Navy's march toward an unbeaten season came to a painful halt today.

Riding the passing arm of reserve quarterback Dan Marino and a harassing defense, 12th-ranked Pittsburgh beat up on the 14th-ranked Midshipmen in the second half in cruising to a 24-7 triumph before 51,332 at Pitt Stadium.

Despite losing two more starters and a key reserve with knee injuries on the treacherous AstroTurf surface, Navy (6-1) held the Panthers at bay in the first half and went to the locker room leading, 7-3.

But the Panthers (6-1) turned the second half into a private show. Led by defensive end Hugh Green, Pitt stopped the Navy offense cold, allowing the visitors a total of 20 yards after intermission. Navy finished the game with 116 yards.

Pitt's offense, meanwhile, took advantage of the slow-reacting Mid secondary in the second half.

Marino, a strong, 6-foot-4 freshman, replaced starting quarterback Rick Trocano, who pulled a hamstring sliding for a first down in the first quarter. Marino completed 22 of 30 passes for 227 yards and two touchdowns.

Navy, after losing tackles Steve Chambers and Mike Matthes and rover back Gregg Milo, was unable to stop the fired-up Panthers.

"The second half, we just came off the ball quicker," said Mark May, Pitt's 6-5, 287-pound offensive tackle. "In the first half, Navy didn't make any mistakes and was always in the right spot. The second half, we wore them down a bit."

On Pitt's second possession of the third period, Marino guided his team 60 yards in 16 plays for the go-ahead touchdown. The poised 18-year-old found split end Ralph Still wide open for gains of 22 and 11 yards to keep the time-consuming march alive.

On a third-and-one from the Navy seven, Marino botched a handoff to Freddie Jacobs, but held the ball and jumped into the pile to get the necessary yardage.

On fourth and goal at the three, Marino rolled to his right and hit Jacobs with a touchdown pass with 2:30 left in the quarter. Mark Schubert made the kick and Pitt was ahead to stay, 10-7.

"I just missed that handoff," Marino said. "Then I thought I better get what I could."

Navy was unable to move on its next possession, and a 58-yard Lex Lauletta punt put the Panthers back to their 33.

But Marino was hot.Hitting 10 straight passes at one point, he found Still, tight end Benjie Pryor and flanker Kenny Bowles for key gains in a 7 1/2-minute touchdown drive.

A pass-interference call on Navy's Chris Boblit brought up first and goal at the Navy four. On the next play, Rooster Jones followed the crunching blocks of May and 6-2, 252-pound guard Emil Boures around the right side for six points. Schuberts kick made it 17-7 with 7:55 left in the game, and Navy was doomed.

"They knocked the hell out of us," said Navy Coach George Welsh. "We had too many injuries on defense. We couldn't hold them off forever. Our offense had a lousy day. We couldn't make a first down."

The punishing hits administered to tail-back Duane Flowers by Green, nose guard Jerry Boyarsky and tackle Bill Neill finally took their toll.

On Navy's 10th play of the second half, Flowers (23 yards on 20 carries) was jolted at the Mid nine-yard line and fumbled. Pitt's Steve Fedel fell on the loose ball at the seven.

After a four-yard loss by fullback Randy McMillan, Marino lofted a high floater over Navy cornerback Charlie Meyers and Still made an over-the-shoulder grab in the end zone.

Schubert's third extra point gave Pitt a commanding lead, 24-7, with just 6:10 to play.

"We couldn't get the ball in the second half," said Navy quarterback Bob Powers, who played at nearby Beaver Falls High School. "Pitt was tough and we didn't get any mileage out of our running game at all. I wanted to throw more but I got some pressure and had to run some. They came at us hard."

Green, in particular, was a pain in Navy's side. The 6-2, 224-pound junior All-America was all over the field making his nine tackles. Ironically, a defensive mistake by Green gave Navy its only touchdown.

Schubert's 39-yard field goal with five seconds left in the opening quarter gave Pitt a 3-0 lead. But Heath bobbled a punt and Ed Meyers recovered for Navy at midfield.

Seven plays later, on fourth and two from the 27, Powers faked to fullback Kevin Tolbert up the middle and tossed a nifty pass to wide-open Carl Hendershot. The tight end caught the ball at the 15 and dragged Heath into the end zone.

"Our defense can't be blamed for that one," Green said. "That was entirely my fault. I had the tight end man to man but I heard someone call off the defense in the huddle so I didn't go with him. Blame me for that one.

"That was about the only mistake we made. Any team that tries to run against us will find it tough. And our secondary is pretty good."

After Steve Fehr's extra point made it 7-3, the Panthers began to show signs of life. On their next possession, they moved the ball to the Navy 31, but Schubert missed a 48-yard field goal.

Another second period Pitt drive fizzled when Marino was sacked by three Navy defenders for a 15-yard loss back to midfield.

But the second half was another story. With Still (eight catches for 104 yards) and Pryor (six catches for 57) running unattended most of the time, Marino was on the way to his best day as a college quarterback. Jones, McMillan and Jacobs got just enough yards on the ground to keep the Mids honest.

"Navy did an excellent job of stopping our running game and that's why we went to the short passing game," said Pitt Coach Jackie Sherrill. "We had to go to the air. Our defense once again played superbly. Green, Neill and the others had excellent games."

Pitt finished with 294 yards and had 20 first downs compared with Navy's six. With as many as seven starters injured, Navy must now regroup for Saturday's clash with Notre Dame.