Translation Of Rocks Research Articles

Testing erosional and kinematic drivers of exhumation in the central Himalaya

This study investigates the influence of fault geometry, kinematics, and displacement on the exhumation history of the central Himalaya using geologic mapping constraints, a new interpreted cross-section, and a suite of 176 thermochronometer ages through the Marsyangdi valley in central Nepal. Guided by the cross-section, we integrate a forward model of fold-thrust belt evolution with a 2D thermokinematic model. Model-predicted and measured thermochronometer ages were compared to evaluate the sensitivity of thermochronometer ages to the geometry and location of structures, and their rates of deformation. Results indicate 84% of the measured data can be reproduced with a largely in-sequence system of faulting where displacement occurs on the Main Central thrust (MCT) from 23-16 Ma, the Ramgarh-Munsiari thrust (RMT) from 16-7.5 Ma, the Trishuli thrust (TT) from 7.5-6 Ma and the Main Boundary thrust (MBT) from 6-3 Ma. Our cross-section solution shows the development of a duplex that initiates at 4 Ma with the TT and MBT as the roof thrust. The duplex is translated over the Main Himalayan thrust (MHT) ramp, concurrent with forward propagation of faulting in the synorogenic Siwaliks from 3 Ma to present. Notably, the 2-0.5 Ma apatite fission-track ages between the MCT and Tethyan strata 45 km north of the MCT are not reproduced by the wide range of in-sequence deformation scenarios we explored. Instead, these data are consistent with simulations that include significant displacement on two out-of-sequence (OOS) faults in the last ∼1 Ma: 10 km of OOS faulting near the MCT followed by 5 km of displacement on a fault 15 km south of the MCT. Modeled OOS thrusting both builds significant topography in the hinterland and flexurally suppresses topography in the foreland. Consequently, our preferred kinematic solution has ∼10 km of final fault motion in the Siwaliks to rebuild topography at the MBT and the active MFT from ∼0.5 Ma to the present.Modeled exhumation rates are highly variable through time, and are highest during translation of rocks over high (∼10 km) ramps and during significant changes in architecture, such as duplex formation and OOS faulting. Notably high exhumation rates include 10.5 mm/yr during the first 4 Myr of MCT motion, 4-6 mm/yr at ∼7 Ma, due to the translation over an ∼10 km high TT ramp, and 7-12 mm/yr from 4 Ma to present driven by the development of the duplex and its translation over the MHT ramp, followed by OOS thrusting. The high (7-12 mm/yr) exhumation rates documented here from 4 Ma, correlate with a time of distinct climate change, including strengthening of the monsoon, and northern hemisphere glaciation, lending support to potential climate-tectonic feedbacks.

Testing local and extraregional sediment sources for the Late Cretaceous northern Nanaimo basin, British Columbia, using 40Ar/39Ar detrital K-feldspar thermochronology

Abstract Detrital K-feldspar 40Ar/39Ar thermochronology was conducted on clastic sedimentary rock samples collected from northern exposures of the Upper Cretaceous Nanaimo Group on Vancouver Island and adjacent Gulf Islands of British Columbia to constrain the denudation history of the local Coast Mountains batholith source region and determine the origin of extraregional sediment supplied to the basin. Strata of the northern Nanaimo Group deposited between 86 and 83 Ma (Comox and Extension formations) exhibit a 130–85 Ma age distribution of detrital K-feldspar 40Ar/39Ar ages that lack age maxima. These are interpreted to have been sourced from the southwestern Coast Mountains batholith. Younger strata deposited between 83 and 72 Ma (Cedar District and De Courcy formations) yield a broader age range (150–85 Ma) with an age maximum near the depositional age. These results indicate focused denudation of deeper-seated rocks east of the Harrison Lake fault. The youngest units deposited after 72 Ma (Geoffrey, Spray, and Gabriola formations) primarily yield younger than 75 Ma detrital K-feldspar ages with pronounced age maxima near the depositional age. This sediment was sourced extraregionally relative to the Coast Mountains batholith. We sought to constrain the origin of the extraregional sediment by measuring the thermal histories of 74 samples of basement rocks from throughout the Pacific Northwest, and by compiling a database of over 2400 biotite 40Ar/39Ar and K/Ar cooling ages from predominantly Cretaceous batholiths along the western North American margin. This analysis focused upon two previously proposed source regions: the Idaho batholith and the Mojave-Salina margin of southern California. The Nanaimo detrital K-feldspar 40Ar/39Ar age distributions favor the peraluminous Late Cretaceous Idaho batholith and its Proterozoic Belt-Purcell Supergroup sedimentary wall rock as the more likely source of the extraregional sediment and disfavor the Baja–British Columbia hypothesis for 2000–4000-km-scale translation of rocks along the margin during the Late Cretaceous.

Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC). Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of the burial on the metamorphism.

Lithospheric processes and products in the southern Canadian Cordillera: a Lithoprobe perspective

Analyses of Lithoprobe and other data from southwestern Canada provide new insights on how this portion of the Cordillera formed during plate convergence along the western margin of North America. Crustal rocks are detached from their mantle lithosphere, which must have been consumed during subduction. Detachment occurred at or near the base of the crust beneath the Intermontane and (or) Omineca belts, probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool, mechanically rigid, North American craton. During the Late Cretaceous and early Tertiary, rotation of detached rocks caught between the North American craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western Cordillera, and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern Cordillera during the same time interval. Transpression associated with rotation in the Foreland and Omineca belts ceased by the early Tertiary because detached allochthonous rocks of the crust became mechanically attached to, and thus physically part of, North America. Continued plate convergence led to regional transtensional shearing and associated crustal extension in the southern Canadian Cordillera, and perhaps as far inboard as northern Montana, where coeval magmatism was probably associated with new, or reactivation of ancient, lithosphere-penetrating fracture systems.