Constraints on the Structure of the Border Ranges Fault System, South-Central Alaska from Integrated 3-D Inversion of Gravity/Magnetic Data (grant 48312-AC8) Diane Doser, Dept. Geological Sciences,Univ. Texas at El Paso
Significance • The Border Ranges Fault System (BRFS) bounds the major petroleum producing region of Cook Inlet Basin • The geometry, timing, sense and amount of movement along BRFS and its influence on basin formation are poorly known • We will test models of BRFS structure and basin evolution using a novel 3-D inversion of gravity and magnetic data
Study Area Year 2
62
Inle
Bo rde r
Kenai Penin.
Ra ng es
PWS
Flt
.Sy s.
Co ok
60
56 -160
-156
Tr en ch -152
le ut ia n
Kodiak Island
A
58
B R FS
Pe ni n.
Gulf of Alaska
A la sk a
latitude
t
Alaska
Study Area Year 1
-148
-144
-140
longitude
Red is BRFS. Boxes show study areas for years 1 and 2. Black square indicates Anchorage.
61.75 Border Ranges Fault System Other faults and folds
61.5
61.25
61
60.75
60.5
60.25 -151.5
-151
-150.5
-150
-149.5
-149
-148.5
gravity data collected 2010 gravity data collected June 2009 gravity data collected March 2009 original gravity data base
In 2009 and 2010 we collected over 800 gravity observations (green, magenta, and black symbols) to help constrain our models of the BRFS. Note sparseness of original data (brown plusses).
Setting up GPS unit in the field (required for precise location of gravity observation points)
61.75
Border Ranges Fault System Other faults and folds Gravity observation
61.5
60 40
61.25
20 0 -20 -40
61
-60 -80 -100 -120
60.75
-140 -160 -180 free air gravity in mGals
60.5
60.25 -151.5
-151
-150.5
-150
-149.5
-149
-148.5
Our latest map of variation in gravitational acceleration across the BRFS.
X
X’
We have collected gravity observations in subregions to estimate variations in nearsurface densities (white boxes). These are comparable to estimates from well logs (blue boxes) or surface samples (tan boxes). Geology/gravity cross section (green line) shown in next slide.
Cross section along X-X’ (see previous slide) showing observed gravity (black dots), calculated gravity (black line) and error (red line). Preliminary geologic/density model shown below. Model is based on well log, seismic reflection, surface geology and other information.
Third Year Tasks • Complete paper on inversion approach used to determine near-surface densities • Conduct 2-D modeling of gravity data to aid in building 3-D model • Build plausible 3-D models to test • Complete testing of forward modeling code for magnetic data • Complete and test inversion code • Determine most viable models for the BRFS
