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The detachment era (1977–1982) and its role in revolutionizing continental tectonics

California Institute of Technology, Division of Geological and Planetary Sciences, 1200 East California Boulevard, Pasadena, CA 91125, USA (e-mail: brian{at}gps.caltech.edu)

	Abstract

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
After the discovery of thrust-and-nappe structure near the turn of the twentieth century, mountain belts were viewed as a direct expression of horizontal shortening of the continental crust, and continental rifting was viewed as a phenomenon distinct from it. By mid-century, broad consensus had emerged, mainly on the basis of physical reasoning, that thrust-and-nappe structure instead reflected gravity sliding secondary to vertical motions of the crust, as embodied in the influential stockwerk folding hypothesis. In a noteworthy period from 1977 to 1982, informally referred to here as the ‘detachment era’, not only did the last vestiges of support for the stockwerk hypothesis evaporate, but large-magnitude extension was discovered throughout the Cordillera, manifest primarily by extensional detachments and metamorphic core complexes. Soon afterward extensional detachments were recognized as a global phenomenon, forcing first-order reinterpretation of field relations in most orogens. Although plate tectonics is indisputably the most profound discovery in Earth sciences in the twentieth century, the detachment era arguably had commensurate impact on field-based interpretations of continental tectonics. Three decades later, controversy persists over the origin of metamorphic tectonites in core complexes, and over the existence and mechanics of slip on shallowly dipping extensional detachments.

These syntheses included the concept of finite horizontal stretching of the crust (e.g. Argand 1924), perhaps of magnitude sufficient to exhume ductilely stretched lower crust and eventually the upper mantle (e.g. Wegener 1929, chapter 10). Even though features now known to be detachments and core complexes had locally been described even earlier in the century (e.g. Ransome et al. 1910), making the connection between the stretching predicted by the mobilists and its expression in continental geology would have to wait more than half a century.

	The fixist era

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
By the advent of plate tectonics in 1968, documentation of large-displacement thrust faults and strike-slip faults in most orogenic belts was extensive. Remarkably, however, few prominent geologists at the time, even including the minority that had earlier advocated continental drift, believed that thrust faults were an expression of crustal contraction, and there was little notion that large-displacement normal faults even existed. Studies of exposed continental rifts were undertaken separately from the study of orogenic belts, and focused on conspicuous regions of active faulting and subsidence such as the Dead Sea rift (e.g. Quennell 1958) and the Basin and Range province (e.g. Thompson 1960). On cross-sections of these rifts, normal faults were generally depicted as steeply dipping structures with displacements of no more than a few kilometres, largely in the vertical component of slip. In light of the thematic focus of many of the papers in this volume – large-displacement normal faults within orogens – at present the pendulum could hardly have swung any farther away from the mid-1960s consensus.

In that consensus, the most widely accepted general model for mountain building was the ‘infrastructure-superstructure concept’, or as it was also known, the stockwerk folding hypothesis, in which the development of major tectonic elements results primarily from vertical motions within the crust (Wegmann 1935). According to this hypothesis, the evolution of orogenic belts begins with a long phase of subsidence and sedimentation of cryptic origin, followed by the buoyant rise of a ‘migmatite front’ toward the cold, strong sedimentary cover (superstructure). Finally, the metamorphic complex (infrastructure) incorporates and deforms part of the lower superstructure by a modest amount of horizontal flow along a complex shear zone (abscherungzone).

The most cited example in support of this hypothesis was the East Greenland Caledonides, which exhibit spectacular exposures of a superstructure of thick Neoproterozoic–Cambrian sediments sheared off of, and locally incorporated into, their migmatitic substrate. During this era, the upward flux of heat and resulting buoyancy of a ‘granitized’ core was widely regarded as the driving force of tectonics. The model was based partly on the work of C. E. Wegmann and John Haller's East Greenland studies, which included detailed mapping and spectacular photo-documentation of key structures (synthesized in English in Haller 1971). But its mainstream acceptance was driven mainly by the physical presumption that thrust sheets are internally too weak to be ‘pushed from behind’, leaving downslope movement under the influence of gravitational body forces as the only viable explanation for thrust-and-nappe structure. The 1965 edition of the most influential geology text of the twentieth century (Holmes 1965), whose author was an early exponent of continental drift and mantle convection, somewhat ironically argues that horizontal shortening in thrust belts is a surficial response to vertical motions (e.g. Van Bemmelen 1954) and rejects the ‘traditional’ view that Alpine-type thrust-and-nappe structure is an expression of convergence between continental platforms on either side of the orogen. Holmes (1965) quotes Harold Jeffreys (p. 1169), an influential founder of the field of geophysics and (also ironically) a proponent of the contracting Earth theory, as stating that the origin of thrust-and-nappe structure is ‘something that is not crustal shortening’.

The stockwerk hypothesis for East Greenland did not survive geochronological testing, which demonstrated that high-temperature deformation of the deepest exposed infrastructural levels predated deposition of the superstructural sediments by more than a billion years (e.g. Henriksen & Higgins 1976). Modern studies of this complex show that rocks that did experience Caledonian metamorphism and deformation are regionally underlain by Archaean and Proterozoic basement with relatively minor Caledonian overprint, and that the abscherunzone is an extensional detachment (e.g. White & Hodges 2002).

	The demise of fixism

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
The geochronology of East Greenland notwithstanding, the end of the fixist synthesis of mountain building came not with the advent of plate tectonics even though it confirmed the mobility of lithospheric blocks on either side of mountain belts. Rather, it came from seismic images of crustal-scale faults within mountain belts. It began with publication of petroleum industry seismic reflection profiles across the southern Canadian Rockies in the mid-1960s suggesting that the Rocky Mountains foreland fold-and-thrust belt was rooted beneath the crystalline core to the west (Bally et al. 1966), leading many geologists to seriously question the fixist synthesis (e.g. Burchfiel & Davis 1968). Despite these important developments, enthusiasm for vertical tectonics and gravity sliding remained strong through the 1970s in both North America and Europe (e.g. De Jong & Scholton 1973).

The coup de grace was administered by the Consortium for Continental Reflection Profiling (COCORP), upon publication of deep seismic reflection profiles across the crystalline cores of two major contractile mountain ranges in the US that vindicated the general conclusions of Bally et al. (1966). A profile across the Laramide Wind River Range in Wyoming demonstrated that its bounding thrust fault cut at moderate to shallow dip to >20 km depth in the crust, and accommodated >20 km of horizontal shortening of the continental basement (Smithson et al. 1978). The profile unambiguously resolved a long-standing debate as to whether or not the Laramide ranges were vertical ‘piston-cored’ uplifts, over which the sedimentary cover had been ‘draped’. A second profile across the southern Appalachian orogen showed that its crystalline core was a detached upper crustal flake that had overthrust the ancient continental platform by at least 260 km (Cook et al. 1979), and so could not be considered any sort of deep-seated ‘infrastucture’ for the Valley and Ridge province fold and thrust belt. The publication of the COCORP profiles accordingly marked the end of serious speculation that either of the imaged faults, or thrust-and-nappe structure in general, were primarily the result of vertical movements.

Despite its demise, the stockwerk folding hypothesis did contain the kernel of the idea that the lower continental crust is hotter and therefore may be weaker than the upper crust, and hence subject to lateral flow, as also anticipated by the early mobilists. During the detachment era (again, coincidentally), this qualitative idea was confirmed and quantified by the synthesis and publication of laboratory data on the brittle and ductile failure strengths of rocks (Brace & Kohlstedt 1980). By 1978, when the author entered graduate school, the kinematic relationship between upper crustal strain via faulting and deep crustal strain via flow mechanisms was a much-debated subject for strike-slip, thrust and normal fault systems, about which there was, and still remains, much controversy.

	Impact of the 1977 Penrose Conference near Tucson, Arizona

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
Even with the demise of fixism and acceptance of continental rifting and sea-floor spreading, the consensus view remained that continental extension was accommodated by steep normal faults, and hence no field evidence or mechanism was known for the mechanical extension of the upper crust much beyond 10% increase in original width (Stewart 1971; Thompson & Burke 1974). Further, it was tacitly assumed by most geologists that the formation and exposure of metamorphic rocks occurred in contractile regimes. However, a handful of field and geochronological studies in the Basin and Range province (Anderson 1971; Armstrong 1972; Wright & Troxel 1973; Proffett 1977), having little to do with the plate tectonics revolution, quietly began to mount a strong challenge to both of these views of continental tectonics.

Put into practice for Basin and Range field relations, the first implied that all low-angle faults, either on geological maps or in subsurface images, were thrust faults. The second implied that all Phanerozoic metamorphism in the Cordillera occurred during either Palaeozoic or Mesozoic episodes of horizontal contraction. Because thrust faulting in the retroarc Cordilleran fold-and-thrust belt ended by Early Tertiary time, major low-angle faults now recognized as Tertiary extensional detachments in the Basin and Range were initially interpreted as either Mesozoic thrusts (e.g. Thorman 1970; Drewes 1978) or alternatively as gravity slides of Mesozoic (Hose & Danes 1973) or Tertiary age (e.g. Compton et al. 1977).

As a typical example, in the Snake Range area of east-central Nevada, a number of large low-angle faults are developed within thick continental shelf deposits of Palaeozoic age that consistently place younger rocks on top of older. Among the best examples of these faults is the Snake Range decollement, which juxtaposes steeply tilted strata as young as mid-Tertiary over Cambrian strata metamorphosed to the amphibolite facies. The fault was originally discovered by Misch (1960) and interpreted to be a Mesozoic thrust fault. Armstrong's (1972) geometric analyses of relationships between Tertiary strata and a number of these faults showed that virtually none of the large-offset low-angle faults in the region could be Mesozoic in age, and that the consistency of younger-on-older relationships were best attributed in some way to extension. (A detailed analysis of this landmark paper may be found in Wernicke & Spencer 1999.)

The 1977 Penrose Conference convened near Tucson, Arizona (Max D. Crittenden, Peter J. Coney and George H. Davis, convenors) marks the beginning of the detachment era. It was attended by a large fraction of the geologists who over the ensuing five years would field-test the premise of the conference: that the Basin and Range was literally awash in both metamorphic tectonite and large faults that post-dated regional contraction of the continental interior. With the noteworthy exception of the mobilistic synthesis of Hamilton & Myers (1966), prior to this era few geologists believed that Cordilleran extension amounted to anything other than modest horizontal stretching along moderate to steeply dipping high-angle faults. This view was not substantially different from structural sections drawn near the turn of the century (e.g. the much-reproduced fig. 1 in Davis 1903), except perhaps for quantification of horizontal extension as greater than zero, but relatively small. By 1982, it was clear on the basis of field relations and geochronology that a profound mid-Tertiary period of extension resulted in the development of detachments with displacements large enough to exhume broad tracts of mid-crustal metamorphic tectonite and associated magmas in their footwalls, and that the net horizontal extension across the province was probably much larger than 10% (e.g. Davis & Coney 1979; Crittenden et al. 1980; Reynolds & Rehrig 1980; Wernicke 1981; Armstrong 1982; Frost & Martin 1982).

The generic template for core complexes that quickly developed was that of a domiform fault surface separating footwall metamorphic tectonites (or in some cases, unmetamophosed, deeper crustal levels) from distended hanging-wall rocks. In this template (e.g. Davis 1980), the maximum elongation direction in the tectonites trends parallel to antiformal axes defined by the detachment, spectacularly exposed in ranges such as the Snake Range, Whipple Mountains of southeastern California, and Catalina-Rincon Mountains of southeastern Arizona, the latter of which was visited and examined by participants of the 1977 conference. The maximum elongation direction in footwall tectonites also tends to parallel the maximum elongation direction of hanging-wall normal fault blocks. Footwall tectonites are often syn-tectonically intruded by granitic magmas, and exhibit progressive overprinting of higher temperature deformation fabrics by lower. Hanging-wall fault blocks are typically steeply tilted, and contain mafic to silicic volcanics interstratified with fluvial–lacustrine sediments and scarp-induced rock avalanche deposits.

The export of this generic template from the Cordillera to the rest of the world was swift. It is so distinctive that the presence of core complexes often became apparent simply by reading existing literature. For example, the first core complexes identified in the Alpine–Himalayan chain were in a previously well studied portion of the Cyclades Islands in the Aegean Sea. These core complexes were identified primarily by review of existing structural and geochronological data, and as was the case in the Cordillera, decades of interpreting low-angle structures in the Aegean region as thrust faults came to an abrupt end (Lister et al. 1984). Comparable revolutions also soon followed in the Alps (e.g. Selverstone 1988), Himalaya–Tibet (e.g. Burchfiel & Royden 1985), Caledonides (e.g. Serrane & Siguret 1987), and for accretionary settings in general (Platt 1986). Core complex elements were also soon identified in passive margin settings (Wernicke 1985; Lister et al. 1986) and in slow-spreading mid-ocean ridges (e.g. Karson & Dick 1983).

	Uniform stretching and its exceptions

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
During the detachment era, a parallel revolution occurred in understanding the physical relationship between extension and vertical motions of the crust. The key breakthrough came from consideration of the isostatic consequences of mechanical extension and ensuing conductive cooling of newly thinned lithosphere. In general, if ‘typical’ continental lithosphere is stretched uniformly, it will lose elevation, because the negative effect of thinning the crust outweighs the positive effect of thinning the subcrustal lithosphere. After mechanical stretching is over, conductive cooling of the boundary layer predicts elevation loss at a rate proportional to the square root of time, analogous to oceanic lithosphere as it moves away from spreading centres (McKenzie 1978). Adding to this framework the effect of sedimentary compaction and flexural strength, relatively simple physical models can be developed that predict the subsidence rate in sedimentary basins produced by a given amount of crustal stretching, most usefully in passive margin sedimentary wedges (e.g. Sclater & Christie 1980; Watts et al. 1982). Because of the increasing availability of seismic reflection profiles and well data from petroleum exploration of passive margins, literally hundreds of studies attempting to apply the uniform stretching model to sedimentary basins were published during and after the detachment era.

In addition to the powerful ‘exportability’ of their distinctive generic template, the discovery of core complexes in the Cordillera also led to the quantification of the amount of horizontal stretching of the upper crust in many parts of the Basin and Range. It was found that stretching was extremely heterogeneous, such that the province was a complicated patchwork of very highly extended regions set among relatively unextended crustal blocks (e.g. Guth 1981; Miller et al. 1983). It was also realised that although the degree of upper crustal extension varied many-fold, there was little or no commensurate variation in crustal thickness, falsifying the assumption of uniform stretching in dramatic fashion (Wernicke 1985). The simplest way to maintain a flat Moho is to treat the deep crust as a fluid or ‘crustal asthenosphere’ within which the upper crustal fault blocks float, a simple tectonic idea dating back to Taber (1927). In the case of the complex three-dimensional patchwork of highly extended areas in the Basin and Range, flow within an inviscid substrate would precisely complement upper crustal extension so as to minimize relief on the Moho (Block & Royden 1990).

Other areas of failure of the uniform stretching model were revealed in detailed analyses of uplift and subsidence patterns within and adjacent to passive margin sedimentary basins, which led many workers to propose a variety of non-uniform stretching models such as simple shear of the whole lithosphere (Wernicke 1985; Ussami et al. 1986) or combinations of simple shear and pure shear (e.g. Kusznir & Ziegler 1992). The notion of an intracrustal asthenosphere initially developed for extensional terrains has since been applied to collisional regimes with thick crust (e.g. Clark & Royden 2000; McQuarrie & Chase 2000). Culshaw et al. (2006) have gone so far as to call for a ‘quantitative revival’ of the infrastructure–superstructure concept, although these authors seemed unaware that the original concept was developed to explain mountain building without resort to horizontal strain of the crust as outlined above.

	The way ahead

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
The breakthroughs in extensional tectonics during the detachment era resulted primarily from professionally courageous interpretations of field relations that strongly contradicted community consensus, and from the advent of geochronological studies of crystalline rocks in core complex footwalls, which demonstrated their mid-Tertiary age. They occurred in the context of a wider consensus that all low-angle faults were either thrust faults or surficial gravity slides, normal faults were of limited displacement and restricted to continental rifts, and metamorphic tectonites form in contractile tectonic settings. The wholesale demolition of this consensus during the detachment era was profoundly transformative of continental tectonics, arguably the single most important advance in the post-plate tectonics era. (A geophysical perspective that instead emphasizes active tectonics and various continuum theories may be found in Molnar 2001.)

The objective of identifying and describing this transformation here is to highlight the importance of scepticism in moving the field ahead. In contrast to most community endeavours where the objective is to forge consensus (e.g. in religious, political or family matters), in science the highest calling of community members is to disembowel it, the wider the better. Further progress in understanding extensional detachments and metamorphic core complexes should therefore focus on scepticism of major hypotheses that in one way or another have weaknesses.

A top candidate for scrutiny is the notion that metamorphic tectonites in core complexes represent the down-dip continuation of the brittle shear plane of the overlying detachment fault (e.g. Wernicke 1981). In most instances, the situation appears to be more complex, to the point that it is questionable whether even a small fraction of footwall tectonite has this origin. Flow within a relatively inviscid fluid layer is a distinct alternative. However, as is clear from several papers in this volume, the formation of footwall tectonites in many instances occurs well before displacement on the overlying detachments, suggesting more complex mechanisms for the development and ‘capture’ of tectonites by the fault may be in play. Improved constraints on the timing of tectonite formation, as amply represented by the research published in this volume, will likely lead to new and surprising conclusions regarding the origin of footwall tectonite. These terrains are clearly the most important laboratories for testing models of the kinematic relationship between upper crustal and deeper crustal layers during tectonism, and the lack of consensus over how they evolve makes them all the more interesting.

An equally important candidate that merits scepticism is the concept that some detachments are active at dips of less than 30°, as suggested by seismic reflection profiles (most notably, the COCORP profile across the Sevier Desert basin in the Utah Basin and Range; Allmendinger et al. 1983) and field relations between detachments and hanging wall fault blocks exposed in a number of core complexes (e.g. Lister & Davis 1989). The close of the detachment era might also be marked by the emergence of a spirited, contrarian minority arguing that even though the extension is large, core complex detachments are not large-displacement faults (e.g. Miller et al. 1983). Although most geologists today, including the author, remain comfortable drawing reconstructions that include low-angle normal faults with large displacements, over the last quarter century a ‘loyal opposition’ has persistently challenged evidence in support of active slip on low-angle planes (e.g. Jackson & White 1989; Anders & Christie-Blick 1994; Anders et al. 2006; Wong & Gans 2008). These authors have properly stressed that an inventory of earthquake focal mechanisms on a par with that of low-angle thrust faults, strike-slip faults, and moderate- to high-angle normal faults has yet to materialize, and that slip on low-angle normal faults is in any event extremely difficult mechanically (e.g. Wills & Buck 1997). A novel modelling approach that accounts for thermal–mechanical feedbacks between brittle and viscous layers demonstrates that elastic strain energy becomes focused into the strongest layers of the lithosphere, producing primary low-angle normal faults without any special assumptions or anisotropy (e.g. Weinberg et al. 2007), and thus may explain the mechanics. Nonetheless, after many decades of seismic monitoring in regions of active extension, the community still awaits ‘the big one’ on an active low-angle normal fault.

The controversy, and its implications for the mechanics of brittle faulting in the crust, has recently stimulated international interest in the establishment of a borehole observatory through the very low-angle (10–12°) detachment imaged beneath the Sevier Desert basin (Christie-Blick et al. 2007), which geodetic data suggest is currently active (Niemi et al. 2004). The establishment of a borehole observatory across this fault is particularly timely in that the mechanical paradox of brittle slip on fault planes oriented at high angle to the maximum principal stress direction is common to both the Sevier Desert detachment and the San Andreas fault. The latter is developed within a transpressional tectonic regime and is currently being monitored by the San Andreas Borehole Observatory at Depth (SAFOD). Preliminary data suggest that the San Andreas is a weak zone of high pore-fluid pressure and strongly variable permeability, embedded within an otherwise much stronger crust (Hickman et al. 2008), along the lines suggested for the San Andreas by Rice (1992) and extended to low-angle normal faults by Axen (1992). Verifying that the Sevier Desert detachment is active, determining the orientations and magnitudes of the stresses within and adjacent to the fault zone and characterizing the pore fluid pressure and composition of fault rocks will provide significant new insight into fault mechanics. In particular, it will provide an opportunity to determine whether the physical processes that enable slip on the San Andreas are the same as those in the radically different tectonic setting of the Basin and Range.

	Acknowledgments

Top Abstract The fixist era The demise of fixism Impact of the 1977... Uniform stretching and its... The way ahead Acknowledgments References

 
Preparation of this account was supported by the Caltech Tectonics Observatory and NSF Grant EAR-0810328. Comments by Gregory A. Davis, George H. Davis and Uwe Ring greatly improved the balance and clarity of the manuscript.

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