumation of the west-central Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics

Crystallization and thermal histories of two plutons in the west-central Alborz (also Elburz, Elburs) Mountains, northern Iran, are combined with crosscutting relations and kinematic data from nearby faults to determine the Cenozoic tectonic evolution of this segment of the youthful Euro-Arabian collision zone. U/Pb, ^(40)Ar/^(39)Ar, and (U-Th)/He data were obtained from zircon, biotite, K-feldspar, and apatite. The Akapol pluton intruded at 56 ± 2 Ma, cooled to ∼150 °C by ca. 40 Ma, and stayed near that temperature until at least 25 Ma. The nearby Alam Kuh granite intruded at 6.8 ± 0.1 Ma and cooled rapidly to ∼70 °C by ca. 6 Ma. These results imply tectonic stability of the west-central Alborz from late Eocene to late Miocene time, consistent with Miocene sedimentation patterns in central Iran. Elevation-correlated (U- Th)/He ages from the Akapol suite indicate 0.7 km/m.y. exhumation between 6 and 4 Ma, and imply ∼10 km of Alborz uplift that was nearly synchronous with rapid south Caspian subsidence, suggesting a causal relation. Uplift, south Caspian subsidence and subsequent folding, reversal of Alborz strike-slip (from dextral to sinistral) and(?) eastward extrusion of central Iran, coarse Zagros molasse deposition, Dead Sea transform reorganization, Red Sea oceanic spreading, and(?) North and East Anatolian fault slip all apparently began ca. 5 ± 2 Ma, suggesting a widespread tectonic event that we infer was a response to buoyant Arabian lithosphere choking the Neo-Tethyan subduction zone.

Variation in styles of rifting in the Gulf of California

Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism—from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism.

Space-time patterns and tectonic controls of Tertiary extension and magmatism in the Great Basin of the western United States

Structural and stratigraphic relations in the Great Basin indicate widespread pre-middle Miocene crustal extension that appears to define two north-trending belts. Most extension in these belts was Oligocene age, but locally it began earlier or lasted into early Miocene time. The eastern belt straddles the Nevada-Utah border and includes the Snake Range, Nevada, area, with its southern end near 37.5°N and its western edge at the Seaman-Butte Mountains breakaway. The southern boundary of the eastern belt is occupied by the 26-15 Ma Caliente caldera complex and approximately coincides with the present east-west-trending margin of the Great Basin north of Saint George, Utah. Crust north of this boundary extended approximately east-west before volcanism began at 30-32 Ma, but to the south, extension began after about 15 Ma. This boundary may have been a rooted zone of left-slip faults that allowed the footwall of the Stampede detachment to move west relative to unextended terrain to the south. The eastern margin of the eastern belt is probably located near the present eastern edge of the Great Basin, but its northern end is poorly defined. The western belt runs from the Funeral and Grapevine Mountains, California, to the Ruby Mountains, Nevada, and north-northeast to the Albion Range, Idaho. Tens of kilometers of crustal extension occurred at least locally in both belts, but magnitude of extension is poorly known for large areas of each. Tertiary volcanism in the Great Basin began in the north in Eocene time with predominantly effusive volcanism and swept southward, ending with voluminous Oligocene-Miocene ignimbrite eruptions from calderas in an irregular, discontinuous belt between Marysvale, Utah, and Reno, Nevada. A result of the south-ward migration of volcanism is that the onset of extension in both belts was syn- or post-volcanic in the north but was pre-volcanic in the south. Late Paleogene extension and crustal magmatism coincided in both time and space only locally, where south-migrating magmatism overlapped active north-south-trending extensional belts. Most calderas in the southern Great Basin formed in previously extended belts or on their margins. Southward migration of ignimbrite sources was apparently blocked by unextended crust to the south. In contrast, volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to, or during, crustal extension. This is consistent with observations in the southern Basin and Range, where volcanism and crustal extension were generally synchronous, and volcanism was dominantly effusive. Thus, caldera formation may be controlled by the distribution of upper-crustal extension, although the physical mechanism for this control remains speculative. The space-time patterns of late Paleogene extension in the Great Basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin, but being driven by gravitational collapse of thick crust. Space-time patterns of Tertiary volcanism in the Basin and Range also appear to conform to patterns of thermal weakening or destruction of the subducted slab. Both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts, because both predict close spatial and temporal association of extension and magmatism, which is not generally observed.

Late Cenozoic shortening in the west-central Alborz Mountains, northern Iran, by combined conjugate strike-slip and thin-skinned deformation

The west-central Alborz Mountains of northern Iran have deformed in response to the Arabia-Eurasia collision since ca. 12 Ma and have accommodated 53 ± 3 km of shortening by a combination of range-parallel, conjugate strike-slip faulting and range-normal thrusting. By our interpretation, ∼17 km of shortening across the Alborz is accommodated by westward relative motion of a crustal wedge bounded by conjugate dextral and sinistral strike-slip fault systems. The Nusha, Barir, and Tang-e-Galu fault zones strike west-northwest, constrain the north side of the wedge, and, prior to ca. 5 Ma, accumulated a total of ∼25 km of dextral slip. The south side of the wedge is bounded by the active sinistral reverse Mosha and Taleghan faults, which merge northwest of Tehran and have a total slip estimate of 30–35 km. A restored cross section across the range indicates a minimum of 36 ± 2 km of fold-and-thrust–related, range-normal shortening. Combined, wedge motion, thrusting, and folding yield a net shortening of 53 ± 3 km across the range, which is within the error of the shortening estimate predicted by assuming that the present-day shortening rate (5 ± 2 mm/yr) has been constant since ca. 12 Ma (∼60 km of predicted shortening). A second restored cross section farther west, which includes the wedge, gives a total shortening of 15–18 km and a long-term shortening rate of 1.25–1.5 mm/yr (constant shortening rate since ca. 12 Ma). These strong along-strike finite-strain and long-term strain-rate gradients are important for our understanding of how long-term strain rates compare with instantaneous strain rates derived from global positioning system (GPS) data, and should be considered when planning mountain belt–scale GPS surveys. Finally, a 60-km-long right-hand bend in the Mosha-Taleghan fault system has driven the development of a transpressional duplex south of the fault. The southern boundary of the duplex is the active Farahzad–Karaj–North Tehran thrust system. The kinematic development of this strike-slip duplex has implications for seismic hazard assessment in the heavily populated Karaj and Tehran areas.

Pore pressure, stress increase, and fault weakening in low-angle normal faulting

Low-angle (dip < 30°) normal faults accommodate much extension of the continental crust. They apparently move under low resolved shear stress and are anomalously weak, characteristics that they share with the San Andreas fault. Structural, textural, and geochemical arguments suggest that low-angle normal faults are weak in both the ductile and brittle regimes, partly or totally due to elevated pore fluid pressure. High pore pressure in detachment zones may be contained by upper-plate strata, mineral precipitation in their hanging walls, formation of low-permeability microbreccia layers, threshold pressure gradients, and low-permeability mylonites below chlorite breccia. Mechanical analysis shows that fault weakening may preclude equality of the regional and fault-zone stress tensors, and predicts reorientation and increase of principal stresses in weak fault zones. These changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks. Fault weakening focuses extension in the upper crust onto low-angle normal ductile-brittle shear zones in the midcrust, promoting propagation of low-angle brittle normal faults into the upper crust.

Chronology of Miocene–Pliocene deposits at Split Mountain Gorge, Southern California: A record of regional tectonics and Colorado River evolution

Late Miocene to early Pliocene deposits at Split Mountain Gorge, California, preserve a record of basinal response to changes in regional tectonics, paleogeography, and evolution of the Colorado River. The base of the Elephant Trees Formation, magnetostratigraphically dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of extension in the southwestern Salton Trough. The oldest marine sediments are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in the Salton Trough and northern Gulf of California region supports a model for localization of Pacifi c‐North America plate motion in the Gulf ca. 6 Ma. The fi rst appearance of Colorado River sand at the Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypothesis for initiation of the lower Colorado River.

LATE MIOCENE-PLEISTOCENE EXTENSIONAL FAULTING, NORTHERN GULF OF CALIFORNIA, MEXICO AND SALTON TROUGH, CALIFORNIA

A belt of low-angle normal (or detachment) faults ∼250 km long extends from the northern end of the Salton Trough, California to southern Laguna Salada, Baja California, Mexico. The detachment system is divided into two principal segments. The northern segment, here termed the “west Salton detachment system,” comprises top-to-the-east detachment faults along the eastern Peninsular Ranges that root under the Salton Trough. The southern segment, here termed the Laguna Salada detachment system, comprises top-to-the-west detachment faults in northeastern Baja California and the Yuha Desert region of the southwesternmost Salton Trough. Detachments of that system root under Laguna Salada and the Peninsular Ranges of northern Baja California. Both of these systems experienced a major episode of activity in late Miocene to Pleistocene time, synchronous with deposition of the Imperial and Palm Spring formations, and the Laguna Salada detachment system may still be active. Thus, their activity temporally overlapped...

Thermal histories from the central Alborz Mountains, northern Iran: Implications for the spatial and temporal distribution of deformation in northern Iran

We integrate new and existing thermochronological, geochronological, and geologic data from the western and central Alborz Mountains of Iran to better constrain the late Cenozoic tectonic evolution of northern Iran in the context of the Arabia-Eurasia collision. New data are presented for two granitic plutons north of the Alborz Range crest. Additional new apatite (U-Th)-He data are also presented for volcanic, intrusive, and detrital apatite grains from two transects south of the range crest. Our most definitive results include zircon and apatite (U-Th)-He and limited K-feldspar ^(40)Ar/^(39)Ar thermal history data from the Cretaceous (ca. 98 Ma) Nusha pluton that reveal that the Alborz basement underwent generally slow denudation (∼0.1 km/m.y.) as late as 12 Ma with more accelerated exhumation (∼0.45 km/m.y.) that likely began shortly after 12 Ma. The Lahijan pluton, a late Neoproterozoic–Cambrian basement exposure near the Caspian shore, records apatite (U-Th)-He closure at 17–13 Ma. Additional (U-Th)-He results from detrital apatites sampled along two separate horizontal transects all consistently yielded latest Miocene to Pliocene apparent ages that imply that even supracrustal cover rocks within the Alborz have undergone significant, regionally extensive exhumation. Overall, our data are consistent with ∼5 km of regionally extensive denudation since ca. 12 Ma. The onset of rapid exhumation in the Alborz at ca. 12 Ma appears to be consistent with other timing estimates that place the onset of the Arabia-Eurasia collision between 14 and 10 Ma.