Orogenic gold and geologic time: a global synthesis

Clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones. Based on a critical appraisal of the geophysical and geological literature, 26 parameters are estimated for each of 39 modern subduction zones. To isolate causal relationships among these parameters, multivariate analysis is applied to this data set. This analysis yields empirical quantitative relations that predict strain regime and strike-slip faulting in the overriding plate, maximum earthquake magnitude, Benioff zone length, slab dip, arc-trench gap, and maximum trench depth. Excellent correlation is found between length of the Benioff zone and the product of convergence rate and age of the downgoing slab. This relationship is consistent with the conductive heating model of Molnar et al. (1979), if the model is modified in one respect. The rate of heating of the slab is not constant; it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate. The structural style in the overriding plate is determined by its stress state. Though the stress state of overriding plates cannot be quantified, one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional (class 1, back-arc spreading) to strongly compressional (class 7, active folding and thrusting). This analysis indicates that strain class is probably determined by a linear combination of convergence rate, slab age, and shallow slab dip. Interplate coupling, controlled by convergence rate and slab age, is an important control on strain regime and the primary control on earthquake magnitude. Arc-parallel strike-slip faulting is a common feature of convergent margins, forming a forearc sliver between the strike-slip fault and trench. Optimum conditions for the development of forearc slivers are oblique convergence, a compressional environment, and a continental overriding plate. The primary factor controlling presence of strike-slip faulting is coupling; strongly oblique convergence is not required. The rate of strike-slip faulting is affected by both convergence obliquity and convergence rate. Maximum trench depth is a response to flexure of the underthrusting plate. The amount of flexural deflection at the trench depends on the vertical component of slab pull force, which is very sensitive to slab age and shallow slab dip. Shallow slab dip conforms to the cross-sectional shape of the overriding plate, which is controlled by width of the accretionary prism and duration of subduction. Deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow. Much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike-slip faulting and strain regime. Empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones.

Relations among subduction parameters

A numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes. Crustal deformation is calculated by a two-dimensional finite element model of deformation in response to subduction and accretion of continental crust. Erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power. The relative importance of the tectonic and erosion processes is given by a dimensionless “erosion number” relating convergence velocity, rock erodibility, and precipitation rate. This number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt. Fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction. These characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt. The two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity. Models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior. Models of the latter case predict a focused zone of exhumation at the margin of the orogen and, at high erosion number, a reversal in the topographic asymmetry. Natural examples of these two cases are presented. The Southern Alps of New Zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate. The asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry. The contrasting example of dominant wind in the direction of subduction motion is provided by the Olympic Mountains of Washington State. In this case, exhumation of deep levels of the Cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900248

Orogeny and orography: The effects of erosion on the structure of mountain belts

Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms ...

/pdf/rheology-of-the-lower-crust-and-upper-mantle-evidence-from-3ttor5zpl7.pdf

Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations

Fieldwork complemented by SPOT image analysis throws light on current crustal shortening processes in the ranges of northeastern Tibet (Gansu and Qinghai provinces, China). The ongoing deformation of Late-Pleistocene bajada aprons in the forelands of the ranges involves folding, at various scales, and chiefly north-vergent, seismogenic thrusts. The most active thrusts usually break the ground many kilometres north of the range-fronts, along the northeast limbs of growing, asymmetric ramp-anticlines. Normal faulting at the apex of other growing anticlines, between the range fronts and the thrust breaks, implies slip on blind ramps connecting distinct active decollement levels that deepen southwards. The various patterns of uplift of the bajada surfaces can be used to constrain plausible links between contemporary thrusts downsection. Typically, the foreland thrusts and decollements appear to splay from master thrusts that plunge at least 15–20 km down beneath the high ranges. Plio-Quaternary anticlinal ridges rising to more than 3000 m a.s.l. expose Palaeozoic metamorphic basement in their core. In general, the geology and topography of the ranges and forelands imply that structural reliefs of the order of 5–10 km have accrued at rates of 1–2 mm yr−1 in approximately the last 5 Ma.



 From hill to range size, the elongated reliefs that result from such Late-Cenozoic, NE–SW shortening appear to follow a simple scaling law, with roughly constant length/width ratio, suggesting that they have grown self-similarly. The greatest mountain ranges, which are over 5.5 km high, tens of kilometres wide and hundreds of kilometres long may thus be interpreted to have formed as NW-trending ramp anticlines, at the scale of the middle–upper crust. The fairly regular, large-scale arrangement of those ranges, with parallel crests separated by piggy-back basins, the coevality of many parallel, south-dipping thrusts, and a change in the scaling ratio (from ≈5 to 8) for range widths greater than ≈30 km further suggests that they developed as a result of the northeastward migration of large thrust ramps above a broad decollement dipping SW at a shallow angle in the middle–lower crust. This, in turn, suggests that the 400–500 km-wide crustal wedge that forms the northeastern edge of the Tibet–Qinghai plateau shortens and thickens as a thick-skinned accretionary prism decoupled from the stronger upper mantle underneath.



 Such a thickening process must have been coupled with propagation of the Altyn Tagh fault towards the ENE because most thrust traces merge northwestwards with active branches of this fault, after veering clockwise. This process appears to typify the manner in which the Tibet–Qinghai highlands have expanded their surface area in the Neogene. The present topography and structure imply that, during much of that period, the Tibet plateau grew predominantly towards the northeast or east-northeast, but only marginally towards the north-northwest. This was accomplished by the rise, in fairly fast succession, of the Arka Tagh, Qiman Tagh, Mahan shan, Tanghenan Shan, and other NW-trending mountain ranges splaying southeastwards from the Altyn Tagh, isolating the Aqqik-Ayakkum Kol, Qaidam, Suhai and other catchments and basins that became incorporated into the highland mass as intermontane troughs. The tectonic cut-off of catchments and the ultimate infilling of basins by debris from the adjacent ranges, a result of tectonically forced internal drainage, have thus been essential relief-smoothing factors, yielding the outstandingly flat topography that makes Tibet a plateau.



 Using Late-Mesozoic and Neogene horizons as markers, the retrodeformation of sections across the West Qilian Ranges and Qaidam basin implies at least ≈150 km of N30°E Neogene shortening. On a broader scale, taking erosion into account, and assuming isostatic compensation and an initial crustal thickness comparable to that of the Gobi platform (47.5±5 km), minimum amounts of Late-Cenozoic crustal shortening on NE sections between the Kunlun fault and the Hexi corridor are estimated to range between 100 and 200 km. In keeping with the inference of a deep crustal decollement and with the existence of Mid-Miocene to Pliocene plutonism and volcanism south of the Kunlun range, such values suggest that the lithospheric mantle of the Qaidam plunged obliquely into the asthenosphere south of that range to minimum depths of the order of 200–300 km. A minimum of ≈150 km of shortening in the last ≈10 Ma, consistent with the average age of the earliest volcanic–plutonic rocks just south of the Kunlun (≈10.8 Ma) would imply average Late-Cenozoic rates of shortening and regional uplift in NE Tibet of at least ≈15 mm yr−1 and ≈0.2 mm yr−1, respectively. Such numbers are consistent with a cumulative sinistral offset and slip rate of at least ≈200 km and ≈2 cm yr−1, respectively, on the Altyn Tagh fault east of 88°E. The fault may have propagated more than 1000 km, to 102°E, in the last 10 Ma.



 Our study of ongoing tectonics in northeast Tibet is consistent with a scenario in which, while the Himalayas-Gangdese essentially ‘stagnated’ above India’s subducting mantle, much of Tibet grew by thickening of the Asian crust, as propagation of large, lithospheric, strike-slip shear zones caused the opposite edge of the plateau to migrate far into Asia. The Asian lithospheric mantle, decoupled from the crust, appears to have subducted southwards along the two Mesozoic sutures that cut Tibet north of the Gangdese, rather than to have thickened. The Bangong-Nujiang suture was probably reactivated earlier than the Jinsha-Kunlun suture, located farther north. Overall, the large-scale deformation bears a resemblance to plate tectonics at obliquely convergent margins, including slip-partioning along large strike-slip faults such as the Altyn Tagh and Kunlun faults. Simple mechanisms at the level of the lithospheric mantle are merely hidden by the broader distribution and greater complexity of strain in the crust.

/pdf/crustal-thickening-in-gansu-qinghai-lithospheric-mantle-1lhm26pbvl.pdf

Richard J. Norris

Papers

Late Quaternary slip rates and slip partitioning on the Alpine Fault, New Zealand

The obliquely-convergent plate boundary in the South Island of New Zealand: implications for ancient collision zones

The structural evolution of active fault and fold systems in central Otago, New Zealand: evidence revealed by drainage patterns

Quartz fabrics in the Alpine Fault mylonites: Influence of pre-existing preferred orientations on fabric development during progressive uplift

Cainozoic history of southern New Zealand: An accord between geological observations and plate-tectonic predictions