The Upper Rhine Graben (URG) revisited: Miocene transtension and transpression account for the observed first‐order structures

Abstract

Although the Upper Rhine Graben (URG) has been studied extensively for years, the origin of some of its first‐order structures is still under debate, particularly the relatively young uplift of the Vosges Mountains (VM) and Black Forest Mountains (BFM). Their uplift appears to be temporally related to the change of the URG into a continental transform, the rapid subsidence of its deep northern basin, and the onset of erosional and nondepositional phase south of the Northern Basin. Recent observations from newly released seismic reflection data, coupled with older geologic and seismic observations, are used to explain this correlation. We suggest that when the URG turned into a continental transform during the early Miocene, not only was its northern basin transtensionally subsiding as previously suggested, but the VM and BFM were transpressionally uplifted. Transpression became weaker with growing distance from the Alpine front, and north of Baden‐Baden the transpression is expressed only by down‐buckling of the sediments, forming a deep, elongated syncline. The largest uplifts and erosion associated with this event occurred along both boundaries of the southern URG. However, the center of the graben was also affected to some extent, causing widespread erosion of pre‐early Miocene sediments and subsequent nondeposition. The arcuate Vosges and Black Forest fault systems, which formed the boundary faults of the URG during the Oligocene, became mechanically unfavorable during the Miocene transpressional regime. Instead, more linear normal faults took over as the dominant boundary faults, forming the western and eastern Rhine Fault systems and assuming a strike‐slip component of motion.