Event 1 in Trench T-19
Event 1 was identified in trench T-19, which was
excavated in an active stream channel of the Shehoret
alluvial fan
and in the central part of the Elat fault zone. The broader central Elat
fault zone is characterized by
sinistral strike-slip faulting
accompanied by local
compressional and
extensional structures,
including grabens,
push-up ridges,
and
flower structures.
Shallow seismic reflection data
show that the Elat fault zone is several hundred
meters wide and consists of several
subparallel,
north-northeast-trending
fault splays,
including flower structures. Amit et al. (2002)
note that these subsurface structures are
generally consistent with the mapped surface
traces across the alluvial-fan surfaces and
playa deposits. The part of the fault zone crossed by trench
T-19 is characterized by a north-trending
asymmetrical graben.
Trench T-19 was excavated across the
extrapolated northern continuation of this structure, where the graben
narrows and transitions into a
localized shear zone.
The sedimentary sequence exposed in trench T-19
is buried beneath the alluvial cover of the
active stream channel but excavations show that there are correlations
with the sequence exposed in trench T-18 to the south. This
correlation is based on the presence of the same
two
paleosols (reg soils)
in both trenches (Reg1 and Reg2) and on the similarity of their
ages. The lower paleosol in trench T-19 (unit Reg1) was
dated to 28.0 ± 2.3
ka,
whereas the upper paleosol (unit Reg2) was
estimated from soil development to date to
ca. 10 ka.
Event 1 represents the earliest recognized
surface-rupturing event in trench T-19 and marks
the onset of tectonic deformation associated
with the northern termination of the Shehoret
graben. Five faults that reach the surface were
identified in the trench. These faults are
mostly vertical and are characterized by rotated
clasts and relatively small vertical
displacements. The major deformation zone occurs
between faults C and D, where the section is
noncorrelative across the deformation zone and where
the two paleosols recognized elsewhere in the
section are absent.
Amit et al.
(2002:Table 1) used
IRSL and
pedogenic interpretation to date Event 1 to
2900-1300 BCE (4.1 ± 0.8
ka). Amit et al. (2002) concluded that the
push-up structure formed during the second stage
of graben development, between 4.5 and 1 ka.
Amit et al. (2002) inferred that sediment within
the fault zone had been exposed to sunlight,
thereby resetting its
luminescence signal. This interpretation
was used to explain the anomalously young age
(1.1 ± 0.1
ka) obtained from OSL Sample 2 collected
approximately 80 cm west of Fault D. Event 1 is
generally presumed to be constrained by the
presence of the two overlying Reg soils that were found in T-18, Reg1 and Reg2. Amit et al. (2002) dated the formation
of Reg soil Reg1 in nearby Trench T-18 to between
approximately 30 and 12
ka which agrees with OSL samples 5 and 6 which pre date Reg1 and date to
23.8 ± 5.1 ka and 28.0 ± 2.3 ka respectively.
As for Reg soil Reg2, this paleosoil was found
overlying, but outside of, the deformed zone
between Faults C and E. Reg2 was dated by OSL
Sample 4 to 4.1 ± 0.8
ka and by soil development to 10 ka.
It is, however, possible that the
1.1 ± 0.1
ka age is itself diagnostic of the event,
in which case Event 1 may have occurred between
approximately 800 and 1000 CE. Under this
interpretation, Event 1 could correlate with
similarly dated events identified in the Avrona
playa, including nearby events within the same
graben system such as Events 8 and 9 in Trench
T-18, both constrained to 500–1500 CE, and
Event 3 in Trench T-20, which was dated to
800–1000 CE.
Conceivably, a determining factor in
assigning an older age to Event 1 was the
date and presence of Reg soil Reg2. In addition to the
OSL age of 4.1 ± 0.8
ka, Amit et al. (2002:195) differentiated
Reg soils into "four different time periods:
late Holocene (2–4 ka), early Holocene
(10 ka), early Holocene to late Pleistocene
(10–14 ka), and Pleistocene (older than
50 ka)." By this standard, Reg soil Reg2 would
be expected to predate by at least 2 ka and possibly 10 ka and therefore be
significantly older than the 1.1 ± 0.1
ka OSL age. This Reg soil chronological framework
was based on "a chronosequence of Reg
(Torrifluvent, Typic Haplogypsid, or Gypsic
Haplosalid) soils" calibrated against dated
alluvial surfaces whose ages were constrained
by radiocarbon, OSL, and prehistoric artifact
dating (Amit and Gerson, 1986; Amit et al.,
1993; Avni et al., 2000)." Ultimately, OSL sample 2 does not fit
with other chronological data from Trench T-19 (and T-18) and the weight of the evidence
suggests Amit et al. (2002)'s date of 2900-1300 BCE for Event 1, although reactivation of
these faults at a later date (e.g. ~1 ka) is possible.
It was not possible to accurately determine the
exact displacement associated with Event 1.
However, the event is characterized by a
minimum vertical displacement of at least 1 m,
based on the vertical separation between the
base of the lower paleosol (R1) on either side
of the uplifted block between faults C and D. The fault
geometry is dominated by vertical fault planes
and
flower structures,
which Amit et al. (2002) interpreted as typical
of strike-slip fault systems. In addition,
reversed displacement on faults A and C suggests
a localized compressional stress regime within
the northern tip of the graben. This change from
extension to the south in Trench T-18 to localized
compression to the north in Trench T-19 resulted in
the development of a
push-up structure
at the northern termination of the graben.
If the
Wells and
Coppersmith (1994)
maximum-displacement relation for
normal faults
is applied, a minimum vertical displacement of
1 m implies an estimated minimum
moment magnitude
of approximately MW 6.6.
Figure 9b