README The digital data of this open-file report are available from the following U.S. Geological Survey URL: http://pubs.usgs.gov/of/2012/1080/ DATA CONTENTS The database includes an ASCII file containing new helicopter aeromagnetic data, two ASCII files with point locations of the local maximum horizontal gradient derived from the aeromagnetic data, and physical property data. The digital dataset consists of one file (helo_mag.xyz) containing new aeromagnetic data. Approximately 266,434 point data were gridded, added to earlier survey data, and contoured to produce aeromagnetic contours on the map. The file contains the principal data fields of the aeromagnetic measurements, with the first line (marked by a "/") that has an abbreviated description for each measurement. The format of the data is described below. Each measurement has a line number, latitude, longitude, date and time information, radar and GPS altitude and various levels of magnetic field measurements. Measurement Description: Fiducial * Fiducial count (flight specific) Line * Line number Direction degrees Direction of flight line Date * Date flown (YMMDD) julian * julian day (YYYYDDD) hr * hour flown (PST) min * minute flown (PST) sec * second flown (PST) lat_wgs84 decimal degrees latitude coordinate (WGS84 datum) lon_wgs84 decimal degrees longitude coordinate (WGS84 datum) utmx84 meters x UTM zone 10 coordinate (WGS84 datum) utmy84 meters y UTM zone 10 coordinate (WGS84 datum) utmx27 meters x UTM zone 10 coordinate (NAD27 datum) utmy27 meters y UTM zone 10 coordinate (NAD27 datum) gps-sec sec GPS daily second counter (0-86399) gps-ht meters Height above sea level radarm meters radar height (meters above ground level) m_base nT measured base station magnetic field, located near Diablo Canyon x volt Fluxgate x-axis y volt Fluxgate y-axis z volt Fluxgate z-axis mag_raw nT raw magnetic total field (uncompensated) mag-comp nT compensated raw magnetic field mag_lag_cor nT Lag corrected mag-comp magnetic field mag-diur-cor nT diurnally corrected compensated magnetic field head-cor nT line to line heading correction mag-head-cor nT heading-corrected mag-diur-cor magnetic field mag-sim-lvl nT Conventionally leveled mag-head-cor mag-mic-lvl nT Microleveled mag-sim-lvl igrf nT Calculated IGRF (International Geomagnetic Reference Field 2005) value mag-igrf-cor nT IGRF corrected magnetic field (mag-mic-lvl) tmi-final nT IGRF corrected magnetic field (mag-igrf-cor) vdv nT/m First order FFT2 vertical derivative Two additional files are also included that contain the horizontal maximum gradients calculated using the method described in Blakely and Simpson (1986). grad_big.xyz contains those gradients larger than the mean gradient; grad_small.xyz contains those gradients that are smaller than the mean gradient. See report for details of the measurements. The format of the file is longitude, latitude, and amplitude of horizontal maximum gradient. Locations are on the NAD27 datum. Physical Properties An additional file named physical_properties.txt contains physical property information from hand samples in the map area. Data are listed as tab delimited values. Densities of rock samples were determined using a precision Sartorius electronic balance. All rocks were weighed dry in air (Wa), saturated in water (Ww), and saturated with water in air (Ws). From these measurements, grain density, dry bulk density, and saturated bulk density were calculated using the following formulas: Grain density = Wa/(Wa-Ws) Dry bulk density = Wa/(Ws-Ww) Saturated bulk density = Ws/(Ws-Ww) Magnetic susceptibilities for samples that do not have paleomagnetic data were measured using a Geophysica KTÜ5 susceptibility meter and are reported to 0.01 x 10-3 SI units. The Geophysica KT-5 calculates volume susceptibility by assuming the sample shape is an infinite half-space. The instrumentÍs ability to measure magnetic susceptibility is affected by surface roughness, weathering, and sample size, all of which can result in an underestimation of a sampleÍs true susceptibility. The magnetic susceptibility values reported represent an average of multiple (4-8) readings on the sample. Diabase and ophiolite paleomagnetic samples were cored from oriented hand samples collected near/around a location with a latitude= 35.226378Á, longitude= 120.853929Á, and elevation is near sea level. The International Geomagnetic Reference Field 2009 for this location has a declination (D)= 13.6Á, inclination (I)= 60Á, field intensity = 50986 nT, and the geo-axial dipole (GAD) = 48Á. All samples were subjected to progressive 10 step alternating field (AF) demagnetization (applying 0, 2, 5, 10, 15, 20, 30, 40, 60, and 80 mT) in order to remove secondary overprint magnetizations and resolve the primary remanent magnetization directions. Both primary and overprint magnetizations directions were fit using standard principal component (PCA) line fits and great circle (GC) analyses and software (Paleomac by CognÚ). Sample directions are determined by fitting the demagnetization data with straight-line fits on Zijderveld plots or great circles. Susceptibilities were run on Bartington MS3 using Bartsoft software. Additional text in data file describes how well these sites provided primary directions. Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, v. 51, p. 1494-1498.