In his testimony to Congress this week, Lt. Col. Oliver L. North has foreshadowed his defense against possible criminal charges that might stem from his role in the Iran-contra affair.

The central theme running through three days of testimony is that everything North did while at the National Security Council was authorized by his superiors. In a political arena, that assertion deflects criticism that North was a "loose cannon" inside the administration. In a legal arena, it suggests that, because he followed orders he considered legal, he did not personally break the law.

North also has argued that, during the two-year period Congress had banned most U.S. military aid to the Nicaraguan contras, under what is known as the Boland Amendment, the administration "had gone in extremis to find a way to live within the constraints and proscriptions of Boland." North said he "worked very, very hard to find legal ways to carry out the policy of the president."

Lawrence E. Walsh, the independent counsel for the Iran-contra affair, has been working on putting together a criminal case focusing on a broad conspiracy to defraud the government that would result in charges ranging from obstruction of justice to the misuse of government funds, according to sources.

North's shredding of NSC documents on the Iran-contra affair is being investigated for a possible obstruction-of-justice charge, sources said. Walsh is also operating on the premise that the profits derived from U.S. arms sales to Iran belonged to the U.S. government and that the diversion of those funds to the contras and elsewhere may have represented a misuse of U.S. funds.

Both North and his attorney, Brendan V. Sullivan Jr., have repeatedly demonstrated during the congressional testimony their sensitivity to Walsh's separate criminal investigation.

Sullivan cited Walsh's inquiry within minutes after North began his testimony Tuesday and said he had never represented a client forced to testify about "all matters that are the subject of a pending indictment."

Yesterday, Sullivan said that North in his testimony is "exercising great care in addition to telling the truth because . . . an independent counsel . . . occupies an office down the street, and he's like a separate, 'super-Justice Department.' . . . He's got 100 people down there looking at Col. North and everything that he did . . . . They're all poring over his testimony."

North said that his plan to act as a "fall guy" was based on the concern that a political controversy -- not a criminal investigation -- might result from the disclosure of the Iran-contra affair. "None of us, at least certainly not me, and no one I ever talked to, ever imagined that we had done anything criminally wrong," North said.

Retired Air Force major general Richard V. Secord, North's chief assistant in the private Iran-contra network, testifed earlier that North joked that if his role was ever revealed he would be pardoned.

Robert Owen, North's liaison to the contras during the aid ban, testified that he and North "would laugh" about going to jail for their activities.

North may have signaled a defense against possible obstruction-of-justice charges by testifying that he shredded documents during his entire stint at the NSC and that he began destroying Iran-contra papers in early October, more than a month before the criminal investigation started.

North said he probably intensified the shredding when he learned last Nov. 21 that Attorney General Edwin Meese III had begun an inquiry. But he added he was told Meese was conducting a fact-finding inquiry "in his role as friend to the president," and "the word 'investigation' was never used."

North also has indicated that he does not consider the Iranian arms sales profits to be U.S. funds, suggesting a line of defense to possible charges of misuse of government funds.

He also insists on calling profits "residuals" which were transfered to the contras, rather than a "diversion."

"The only thing we did was divert money out of {Iranian middleman Manucher} Ghorbanifar's pocket and put it to a better use," North said.