Cross-strike discontinuities and along-strike variability in continental collision zones

虽然山带是我们理解造山作用（造山过程）的坩埚，但大多数山带只在一个方向上进行研究：平行于构造搬运。该方法揭示了从变质腹地到前陆逆冲系统的重要走向变化，并在造山带模型中取得了突破性进展，但淡化了在大多数造山带中观察到的重大走向变化。同样未被重视的是，在整个造山带旋回中，继承的（造山前）构造不可否认地影响着造山带构造的发展。因此，为了充分了解造山过程，有必要通过对碰撞板块的造山前构造的清晰描述来补充对造山系统的理解。喜马拉雅系统之所以被选为自然实验室，是因为其壮观的、被分解的保存，公认的前喜马拉雅基底交叉结构，已知的构造板块配置，收敛速度和最近的演化，以及有限的构造叠加，使得喜马拉雅和前喜马拉雅变形之间的区别得以区分。

Although mountain belts are the crucible of our understanding of orogenesis (mountain building processes), most mountain belts are only studied in one direction: parallel to the tectonic transport. This approach has revealed important across-strike variations from metamorphic hinterlands towards foreland thrust systems and led to ground-breaking advancements in orogenic models, but downplayed significant along-strike variations observed within most orogens. Equally underappreciated is the fact that inherited (pre-orogenic) structures undeniably influence the development of the orogenic architecture throughout the orogen’s cycle. To fully understand mountain building processes, it is therefore necessary to complement the understanding of the orogenic system with a clear depiction of the pre-orogenic configuration of the colliding plates. The Himalayan system is chosen as a natural laboratory because of its spectacular and unravelled preservation, recognized pre-Himalayan basement cross-structures, known tectonic plate configuration, convergence rates and recent evolution, and consequently limited tectonic overprinting enabling the distinction between Himalayan and pre-Himalayan deformation. The Himalayan system is commonly perceived as laterally continuous; its main geological subdivisions have been viewed as uninterrupted along its entire length, despite significant along-strike variation in topography and relief, orogen structure, and convergence rates. In this grant cycle, I specifically aim to identify and analyse lateral variations in the Himalayan orogen, and to elucidate how such variations may be linked with the pre-collision internal configuration of the underthrusting plate. In this proposal, it is hypothesized that NE-trending Precambrian basement ridges buried beneath the Indo-Gangetic Plain south of the Himalayan Frontal thrust continue beneath the Himalayan orogen and may have contributed to these along-strike changes. Most importantly, the possible link between these ridges and deep-seated structures that may have been repeatedly reactivated through time has yet to be established. If these ridges interacted with the orogen early in its evolution, this may have lead to orogen-parallel strain partitioning, expressed within the metamorphic core as lateral ductile strain variations, in the foreland fold-thrust belt as lateral ramps, lateral differential exhumation, and/or sediment thickness variations, or even in the present-day along-strike partitioning of seismicity. These potential interactions will be explored through several HQP-led projects involving: 3D reconstruction of the foreland fold-thrust belt using seismic data; field and lab investigations to assess along-strike variation in strain, metamorphic peak conditions, and exhumation of the Himalayan metamorphic core; numerical stress modelling linking present-day seismicity, stress trajectories, and cross-strike basement ridges; and dynamically-scaled centrifuge analogue modelling of deformation. It is anticipated that this program will ultimately provide key dynamic links to understand basement cross-structures in the evolution of older mountain belts, such as the Appalachians, the Canadian Cordillera, and the Grenville orogen. Such lateral discontinuities have been interpreted to control earthquake ruptures, influence lateral sedimentary thickness variations and fold-thrust belt geometry, and may be related to deep-seated structures that can localize mineralizing fluids and control hydrocarbon migration through the foreland basin, such as what is observed in the Western Canadian sedimentary basin. In the case of the Himalayan system, such hidden geohazard controls can have tremendous repercussions for one of the most densely-populated regions on Earth.