Crews were starting to get ahead of a vegetation fire that erupted Wednesday afternoon near the Altamont Pass, quickly burning through 500 acres, authorities said.

The Grant Fire — actually a mass of at least seven fires — was about 35 percent contained by 9 p.m., according to the California Department of Forestry and Fire Protection.

Fanned by high winds, the blazes near Byron blew up just after 2 p.m., at Grant Line and Mountain House roads. There were soon two fires in Alameda County and five in Contra Costa County, said Cal Fire Battalion Chief Jonathan Cox.

Multiple structures were threatened, Cox said. There were no immediate reports of evacuations.

Mountain House and West Grant Line roads were closed in both directions, according to the Alameda County Sheriff’s Office. Both were expected to remain closed at least through Thursday afternoon.

The causes of the fires are under investigation.

Five agencies in total, including Cal Fire, responded to the blazes. They include fire departments for Alameda County, Contra Costa County, Livermore and Tracy.

A Pacific Gas and Electric Co. spokesman said the company had no indication that its power lines had been involved in sparking the fires or had suffered any damage from them. PG&E had not received any reports of downed lines or utility poles.

Last week, Cal Fire blamed four of October’s Northern California wildfires on trees coming into contact with PG&E power lines during an intense windstorm.

PG&E has adopted a new policy of shutting down power lines in advance of the most dangerous fire conditions — high winds and very low humidity — to prevent the lines from sparking fires. But spokesman Matt Nauman said the utility had not taken that step Wednesday.

“We have neither proactively de-energized any of our lines, nor have we deactivated them at the request of firefighters,” he said.

Chronicle staff writers David R. Baker and Erin Allday contributed to this report.

Sarah Ravani is a San Francisco Chronicle staff writer. Email: sravani@sfchronicle.com Twitter: @SarRavani