A brush fire spread across 140 acres of open terrain on the grounds of Camp Pendleton near Fallbrook late Thursday afternoon. Base officials said the blaze broke out in the eastern reaches of the sprawling Marine Corps installation shortly after 5 p.m. No structural threats were reported, and the fire was 40 percent contained by 7 p.m. A team of 75 civilian firefighters were helping military personnel extinguish the blaze. “Fire activity has decreased and firefighters expect to have full containment by midnight tonight,” Cal Fire reported. The cause of the fire was not immediately clear.