River delta avulsions are a primary mechanism to distribute sediment and
build coastal land. Experiments show that an avulsion can generate a new
delta lobe, and subsequent avulsions yield multiple lobes that
amalgamate to produce a semi-circular fan deposit. For channels that are
actively building lobes, a condition of sediment transport equilibrium
develops, termed alluvial grade, which is characterized by material
bypassing the delta topset and dispersing to the delta foreset. Previous
studies have examined alluvial grade under conditions of steady
subsidence and uniform basin depth. However, on tectonically active
margins, deltas are affected by punctuated subsidence and lobes prograde
into basins with variable depth. Both conditions disrupt alluvial grade,
which in turn affects avulsion timescales and thus delta morphology. We
explore these interrelated processes using measurements of delta and
basin morphology based on field surveys and remote sensing collected
from the Selenga Delta, which is located along the Baikal Rift Zone.
Major earthquakes, affiliated with normal faulting and possessing
recurrence intervals of several millennia, lower large portions of the
subaerial delta several meters below mean lake level. This results in an
increased regional gradient that triggers lobe-scale avulsions.
Moreover, the timescale for these events is shorter than that predicted
via autogenic lobe switching. Additionally, during periods tectonic
quiescence, smaller channel-scale avulsions occur every 10--90 yrs,
which produces sedimentation that compensationally fills embayments
located between distributary channels. This process gives rise to the
delta's fan-shape morphology. Stratigraphically, tectonically driven
subsidence events are expected to preserve discrete sedimentary units
that represent deposition and reworking associated with short-term
channel avulsions. Understanding the interplay between discrete,
tectonically driven subsidence events and autogenic sediment
accumulation patterns of a delta prograding into a tectonically active
basin will improve interpretations of stratigraphy of ancient systems.