page_title: Half-graben
section_title: Sedimentation
hierarchical_section_title: Half-graben / Sedimentation
caption_attribution_description: English: Schematic diagram of sedimentation in a lake-filled half-graben sub-basin based loosely on diagrams by Mike Leeder and Rob Gawthorpe [1] and on Lake Baikal
context_page_description: A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults.
context_section_description: Four zones of sedimentation can be defined in a half-graben.
  The first is "escarpment margin" sedimentation, found along the major border faults bounding the half graben, where the deepest part of the basin meets the highest rift-shoulder mountains.
  Comparatively little sediment enters the half-graben across the major bounding fault, since uplift of the footwall causes the land on the footwall side to slope away from the fault.
  Rivers on that side therefore carry sediment away from the rift valley. 
  But as the lowest part of the basin with the greatest rate of subsidence, the escarpment margin experiences the highest rates of sedimentation, which may accumulate to several kilometers in depth.
  This sedimentation often includes very coarse debris such as huge blocks from rock falls, as well as fans of sediment from the basin wall. 
  Other material is transported across or along the basin to the deep water parts of a rift lake along the escarpment margin.
  Most of the sediment will enter the half-graben down the unfaulted hanging wall side.
  On the side of the basin opposite the main border fault, sedimentation occurs along the "hinged margin", which may also be called the "shoaling margin" or the "flexural margin". In this part of the basin, slopes are usually gentle and large river systems may carry sediment into the basin, which may be stored in deltas where they enter a rift valley lake. Littoral and sub-littoral carbonate deposits may well be accumulated in these conditions.
  The "axial margins" at the ends of basins often include low-gradient ramps where major rivers enter the basin, building deltas and creating currents within a rift lake that can carry sediment from one end to the other.
  Between adjacent half grabens there will be "accommodation zones" that may include local extension, compression or strike-slip faulting. These can create complex morphologies within which different mechanisms affect sedimentation.
  The types of sedimentation in half grabens also depends on lake levels in the rift, the climate (e.g. tropical versus temperate) in which the sediments form and the water chemistry.
  Lake Baikal is an unusually large and deep example of half-graben evolution.
  The lake is 630 by 80 kilometres (391 by 50 mi), with a maximum depth of 1,700 metres (5,600 ft).
  Sediment in the depression may be up to 6,000 metres (20,000 ft) in depth. The system also includes some small Quaternary volcanoes.
  In this lake, at first a series of half-grabens were linked in a linear chain. As the rift valley aged, extensive deformation developed on both sides of the lake, converting them into asymmetric full grabens.
caption: Lake-filled half-graben showing sedimentation dominantly from the 'hinge' margin
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