Dead Sea Research
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The Dead Sea Transform Fault:
Its expression in the magnetic field
A high-resolution airborne magnetic (HRAM) survey was successfully conducted during 10 days in October
2003 aboard a Jordanian military helicopter piloted by Jordanian military personnel. It covered a 120
km long section of Wadi Araba (as it is called in Jordan) or Arava Valley (The Israeli name) from Aqaba
and Elat in the south up to Zofar in the north. Data were collected along lines crossing the rift
valley from side to side at a line spacing of 300 meters, which required frequent crossing by the
helicopter between Israeli and Jordanian air spaces. The work proceeded smoothly with full coordination
between all the civilian and military authorities. This is the first time that such an operation has
been conducted and the first time that a foreign, let alone an Arab, aircraft conducted a routine
survey over Israeli territory. Good relationships were forged between scientists, technicians and other
military and civilian personnel involved in coordinating and executing the survey.
The helicopter flew at an altitude of 128±20 m above terrain. The magnetometer was mounted in a
fiberglass “bird” and towed roughly 25 m below the helicopter; hence, the mean terrain clearance
was about 100 m. The flying speed was on average 40 m/s (144 km/h). Magnetic measurements were recorded
every 0.5 seconds, nominally at 20 m intervals. A total of 209,350 magnetic measurements were
collected along 4,187 km of survey lines.
The high measurement density, low flying altitude and the measurement quality resulted in an
unprecedented detail of the subsurface in the area. The map clearly delineates the trace of
the major fault in this area, which is part of the Dead Sea fault system. The fault trace was
hitherto known only in a few places. The magnetic map shows areas with thicker sedimentary cover,
which often coincide with water aquifers, and older faults, which act as permeability boundaries.
The magnetic map corresponds well with other geological and geophysical information, where
available, and can be used for infrastructure projects in the rift valley. The survey was also
used as a pilot study and proof-of-concept for similar surveys in Jordan and Israel that will
be studying and mapping mineral resources, aquifers, and faults.
The survey sheds light on the shallow structure of the fault zone and on the kinematics of the
plate boundary. An almost continuous transform fault follows the axis of the transform valley,
and is expressed by a magnetic anomaly several hundred meters wide. Despite infrequent seismic
activity and only intermittent surface exposure, the fault is delineated clearly on a map of
the first vertical derivative of the magnetic intensity field, indicating that the source of
the magnetic anomaly is shallow (the first vertical derivative map emphasizes magnetic anomalies
of shallow origin). The fault is manifested by a 10-20 nannoTesla negative anomaly
in areas where the fault cuts through magnetic basement, and by < 5 nannoTesla positive anomaly
in other areas. Modeling suggests that the shallow fault is several hundred meters wide, in
agreement with other geophysical and geological observations. A magnetic expression is observed
only along the active trace of the fault, and may reflect alteration of magnetic minerals due
to fault-zone processes or groundwater flow. The general lack of surface expression of the fault
in many areas along the valley may reflect the absence of surface rupture during earthquakes.
The magnetic data also indicates that unlike the San Andreas Fault, the location of this part
of the plate boundary was stable throughout its history. Magnetic anomalies also support a
total left-lateral offset of 105-110 km along the plate boundary. Finally, despite previous
suggestions of transtensional motion along the Dead Sea Transform, we did not identify any
igneous intrusions related to the activity of this fault segment.
(The reader is directed to USGS Fact Sheets
FS-236-95
and
FS-237-95
for a more detailed description of the magnetic method).
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U.S. Department of the Interior |
U.S. Geological Survey
